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The role of generalist predators in suppressing herbivores in complex food webs is controversial because alternative prey can modulate predation on focal prey through various indirect interactions, e.g. by distracting or enhancing predation (i.e. apparent mutualism and apparent competition, respectively). We predict that temporal synchrony (i.e., whether the two prey items co-occur at the same time) mechanistically shifts the indirect predator-mediated effect from mutualism in the short-term to competition in the long-term. The impact of alternative prey on population growth of soybean aphid (Aphis glycines), a key invasive pest in Northern American soybean fields, was tested over two years in 520 replicated open field plots. Specifically, we tested short- and long-term predator-mediated indirect interactions, respectively, by quantifying these relationships instantaneously vs. implementing a time lag with alternative prey preceding aphid trials by two weeks. This allowed us to statistically disentangle behavioral effects of prey preferences from population-level effects mediated by numerical responses of natural enemies. The predator, Orius insidiosus, constituted >90% of the aphidophagous predator community and was thus considered the driver of indirect interactions between pests. Of the alternative prey, soybean thrips, Neohydatothrips variabilis, was the dominant species and predicted variation in aphid population growth rates in three of four analyses. Notably, thrips increased aphid growth when co-occurring in the short-term, presumably by satiating O. insidiosus, but negatively affected aphids when tested with a time lag (i.e., asynchronous dynamics). Mites also weakened aphid suppression in the short-term during one of two years. These data suggest that indirect effects in food webs are not static, but fluctuate between positive and negative depending on the time scale over which interactions are tracked. Consequently, alternative foods can be simultaneously beneficial and detrimental to prey suppression with the net effect likely driven by the degree of phenological synchrony exhibited by the co-occurring herbivores.

1. Protection mutualisms between ants and honeydew-producing insects influence arthropod food webs by altering the behaviour, activity and local abundance of ants on plants. Ants often tend multiple species of honeydew-producing insects; however, studies that examine such effects typically consider only pairwisemutualisms. This study investigates how multi-species mutualisms between ants and honeydew-producing insects structure arthropod food webs in citrus.2. In an organic lemon orchard in San Diego County, California, U.S.A., ants or honeydew-producing insects (or neither) were experimentally removed from individual, mature lemon trees and then abundances of the following were estimated over a 2-year period: the Argentine ant, five species of honeydew-producing hemipterans, and California red scale. Red scale produces no honeydew but indirectly benefits from the presence of ants, which disrupt parasitism by Aphytis wasps.3. Mutualism between ants and honeydew-producing hemipