Search

Neonicotinoid insecticides are often detected in pollen and nectar, which are the main food resources for honey bees. Sub-lethal and chronic exposure to these insecticides may have a prolonged effe...

Climate change is associated with increases in temperature and the frequency and intensity of abiotic disturbances such as drought that can affect forest resilience at multiple scales. Higher temperatures leading to extended growing seasons alter the phenology, distribution, and population dynamics of herbivorous insects. Bark beetles (Coleoptera: Curculionidae: Scolytinae) are among the most ecologically and economically important herbivorous insect taxa and can undergo population eruptions and cause widespread coniferous tree mortality. Specifically, Ips bark beetles preferentially attack conifers that are stressed, dying, or dead—such tree conditions are predicted to become more common due to climate change. Our chapter objectives were to synthesize knowledge on the effects of drought on (1) the incidence of Ips beetle outbreaks and (2) changes in conifer host physiology that increase their susceptibility to bark beetles. We documented 132 Ips beetle outbreaks globally between 1919 and 2018 that were reported in association with drought or water stress. However, very few studies quantitatively demonstrated this relationship and/or defined the mechanisms using experiments. Alterations in host physiology that lead to tree stress will increase colonization by Ips beetles, but their responses may be dependent on attributes at the tree, stand to landscape level. Given the shifts in precipitation and temperature regimes worldwide, we argue for more in-depth studies that quantify the damage and drivers of Ips beetle outbreaks, assessing simultaneous changes in host trees and Ips beetles, ways to mitigate damage for beetle outbreaks, and urgently needed proactive adaptive management strategies in drought-prone forests worldwide.

Recent studies demonstrated pyrethroid resistance associated with voltage-gated sodium channel mutations in populations of the epibenthic amphipod, Hyalella azteca. Resistant populations were able to tolerate and bioconcentrate pyrethroids at concentrations significantly higher than toxic levels for non-resistant populations. In conjunction with elevated bioconcentration potential, environmental alteration particularly as a result of global climate change is anticipated to significantly alter abiotic parameters including temperature and salinity. These changes are expected to influence uptake and biotransformation of contaminants. Thus, the aims of the current study were a) to examine the bioconcentration potential of permethrin in two pyrethroid-resistant clades of H. azteca and b) assess the influence of temperature and salinity changes on toxicokinetic parameters. Two pyrethroid-resistant clades of H. azteca were exposed to

Fluvalinate has been extensively used in the United States to combat honey bee colony loss due to Varroa destructor mites. Our objectives were to investigate the extent of fluvalinate contamination in commercially available wax and to define exposure pathways to larval and adult honey bees. All the commercial wax tested contained elevated fluvalinate concentrations, indicating a need for regulation of the sources of wax being rendered for resale. Based on the negative logarithm of the partition coefficient between wax and pollen (-0.54), it is evident that fluvalinate has the potential to actively transfer from the contaminated wax into hive matrices. This was confirmed by adding fluvalinate-dosed wax, fluvalinate-impregnated strips, or a combination of the 2 to hives. Larvae and adult bees were checked for fluvalinate exposure using gas chromatography-mass spectrometry analysis. Larvae had detectable concentrations of fluvalinate in all treatments. Bioaccumulation in adult bees was significantly affected by the interaction between treatment type and application time. In other words, residues from hives that only had fluvalinate-dosed wax were comparable to residues in hives that were actively being treated, suggesting that transfer of fluvalinate from wax into adult bees was an important exposure route. In conclusion, exposure of fluvalinate from contaminated wax and treatment strips to honey bees needs to be considered when the risk for colony loss in hives is being evaluated. Environ Toxicol Chem 2019;38:1356-1363. © 2019 SETAC.

Spatial synchrony and cycles are common features of forest insect pests, but are often studied as separate phenomenon. Using time series of timber damage caused by Dendroctonus frontalis Zimmermann (Coleoptera: Curculionidae) (southern pine beetle) in 10 states within the southern United States, this study examines synchrony in D. frontalis abundance, the synchronizing effects of temperature extremes, and the evidence for shared cycles among state populations. Cross-correlation and cluster analyses are used to quantify synchrony across a range of geographic distances and to identify groups of states with synchronous dynamics. Similar techniques are used to quantify spatial synchrony in temperature extremes and to examine their relationship to D. frontalis fluctuations. Cross-wavelet analysis is then used to examine pairs of time series for shared cycles. These analyses suggest there is substantial synchrony among states in D. frontalis fluctuations, and there are regional groups of states with similar dynamics. Synchrony in D. frontalis fluctuations also appears related to spatial synchrony in summer and winter temperature extremes. The cross-wavelet results suggest that D. frontalis dynamics may differ among regions and are not stationary. Significant oscillations were present in some states over certain time intervals, suggesting an endogenous feedback mechanism. Management of D. frontalis outbreaks could potentially benefit from a multistate regional approach because populations are synchronous on this level. Extreme summer temperatures are likely to become the most important synchronizing agent due to climate change.

Large-scale honey bee colony loss threatens pollination services throughout the United States. An increase in anthropogenic pressure may influence the exposure of hives to household and agricultural pesticides. The objective of this survey was to provide an assessment of the risk of exposure to commonly used pesticides to honey bee colonies in Virginia in relation to land use. Adult honey bee, pollen, and wax samples from colonies throughout Virginia were evaluated for pyrethroid, organophosphate, organochlorine, and triazine pesticides using gas chromatography-mass spectrometry analysis. Of the 11 pesticides analyzed, nine were detected in one or more hive matrices. The probability of detecting a pesticide in pollen was less in forests than in pasture, agriculture, or urban landscapes. Coumaphos and fluvalinate were significantly more likely to be detected across all matrices with concentrations in wax as high as 15500 and 6970 ng/g (dry weight), respectively, indicating the need for further research on the potential effects of miticide accumulation in wax to larval and adult bees.

Although theoretical models have demonstrated that predator-prey population dynamics can depend critically on age (stage) structure and the duration and variability in development times of different life stages, experimental support for this theory is non-existent. We conducted an experiment with a host-parasitoid system to test the prediction that increased variability in the development time of the vulnerable host stage can promote interaction stability. Host-parasitoid microcosms were subjected to two treatments: Normal and High variance in the duration of the vulnerable host stage. In control and Normal-variance microcosms, hosts and parasitoids exhibited distinct population cycles. In contrast, insect abundances were 18-24% less variable in High- than Normal-variance microcosms. More significantly, periodicity in host-parasitoid population dynamics disappeared in the High-variance microcosms. Simulation models confirmed that stability in High-variance microcosms was sufficient to prevent extinction. We conclude that developmental variability is critical to predator-prey population dynamics and could be exploited in pest-management programs.

The woodwasp, Sirex nigricornis (Hymenoptera: Siricidae), is solitary and utilizes a symbiotic fungus to extract nourishment from pine trees to feed its larvae. The woodwasp has a brief adult life, but the larvae develop for 1–3 years in the tree xylem. Infections with the nematode Deladenus proximus have been documented in the native woodwasp, S. nigricornis in the eastern United States and Canada. These nematodes appear to sterilize female woodwasps; however, the extent of the pathology and other aspects of the biology of D. proximus remain unknown. In this study we examined the effects of D. proximus on S. nigricornis using fresh – not preserved – specimens. Between 2009 and 2012, a total of 1639 woodwasps were examined for internal nematodes from emerging sites in Illinois, Louisiana and South Carolina. From this total, only 112 individuals were infected with the nematode D. proximus , with varying prevalence across localities and years. Nematodes were found inside every egg of infected females, as well as the hemocoel and the mycangia. Morphometric analyses of mycetophagous reared adult nematodes suggest that a single species is present in localities from Arkansas, Illinois, Louisiana, New York and South Carolina. The screening of the mitochondrial cytochrome oxidase subunit 1 ( cox1 ) of these organisms is consistent with this pattern in that all of these individuals belong to a single clade. Deladenus proximus appears to be an efficient sterilizer, yet its prevalence is relatively low. Experimental infections of the invasive Sirex noctilio are recommended to test the viability of using this nematode as a biological control agent.

Many of our advances regarding the spatial ecology of predators and prey have been attributed to research with insect parasitoids and their hosts. Host–parasitoid systems are ideal for spatial-ecological studies because of the small size of the organisms, the often discrete distribution of their resources, and the relative ease with which host mortality from parasitoids can be determined. We outline an integrated approach to studying host–parasitoid interactions in heterogeneous natural landscapes. This approach involves conducting experiments to obtain critically important information on dispersal and boundary behavior of the host and parasitoid, large-scale manipulations of landscape structure to reveal the impacts of landscape change on host–parasitoid interactions and temporal population dynamics, and the development of spatially realistic, behavior-based landscape models. The dividends from such an integrative approach are far reaching, as is illustrated in our research on the prairie planthopper Prokelisia crocea and its egg parasitoid Anagrus columbi that occurs in the tall-grass prairies of North America. Here, we describe the population structure of this system which is based on a long-term survey of planthoppers and parasitoids among host–plant patches. We also outline novel approaches to experimentally quantify and model the movement and boundary behavior of animals in general. The value of this information is revealed in a landscape-level field experiment that was designed to test predictions about how landscape change affects the spatial and temporal population dynamics of the host and parasitoid. Finally, with these empirical data as the foundation, we describe novel simulation models that are spatially realistic and behavior based. Drawing from this integrated approach and case study, we identify key research questions for the future.

1 Quantifying dispersal in predator–prey systems can improve our understanding of how these species interact in space and time, as well as their relative distributions across complex landscapes. 2 We measured the dispersal abilities of three forest insects associated with red pine decline: the eastern five spined pine engraver Ips grandicollis (Coleoptera: Curculionidae), its main predator Thanasimus dubius (Coleoptera: Cleridae) and the basal stem and root colonizer Dendroctonus valens (Coleoptera: Curculionidae). We also examined the edge behaviours of these species and the predator Platysoma spp (Coleoptera: Histeridae) between red pine stands (habitat) and clearings (nonhabitat). 3 Thanasimus dubius dispersed 12 times farther than its prey I. grandicollis, with 50% of predators dispersing farther than 1.54 km. This profound difference in dispersal behaviour between prey and predator may contribute to the clumped distribution of I. grandicollis. 4 Most T. dubius and D. valens were confined in the pine forest, thus showing strong edge behaviour. This differed from I. grandicollis and Platysoma spp., which were commonly found in open areas adjacent to red pine plantations. 5 The bark beetle I. grandicollis and one of its main predators, T. dubius, exhibited different patterns of movement within a fragmented landscape. Despite a greater dispersal ability of T. dubius within forests, the spatial distribution of this predator may be restricted by fragmentation of its habitat, and provide an opportunity for partial escape of its prey. 6 The present study contributes to our knowledge of top-down forces within red pine stands undergoing decline. Differences of dispersal patterns and edge behaviour could contribute to the initiation of new pockets of decline, as well as the connectedness among existing ones.

The southern pine beetle, Dendroctonus frontalis (Coleoptera: Curculionidae) (SPB), is known to be a major bark beetle pest of pines throughout the southeastern United States. A common predator of bark beetles, Thanasimus dubius (Coleoptera: Cleridae), has been suggested to play a prevalent role on SPB dynamics. Evaluations of T. dubius have been limited by rearing methods; an artificial diet for larval T. dubius exists, and preservatives such as sorbic acid could help to maximize diet shelf-life and enhance the efficiency of the rearing system. The effects of sorbic acid at different concentrations (0%, 0.1% and 0.2%) in the larval diet for T. dubius were measured, and the effects of increased feeding time intervals (2–3 vs . 5 days) on predator performance evaluated. In addition, an experimental bioassay was conducted where newly hatched T. dubius larvae were released at four densities (0, 50, 100, and 200 per log) on pine logs infested by SPB. Sorbic acid in the diet reduced female fecundity (by 20–40%), but did not affect adult T. dubius size or longevity. However, using this preservative may not be necessary because it had no effect on the overall efficiency of the rearing system, while refreshing the larval diet every 5 days (compared with 2–3 days) did improve its efficiency, even without sorbic acid. The release of larval T. dubius resulted in a highly significant effect on the SPB ratio of increase ( RI ). This experiment was facilitated by the improvements in our rearing methods.

A typical feature of forest insect pests is their tendency to undergo large fluctuations in abundance, which can jeopardize the persistence of their predaceous natural enemies. One strategy that these predators may adopt to cope with these fluctuations would be to respond to sensory cues for multiple prey species. Another possible adaptation to temporal variation in the prey community could involve the learning of prey cues and switching behavior. We conducted three experiments to investigate the ability of the generalist bark beetle predator Thanasimus dubius (F.) (Coleoptera: Cleridae) to respond to different prey signals and to investigate the effect of olfactory experience. We first conducted a field choice test and a wind tunnel experiment to examine the kairomonal response of individual predators toward prey pheromone components (frontalin, ipsenol, ipsdienol, sulcatol) along with the pine monoterpene α-pinene, which is a volatile compound from the host of the prey. We also presented semiochemically naive predators with two prey pheromone components, frontalin and ipsenol, alone or associated with a reward. Our results showed that T. dubius populations are composed of generalists that can respond to a broad range of kairomonal signals. Naive T. dubius also were more attracted to ipsenol following its association with a reward. This work constitutes the first evidence that the behavior of a predatory insect involved in bark beetle population dynamics is influenced by previous olfactory experience, and provides a potential explanation for the pattern of prey switching observed in field studies.

1 The checkered beetle Thanasimus dubius (Fabricius) (Coleoptera: Cleridae) is a common predator in North America that feeds on bark beetles (mostly Ips and Dendroctonus spp.) (Coleoptera: Curculionidae: Scolytinae). Despite numerous field-trapping studies on this predator, the response of individual T. dubius to bark beetle pheromones, host tree volatiles and visual stimuli has seldom been studied in the laboratory. 2 We present a detailed description of T. dubius chemotactic behaviour using wind tunnel observations. We investigated the effect of a synomone, the tree volatile α-pinene, on the T. dubius response towards two kairomones, ipsenol and frontalin, which are, respectively, principal components of Ips grandicollis (Eichhoff) and Dendroctonus frontalis (Zimmermann) prey pheromones. We also compared the chemotactic responses of predators to vanilla (an artificial flavour) and α-pinene, and the response to black and white silhouettes presented near the volatile source. 3 The mean rate of one antennal behaviour (called ‘stand up’) was significantly higher when adult T. dubius were exposed to a semiochemical relative to a blank (no chemical) or to vanilla. The percentage of upwind flights (insects landing upwind and performing at least one ‘stand up’ antennal behaviour) was also significantly higher for semiochemical treatments. The presence of α-pinene did not increase significantly the T. dubius response to ipsenol or frontalin. Predators landed more frequently near black silhouettes but the difference was not significant. 4 Percentages of predators landing closer than 40 cm to the semiochemical source increased significantly during the second trial (43–63%), suggesting a possible effect of experience on chemotactic behaviour.

Insect host-parasitoid systems are often modeled using delay-differential equations, with a fixed development time for the juvenile host and parasitoid stages. We explore here the effects of distributed development on the stability of these systems, for a random parasitism model incorporating an invulnerable host stage, and a negative binomial model that displays generation cycles. A shifted gamma distribution was used to model the distribution of development time for both host and parasitoid stages, using the range of parameter values suggested by a literature survey. For the random parasitism model, the addition of biologically plausible levels of developmental variability could potentially double the area of stable parameter space beyond that generated by the invulnerable host stage. Only variability in host development time was stabilizing in this model. For the negative binomial model, development variability reduced the likelihood of generation cycles, and variability in host and parasitoid was equally stabilizing. One source of stability in these models may be aggregation of risk, because hosts with varying development times have different vulnerabilities. High levels of variability in development time occur in many insects and so could be a common source of stability in host-parasitoid systems.

Animals that store food items in scattered sites must decide how to distribute their caches in space. Our theoretical approach is based on the assumption that such animals disperse their caches in a manner that maximizes the long-term rate of storage of recoverable (surviving) food items in the habitat. We investigate the cache-spacing behavior of theoretical scatter hoarders that encounter food sources differing in the energetic content of the items they contain. We then describe a field experiment in which gray jays (Perisoreus canadensis) were presented with both small- and large-item food sources. The jays compensated for source type by spacing larger-item caches more widely, a compensation that would tend to yield a high rate of storage of recoverable food energy over the long term and throughout the territory. Previous models do not adequately account for the observed patterns of cache dispersion.

Summary 1. Changes in the density of an organism near a boundary elements (edge effects) are often thought to be generated by changes in movement behaviour, but in most cases the mechanism underlying these effects is unknown. 2. We quantified the movement behaviour of a minute parasitic wasp, Anagrus columbi, in relation to edges in its habitat. This wasp attacks eggs of the planthopper Prokelisia crocea, which inhabits a wet prairie ecosystem composed of patches of its host plant prairie cordgrass (Spartina pectinata) interspersed within a matrix of mudflat, smooth brome (Bromus inermis) and native grasses. Two edge types are common in this system, cordgrass-mudflat and cordgrass-brome. 3. We conducted mark–recapture experiments in which wasps were released at the cordgrass-matrix edge and 50 cm within cordgrass or matrix, for both edge types. The marked wasps were recaptured using a grid of sticky traps. We fitted an advection-diffusion model to these data, yielding estimates of the diffusion rate and advection coefficient for cordgrass and matrix, for each release position and edge type. 4. The spatial distribution of wasps was well-described by the advection-diffusion model. The pattern suggests that marked wasps strongly biased their movements towards the edge when released in matrix, and to a lesser extent when released in cordgrass, while edge releases showed little bias. The advection coefficients were similar for the two edge types, as were the diffusion rates for the three substrates (cordgrass, brome, mudflat). The diffusive and advective components of movement were of comparable magnitude for matrix and cordgrass releases, suggesting equal amounts of directed and random movement. 5. Our results suggest the wasps are attracted to cordgrass patches across short distances, and that bias in their movements may concentrate them at the patch edge. Their edge behaviour is qualitatively different from that of the host insect. 6. The methodology described here could be readily adapted to other systems, where direct observations of movement are difficult but mark–recapture studies are feasible.

Bark beetles and their predators are useful systems for addressing questions concerning diet breadth and prey preference in arthropod natural enemies. These predators use bark beetle pheromones to locate their prey, and the response to different pheromones is a measure of prey preference. 2. Trapping experiments were conducted to examine geographic variation in the response to prey pheromones by two bark beetle predators, Thanasimus dubius and Temnochila virescens. The experiments used pheromones for several Dendroctonus and Ips prey species (frontalin, ipsdienol, and ipsenol) and manipulated visual cues involved in prey location (black vs. white traps). The study sites included regions where the frontalin-emitter Dendroctonus frontalis was in outbreak vs. endemic or absent. 3. There was significant geographic variation in pheromone preference for T. dubius. This predator strongly preferred a pheromone (frontalin) associated with D. frontalis at outbreak sites, while preference was more even at endemic and absent sites. No geographic variation was found in the response by T. virescens. White traps caught fewer insects than black traps for both predators, suggesting that visual cues are also important in prey location. 4. The overall pattern for T. dubius is consistent with switching or optimal foraging theory, assuming D. frontalis is a higher quality prey than Ips. The two predator species partition the prey pheromones in areas where D. frontalis is abundant, possibly to minimise competition and intraguild predation.

Excretion rate estimates are important for linking consumers to biogeochemical processes. Short-term incubations in chambers are a common approach for studies. This, however, may result in inaccuracies due to a welldocumented decline in excretion with time, which is often attributed to fasting. An alternative explanation, however, is that excretion slows during recovery from handling stress. Whereas shorter incubations may reduce fasting effects, longer incubations can allow for recovery from handling. Although both views have merit and are not mutually exclusive, attempts to identify optimal incubation times are lacking. We examined effects of handling stress and fasting on NH4+ excretion by omnivorous tadpoles and predatory fish. Fasting did not influence tadpole excretion for at least 6 h. Rather, handling stress appeared to cause initially elevated excretion, and thus longer incubations were appropriate. Fasting and handling stress influenced fish excretion, and intermediate incubations (~30 min) appeared most appropriate. We also found that ammonium measurements were confounded by increasing concentrations of excretory products in longer incubations. We recommend that studies include incubation time trials and correct for potential matrix effects. Effects of handling and fasting likely vary with organism and experimental procedures, and both can be accounted for using the procedures we describe.

We quantified relative extent of pelage staining in Scalopus aquaticus (eastern mole) as an indicator of scent-gland marking, and evaluated whether staining was associated with colored pelage spots and patches often prevalent on the snout and ventral surface of individuals. Moles were collected from southern Illinois (n = 91) and from Cincinnati, OH (n = 152). Adult moles scent-marked more than juveniles, but pelage staining was independent of breeding season for males and females. Pelage spotting occurred in 33.7% of the sample and was not associated with pelage staining from glandular secretions, as has been suggested by some previous investigators. Pelage spots were most prevalent on the ventral surface. Ventral spotting occurred more often in males than females (P < 0.001). Mean area of ventral spots was 2.81 cm2 with no differences in area related to sex or age.

Conversion to agriculture, habitat fragmentation, and the loss of native grazers have made tallgrass prairie one of the most endangered ecosystems. One management option for the remaining prairie parcels, patch-burn grazing (PBG), applies a controlled burn to a portion of the prairie to attract cattle, creating a mosaic of more- and less-grazed patches. Although beneficial to cattle and grassland birds, the potential impacts of PBG on streams have not been studied, and a holistic approach is needed to ensure against adverse effects. We used a Before-After-Control-Impact design to assess potential impacts of PBG with and without riparian protection on tallgrass prairie headwater streams. We sampled stream macroinvertebrates and benthic organic matter 2 yr before and 2 yr during PBG treatments on two grazed watersheds with riparian fencing (fenced), two unfenced grazed watersheds (unfenced), and two ungrazed (control) watersheds. Very fine benthic organic matter increased significantly (51%) in unfenced streams compared with controls (0.007), and fine particulate organic matter (1 mm and250 µm) increased 3-fold in the unfenced streams compared with controls ( = 0.008). The contribution of fine inorganic sediments to total substrata increased 28% in unfenced streams during PBG, which was significantly different from controls ( = 0.03). Additionally, the abundance of Ephemeroptera, Plecoptera, and Trichoptera taxa decreased from 7635 to 687 individuals m in unfenced streams, which was significantly lower than in control streams ( = 0.008). Our results indicate that PBG adversely influences prairie streams through sediment inputs and reductions in sensitive invertebrate taxa, but riparian fencing can alleviate these impacts.

The checkered beetle, Thanasimus dubius F., is an important predator of scolytid bark beetles that attack conifers. Relatively few studies exist that have addressed the population genetics of predatory beetles, especially those with potential as biological control agents. This study was conducted to investigate the population genetics of T. dubius across a large part of its range in the eastern United States. A 464-base pair portion of the mitochondrial cytochrome c oxidase subunit I was sequenced for 85 individuals resulting in 60 haplotypes. Analysis of molecular variance was conducted on the resulting haplotypes for all populations and as a hierarchical analysis between populations defined as broad-scale northern and southern groups. Results indicate a significant overall Phi ST = 0.220 (P < 0.001) for all populations with the hierarchical analysis revealing that this significant Phi ST is due to structuring of the populations between northern and southern regions (Phi CT = 0.388, P < 0.009). The observed genetic structure is possibly due to the discontinuous distribution of pine trees, which act as hosts for the prey of T. dubius, which has occurred historically in the central region of the United States that has been covered by prairie.

A growing body of literature points to a large-scale research approach as essential for understanding population and community ecology. Many of our advances regarding the spatial ecology of predators and prey can be attributed to research with insect parasitoids and their hosts. In this review, we focus on the progress that has been made in the study of the movement and population dynamics of hosts and their parasitoids in heterogeneous landscapes, and how this research approach may be beneficial to pest management programs. To date, few studies have quantified prey and predator rates and ranges of dispersal and population dynamics at the patch level—the minimum of information needed to characterize population structure. From host–parasitoid studies with sufficient data, it is clear that the spatial scale of dispersal can differ significantly between a prey and its predators, local prey extinctions can be attributed to predators and predator extinction risk at the patch level often exceeds that of the prey. It is also evident that populations can be organized as a single, highly connected (patchy) population or as semi-independent extinction-prone local populations that collectively form a persistent metapopulation. A prey and its predators can also differ in population structure. At the landscape level, agricultural studies indicate that predator effects on its prey often spill over between the crop and surrounding area (matrix) and can depend strongly on landscape structure (e.g. the proportion of suitable habitat) at scales extending well beyond the crop margins. In light of existing empirical data, predator–prey models are typically spatially unrealistic, lacking important details on boundary responses and movement behaviour within and among patches. The tools exist for conducting empirical and theoretical research at the landscape level and we hope that this review calls attention to fertile areas for future exploration.

Relationships between riparian land cover, in-stream habitat, water chemistry, and macroinvertebrates were examined in headwater streams draining an agricultural region of Illinois. Macroinvertebrates and organic matter were collected monthly for one year from three intensively monitored streams with a gradient of riparian forest cover (6, 22, and 31% of riparian area). Bioassessments and physical habitat analyses were also performed in these three streams and 12 other nearby headwater streams. The intensively monitored site with the least riparian forest cover had significantly greater percent silt substrates than the sites with medium and high forest cover, and significantly higher very fine organics in substrates than the medium and high forested sites. Macroinvertebrates were abundant in all streams, but communities reflected degraded conditions; noninsect groups, mostly oligochaetes and copepods, dominated density and oligochaetes and mollusks, mostly Sphaerium and Physella, dominated biomass. Of insects, dipterans, mostly Chironomidae, dominated density and dipterans and coleopterans were important contributors to biomass. Collector-gatherers dominated functional structure in all three intensively monitored sites, indicating that functional structure metrics may not be appropriate for assessing these systems. The intensively monitored site with lowest riparian forest cover had significantly greater macroinvertebrate density and biomass, but lowest insect density and biomass. Density and biomass of active collector-filterers (mostly Sphaerium) decreased with increasing riparian forest. Hilsenhoff scores from all 15 sites were significantly correlated with in-stream habitat scores, percent riparian forest, and orthophosphate concentrations, and multiple regression indicated that in-stream habitat was the primary factor influencing biotic integrity. Our results show that these "drainage ditches" harbor abundant macroinvertebrates that are typical of degraded conditions, but that they can reflect gradients of conditions in and around these streams.

The morphology of insect eyes often seems to be shaped by evolution to match their behaviour and lifestyle. Here the relationship between the nuptial flight behaviour of 10 Atta species (Hymenoptera: Formicidae) and the eye size of male and female alates, including the compound eyes, ommatidia facets, and ocelli were examined. These species can be divided into two distinct groups by nuptial flight behaviour: those that initiate the nuptial flight during the day and those that initiate it at night. The most striking difference between day- vs night-flying alates was in ocellus area, which was almost 50% larger in night-flying species. Night-flying species also had significantly larger ommatidia facets than day-flying species. A scaling relationship was also found between compound eye area, facet diameter, and ocellus area vs overall body size. Detailed observations are also presented on the nuptial flight behaviour of a night- vs day-flying species, A. texana and A. sexdens , respectively. The pattern in A. texana is for a single large and precisely timed nuptial flight before dawn, while flights of A. sexdens last for several hours, beginning at midday. Further observations suggest that the timing of the nuptial flight in A. texana is easily disrupted by light pollution.

Amphibian populations are declining on all continents on which they occur, but not all species have been affected equally. Populations of some species are extirpated, others have declined but survive, some have not obviously declined, and some are increasing. If amphibian populations at multiple sites were affected by the same factors, then surviving species should share traits that promote persistence, whereas declining species should share traits that promote susceptibility. Identifying these traits can help diagnose potential causes and thus help to direct conservation actions. Using logistic regression, we quantified the vulnerability of amphibian populations in four areas in Central America. We analyzed a species-specific database of taxonomic identity, geographic and elevational range, elevational distribution, adult and larval habitat, activity period, and maximum adult body size. We found that ( 1 ) all four sites exhibited the same pattern of decline ( there were no interactions between site and other variables ); ( 2 ) declining populations shared aquatic habitats, restricted elevational ranges, and large body sizes; and ( 3 ) there was an interaction between body size and elevational range. The most significant variable in the model was lifetime aquatic index, a factor unrelated to demographic vulnerability and one that therefore might indicate the potential causative agent( s ). Our results provide a predictive model with which to assess potential causes of population declines in other areas, and we generated a list of 52 species predicted to decline at a currently unaffected site in central Panama.

Clerid beetles are common natural enemies of bark beetles, and could potentially be used as biological control agents if they could be reared in sufficient numbers. We developed an artificial diet devoid of insect components for rearing Thanasimus dubius (Fabricius), a clerid that attacks several economically important bark beetles in eastern North America. We reared larvae of this predator using the artificial diet, and then used either natural or factitious prey to feed the adults so produced. Two different methods of presenting the diet were also examined. We then compared the performance of T. dubius reared on the artificial diet with newly-emerged wild individuals collected from the field. Our results suggest that adult predators reared on the diet are near in quality to wild ones, and high R 0 values can be obtained. No difference in prey preference was found between wild and diet-reared individuals after five generations in the laboratory. Sufficient numbers of predators could be generated using these techniques to permit limited field trials of augmentative biological control.

1 The emergence pattern of Thanasimus dubius (F.) (Coleoptera: Cleridae), a common predator of the southern pine beetle, Dendroctonus frontalis Zimmermann (Coleoptera: Scolytidae), was studied under field conditions across different seasons. A simple statistical model was then developed to characterize the emergence data, using the truncated geometric distribution. Data are also presented on the mortality of T. dubius eggs at various temperatures and humidities in an effort to explain certain aspects of emergence behaviour. 2 Emergence of T. dubius from a given tree usually occurred in several discrete episodes across a two-year period, with most individuals emerging in spring or autumn. Almost no emergence occurred in July and August, which may be an adaptation to avoid high temperature mortality. Emergence patterns appeared similar across seasons, with the time of year serving mainly to shift the pattern through time. 3 Cycles in D. frontalis abundance may be the result of delayed density dependence generated by its natural enemy complex. The predator T. dubius is likely to be an important component of this delayed density dependence, because of its lengthy development time and apparent impact on D. frontalis.

Theoretical studies of predator-prey population dynamics have increasingly centered on the role of space and the movement of organisms. Yet, empirical studies have been slow to follow suit. Herein, we quantified the long-range movement of a checkered beetle, Thanasimus dubius, which is an important predator of a pernicious forest pest, the southern pine beetle, Dendroctonus frontalis. Adult checkered beetles were marked and released at five sites and subsequently recaptured at traps baited with pine and pine beetle semiochemicals and located at distances up to 2 km away from the release point. While the pattern of recaptures-with-distance at each site provided a modest fit to a simple random-diffusion model, there was a consistent discrepancy between observed and expected recaptures: a higher than expected proportion of beetles were recaptured at the more distant traps. To account for this deviation, we developed a model of diffusion that allowed for simple heterogeneity in the population of marked beetles; i.e., a slow and fast moving form of the checkered beetle. This model provided a significantly better fit to the data and formed the basis for our estimates of intra-forest movement. We estimated that on average, one half of the checkered beetles dispersed at least 1.25 km, one third dispersed > 2 km, and 5% dispersed > 5 km. The source of the heterogeneous dispersal rates were partially due to differences in beetle size: smaller beetles (for both males and females) were more likely to be recaptured away from the release site than larger beetles. The southern pine beetle (prey for the checkered beetle) exhibited no significant heterogeneity in dispersal ability and provided a very good fit to the simple diffusion model. The only difference in dispersal between these two species was that checkered beetles were undergoing greater long-distance dispersal than the pine beetles (the radius containing 95% of the dispersing individuals was 5.1 km for the checkered beetle and 2.3 km for the pine beetle). Data on the movement of these two species is used to evaluate a general model of spatial pattern formation in a homogeneous environment, and the potential of the checkered beetle as a biological control agent for the southern pine beetle.

1. The nature of intraspecific competition was investigated in the southern pine beetle, Dendroctonus frontalis, a highly destructive pest of pine forests in the southern U.S.A. Data were analysed from an observational study of naturally-attacked trees, and from field experiments where attack density was manipulated by adding different numbers of beetles to caged trees. 2. The effect of attack density on gallery construction, oviposition, brood survival, and the overall rate of increase was examined, and a flexible model of intraspecific competition used to classify the type of competition (contest or scramble) at different points in the life cycle. 3. The results of these analyses suggest that contest competition occurs during gallery construction and oviposition, in accord with previous work on D. frontalis. Strong scramble competition occurs later on in development, however, and the overall competitive process is better characterized as scramble competition, similar to other bark beetles. Trees with attack densities sufficiently high to produce significant competition are common in the field.

Bark beetle populations may undergo dramatic fluctuations and are often important pests in coniferous forests. Their dynamics are thought to be primarily driven by factors affecting the resistance of the host tree to attack, i.e., bottom-up forces, while natural enemies are usually assigned a minor role in these systems. I present behavioral experiments that suggest that the clerid beetle Thanasimus dubius may be an important source of mortality for the bark beetle Dendroctonus frontalis during attack of the host tree, and determine the nature of the functional response of T. dubius under conditions close to natural. I also examine the numerical response of T. dubius to large-scale fluctuations in D. frontalis density, and the relationship between bark beetle population trends and predator density, and find that beetle populations tend to decline when predator densities are high. Combined with the effects of clerid larvae on bark beetle broods, these results suggest that top-down forces generated by natural enemies could also be an important component of bark beetle dynamics. The implications of these results for bark beetle dynamics are discussed in relation to the prolonged life-cycle of clerid beetles.

Thanasimus dubius (F.) is an important predator of the southern pine beetle, Dendroctonus frontalis Zimmermann, a major pest of pine forests in the southern United States. We examined the development of T. dubius in the field using emergence traps, and by sampling the bark of trees previously attacked by D. frontalis. Over a 2-year period, several distinct episodes of T. dubius emergence occurred in trees enclosed by emergence traps, and bark sampling of other trees uncovered many T. dubius immatures almost 2 years after attack by D. frontalis. These results indicate that T. dubius development may be significantly longer and more variable under natural conditions than previously thought, and suggest that some individuals may undergo a diapause.

California red scale is suppressed to very low densities by the parasitoid Aphytis melinus. The system also appears stable. We report on an experimental test of the hypothesis that stability is caused by a refuge for scale. In a grapefruit grove in southern California in 1984-1985, the bark in the interior part of the tree provided a partial refuge from parasitism. Scale were -100 times denser there than in the exterior of trees. In a field experiment, we removed Argentine ants from some blocks of trees to test whether (1) ants caused the refuge by interfering with Aphytis and (2) the expected reduction in scale density in the refuge would lead to an unstable interaction in the exterior. We also tested for density- dependent parasitism, host mutilation, and predation by analyzing data from samples and from scale placed in the field. The temporal variability of the scale was at the low end of the range recorded in field populations. The experiment provided some evidence in support of the refuge hypothesis. The population in the refuge fluctuated much less than that in the exterior. Ant exclusion led to increased parasitism and lower scale density in the interior, and to increased fluc- tuations in abundance in the refuge and exterior. However, these changes were relatively small and perhaps temporary, suggesting that (1) ants are not the main cause of the refuge and that (2) we did not reduce the refuge density enough to determine whether the system would go unstable in the absence of the refuge population. Parasitism, host mutilation, and predation rates on scale showed no temporal density dependence, either direct or delayed, though detection of such patterns is difficult. Possible alternative stabilizing mechanisms include size-dependent interactions between red scale and Aphytis.

While most bark beetles attack only dead or weakened trees, many species in the genus Dendroctonus have the ability to kill healthy conifers through mass attack of the host tree, and can exhibit devastating outbreaks. Other species in this group are able to successfully colonize trees in small numbers without killing the host. We reconstruct the evolution of these ecological and life history traits, first classifying the extant Dendroctonus species by attack type (mass or few), outbreaks (yes or no), host genus (Pinus and others), location of attacks on the tree (bole, base, etc.), whether the host is killed (yes or no), and if the larvae are gregarious or have individual galleries (yes or no). We then estimated a molecular phylogeny for a data set of cytochrome oxidase I sequences sampled from nearly all Dendroctonus species, and used this phylogeny to reconstruct the ancestral state at various nodes on the tree, employing maximum parsimony, maximum likelihood, and Bayesian methods. Our reconstructions suggest that extant Dendroctonus species likely evolved from an ancestor that killed host pines through mass attack of the bole, had individual larvae, and exhibited outbreaks. The ability to colonize a host tree in small numbers (as well as gregarious larvae and attacks at the tree base) apparently evolved later, possibly as two separate events in different clades. It is likely that tree mortality and outbreaks have been continuing features of the interaction between conifers and Dendroctonus bark beetles.

The southern pine beetle, Dendroctonus frontalis Zimmerman (Coleoptera: Curculionidae) is an economically important pest species throughout the southeastern United States, Arizona, Mexico, and Central America. Previous research identified population structure among widely distant locations, yet failed to detect population structure among national forests in the state of Mississippi. This study uses microsatellite variation throughout the southeastern United States to compare the southern pine beetle's pattern of population structure to phylogeographic patterns in the region, and to provide information about dispersal. Bayesian clustering identified east and west genetic groups spanning multiple states. The east group had lower heterozygosity, possibly indicating greater habitat fragmentation or a more recent colonization. Significant genetic differentiation (θST = 0.01, p < 0.0001) followed an isolation-by-distance pattern (r = 0.39, p < 0.001) among samples, and a hierarchical AMOVA indicated slightly more differentiation occurred between multi-state groups. The observed population structure matches a previously identified phylogeographic pattern, division of groups along the Appalachian Mountain/Apalachicola River axis. Our results indicate that the species likely occurs as a large, stable metapopulation with considerable gene flow among subpopulations. Also, the relatively low magnitude of genetic differentiation among samples suggests that southern pine beetles may respond similarly to management across their range.

Diffusion models have been found in various applications in the study of spatial population dynamics for modeling the species dispersal process in natural environments. Diffusion coefficient is a critical parameter in diffusion equations. In this paper, a new method for estimating the diffusion coefficient of insects is presented in terms of occupancy time and the method can produce any desired accuracy. The study of modeling biological organism movement behaviors in a nonhomogeneous landscape is critical in investigating the interplay between environmental heterogeneity and organism movements. By constructing a set of eigenvalues, we can characterize the insect biased movement when insect crosses the intersection of two different type of landscape elements. Some numerical examples are provided to illustrate the theoretical outcomes obtained in the paper.

Plant-mediated egg mortality varied greatly for the oligophagous leafhopper, Carneocephala floridana, a common inhabitant of the salt marshes along Florida's Gulf coast. In the laboratory, survivorship from egg to first instar was generally greater on two herbaceous plants. Borrichia frutescens and Salicornia virginica (80 and 81% respectively) than it was on two grasses, Distichlis spicata and Spartina alterniflora (41 and 77% respectively). Although C. floridana laid significantly more eggs per clutch on Sp. alterniflora than on the two herbaceous species, this increase in egg number was offset by the higher mortality of eggs laid on this species. The greatest source of identifiable plant-mediated egg mortality was death due to desiccation of the leaf tissue surrounding the egg clutch. In addition, field experiments in which the four host species were infested with eggs of C. floridana at natural densities consistently yielded higher parasitism rates by two mymarid wasps on the grasses, regardless of the background plants. C. floridana switches host plants seasonally, using the grass species mainly during the winter months, when the herbaceous species experience a substantial dieback. The herbaceous species are nutritionally superior to the two grass species as a food source for C. floridana. C. floridana reared on the herbs produce larger, more fecund, adults than they do on the grasses (Rossi 1991). However, during cold winter snaps the grasses, while less "attractive", may provide the most abundant nutritional and ovipositional resources available to C. floridana. The results of this study indicate how variable rates of plant-and parasitoid-mediated egg mortality may work synergistically with the interspecific nutritional status of the plants to explain host switching for this insect.

We examine a salt marsh host-parasitoid system, consisting of the planthop- per Prokelisia marginate and its egg parasitoid Anagrus delicatus, for evidence of stabilizing parasitoid behavior. We first determine if there is sufficient parasitoid aggregation to po- tentially stabilize the Prokelisia-Anagrus interaction, using methods that infer parasitoid behavior from the spatial pattern of parasitism. We then test a basic assumption implicit in theory, that the degree of parasitoid aggregation is invariant with respect to changes in overall host and parasitoid density, and through time. Our results suggest that Anagrus behavior should, in theory, exert a consistent stabilizing effect on Prokelisia-Anagrus dy- namics. Using field experiments, and direct sampling of the spatial distribution of parasitoids, we then try to determine if parasitoid aggregation is in fact responsible for the heterogeneous pattern of parasitism observed in the field. The results of these studies indicate that there is insufficient parasitoid aggregation to account for this heterogeneity. Instead, factors such as the degree of tidal inundation, or the age of host eggs, may cause host patches to differ in their vulnerability to parasitoid attack.

One of the fundamental goals of ecology is to examine how dispersal affects the distribution and dynamics of insects across natural landscapes. These landscapes are frequently divided into patches of habitat embedded in a matrix of several non-habitat regions, and dispersal behavior could vary within each landscape element as well as the edges between elements. Reaction-diffusion models are a common way of modeling dispersal and species interactions in such landscapes, but to apply these models we also need methods of estimating the diffusion rate and any edge behavior parameters. In this paper, we present a method of estimating the diffusion rate using the mean occupancy time for a circular region. We also use mean occupancy time to estimate a parameter (the crossing probability) that governs one type of edge behavior often used in these models, a biased random walk. These new methods have some advantages over other methods of estimating these parameters, including reduced computational cost and ease of use in the field. They also provide a method of estimating the diffusion rate for a particular location in space, compared to existing methods that represent averages over large areas. We further examine the statistical properties of the new method through simulation, and discuss how mean occupancy time could be estimated in field experiments.

We developed a model that concerns, in part, how long a scatterhoarder should persist in caching food from an ephemeral, locally abundant source ('bonanza') before moving on in search of other sources. The model assumes that an animal scattering food caches for later use behaves in a manner that maximizes the rate at which it stores recoverable (surviving) food in its habitat. It is shown theoretically that under some conditions it is better not to cache all available food but instead to move on in search of other food sources. This 'source-departure decision' for scatterhoarders is analogous to the patch-departure decision for animals that forage among food patches. It is shown that whether, and at what point, a 'moving-on threshold' is reached should depend on the size of the source, the strength of density-dependent cache theft, and the abundance of sources in the habitat. A field experiment was performed to test the qualitative prediction that gray jays, Perisoreus canadensis, should not persist as long in caching food when a day-long opportunity for caching is restricted to a single source (single-source treatment) as when such an opportunity is divided among a series of disjunct sources (multiple-source treatment). In the single-source treatment, jays tended to cache at lower overall rates, transport food items to more distant cache sites, and spend less time caching. These tendencies became more pronounced later in the day. However, although the rate of caching approached zero in the single-source treatment, the jays did not completely cease caching. This apparent violation of the model is attributed to the behaviour of recaching, a facet of gray jay scatterhoarding behaviour that was not built into the model. This retrieval and redistribution of previous caches resulted in the stabilization of the density of caches near the source. In addition, this switch from single- to multiple-load caching trips arguably would make it economical for gray jays to continue to make caching trips from a source beyond the source-departure point predicted by the model. Our theoretical and empirical results begin to show how scatterhoarders may behave in a manner that maximizes the long-term average rate of storage of recoverable food energy throughout their habitat.

1. Animals move commonly through a variety of landscape elements and edges in search of food, mates and other resources. We developed a diffusion model for the movement of an insect herbivore, the planthopper Prokelisia crocea, that inhabits a landscape composed of patches of its host plant, prairie cordgrass Spartina pectinata, embedded in a matrix of mudflat or smooth brome Bromus inermis. 2. We used mark-release-resight experiments to quantify planthopper movements within cordgrass-brome and cordgrass-mudflat arenas. A diffusion model was then fitted that included varying diffusion rates for cordgrass and matrix, edge behaviour in the form of a biased random walk and heterogeneity among planthoppers (sessile vs. mobile). The model parameters were estimated by maximum likelihood using the numerical solution of the diffusion model as a probability density. Akaike's information criterion (AIC) values were used to compare models with different subsets of features. 3. There was clear support for models incorporating edge behaviour and both sessile and mobile insects. The most striking difference between the cordgrass-brome and cordgrass-mudflat experiments involved edge behaviour. Planthoppers crossed the cordgrass-brome edge readily in either direction, but traversed the cordgrass-mudflat edge primarily in one direction (mudflat to cordgrass). Diffusion rates were also significantly higher on mudflat than for cordgrass and brome. 4. The differences in behaviour for cordgrass-brome vs. cordgrass-mudflat edges have implications for the connectivity of cordgrass patches as well as their persistence. Higher dispersal rates are expected between cordgrass patches separated by brome relative to mudflat, but patches surrounded by mudflat appear more likely to persist through time. 5. The experimental design and diffusion models used here could potentially be extended to any organism where mass mark-recapture experiments are feasible, as well as complex natural landscapes.

The southern pine beetle, Dendroctonus frontalis Zimmerman, is the most destructive insect pest of pine forests in the southeastern United States, Mexico, and Central America. Southern pine beetle aggressively attacks pine trees, and when in epidemic stages, they are capable of killing even the most healthy pine trees in a short period of time. Despite the amount of destruction caused by the southern pine beetle and the amount of monetary loss faced by the timber industry and recreation, the population genetics of this species has been limited to comparisons among distant geographic locations. This study investigates the fine-scale genetic population structure of the southern pine beetle in Mississippi. Very little genetic differentiation was observed among samples. Bayesian assignment testing failed to detect multiple groups within all samples; estimates of genetic differentiation and genetic distance were very low in magnitude; and a Mantel test did not reveal a significant relationship between genetic distance and geographic distance. These results suggest that management of the southern pine beetle needs to consider the potential movements of individuals within and among national forests and should be focused on a large scale, at least as big as continuously forested areas and possibly even multiple forests. These results further suggest that removal of beetle-infested trees is important.

Insect parasitoids are ubiquitous members of natural insect communities, as well as being important agents of biological control. Because of their economic importance and relatively simple life cycles, host-parasitoid systems have received much theoretical effort. In this essay, I first review the theory available for hostparasitoid systems, and in particular explore the effects of environmental variability, spatial subdivision, and migration. These factors are often thought to influence dynamics in the field, but until recently have not received much attention in theory. I then make some recommendations for empirical work that could assess the effects of these factors in the field, and especially how they influence the persistence (through time) of host and parsitoid populations. Using the unstable Nicholson-Bailey model (Nicholson and Bailey 1935) as a starting point, theory has sought mechanisms that stabilize its dynamics; the rationale is that stability should ensure the persistence of the host and parasitoid (an apparent feature of real systems, at least on large spatial scales). Many stabilizing mechanisms have been identified, including mutual interference among parasitoids (Hassell and Varley 1969), several types of parasitoid aggregation (Hassell and May 1973, 1974, May 1978, Hassell 1984, Chesson and Murdoch 1986), density-dependent parasitoid sex ratios (Hassell et al. 1983, Comins and Wellings 1985), and competition among parasitoid larvae (Taylor 1988a). However, Morrison and Barbosa (1987) have questioned the idea that stability leads to persistence, when the population is subject to environmental variability. They used a standard host-parasitoid model, the negative-binomial model (May 1978), and simulated a fluctuating environment by allowing parameters in the model to be random variables. They found that perilously low densities could arise even when the model was strongly stable. Although stability ensures that

Pathogens rarely cause extinctions of host species, and there are few examples of a pathogen changing species richness and diversity of an ecological community by causing local extinctions across a wide range of species. We report the link between the rapid appearance of a pathogenic chytrid fungus Batrachochytrium dendrobatidis in an amphibian community at El Copé, Panama, and subsequent mass mortality and loss of amphibian biodiversity across eight families of frogs and salamanders. We describe an outbreak of chytridiomycosis in Panama and argue that this infectious disease has played an important role in amphibian population declines. The high virulence and large number of potential hosts of this emerging infectious disease threaten global amphibian diversity.

Traps baited with semiochemicals are often used to investigate the chemical ecology of scolytids and associated insects. One statistical problem frequently encountered in these studies are treatments that catch no insects and, thus, have zero mean and variance, such as blank or control traps. A second problem is the use of multiple comparison procedures that do not control the experimentwise error rate. We conducted a literature survey to determine the frequency of these two statistical problems in Journal of Chemical Ecology for 1990-2002. Simulations were then used to examine the effects of these problems on the validity of multiple comparison procedures. Our results indicate that both statistical problems are common in the literature, and when combined can significantly inflate both the experimentwise and per comparison error rate for multiple comparison procedures. A possible solution to this problem is presented that involves confidence intervals for the treatment means. Options to increase the statistical power of trapping studies are also discussed.

The southern pine beetle, Dendroctonus frontalis Zimmermann (Coleoptera: Scolytidae), is an economically important pest of pine forests in the southern United States (Price et al. 1992). This native bark beetle is able to attack and kill living trees, typically loblolly (Pinus taeda L.) or shortleaf (Pinus echinata Mill.) pine, through a process of mass attack coordinated by pheromones emitted by the beetle (Payne 1980). During the attack process, thousands of beetles bore through the outer bark of the tree and begin constructing galleries in the phloem layer. Trees can respond to beetle attack by exuding resin from a network of ducts, but the large number of simultaneous attacks usually overcomes this defense, literally draining the resin from the tree. Oviposition and brood development then occur in the girdled (and ultimately dead) tree. Once a tree is fully colonized the attack process shifts to adjacent trees, often resulting in a cluster of freshly attacked trees, trees containing developing brood, and dead and vacated trees (Coulson 1980). These infestations can range in size from a single tree to tens of thousands, although the latter only occur in areas where no control methods are applied. Approximately six generations can be completed in a year in the southern United States (Ungerer et al. 1999). Like many other forest insect pests, D. frontalis populations are characterized by a considerable degree of fluctuation. The longest time series available are Texas Forest Service records of infestations in southeast Texas since 1958 (figure 5.la). These data suggest that the fluctuations have at least some periodic component, with major outbreaks occurring at intervals of 7-9 years (1968, 1976, 1985, and 1992). A variety of different analyses, including standard time series analysis and response surface methodology (Turchin 1990, Turchin and Taylor 1992), suggest that D.frontalis dynamics are indeed cyclic and appear governed by some kind of delayed negative feedback acting on population growth (see chapter 1). This effect can be seen by plotting the realized per-capita rate of growth (R-values) over a year against population density in the previous year (figure 5.1b).

Population cycles occur frequently in forest insects. Time-series analysis of fluctuations in one such insect, the southern pine beetle ( Dendroctonus frontalis ), suggests that beetle dynamics are dominated by an ecological process acting in a delayed density-dependent manner. The hypothesis that delayed density dependence in this insect results from its interaction with predators was tested with a long-term predator-exclusion experiment. Predator-imposed mortality was negligible during the increase phase, grew during the year of peak population, and reached a maximum during the period of population decline. The delayed nature of the impact of predation suggests that predation is an important process that contributes significantly to southern pine beetle oscillations.

Dendroctonus frontalis Zirnm. (southern pine beetle (SPB)) has caused over $900 million in damage to pines in the southern United States between 1960 and 1990 (Price et al., 1992). The damage of SPB to loblolly (Pinus taeda L.), shortleaf (Pinus echinata Mill.), and pitch (Pinus rigida Mill.) pine has long been established (Hopkins, 1899), however, extensive mapping of SPB infestations has only existed since 1960 (Price and Doggett, 1982). Early detection of SPB outbreak areas is essential to controlling population increases (Swain and Remion, 1981), but the range of SPB is large, SPB have six to eight generations per year, and there is inconsistency in the monitoring methods used to measure SPB populations across its range. Therefore, various models have been developed that attempt to predict SPB outbreak severity across the region (Hansen et al., 1973; Kalkstein, 1974; Michaels, 1984).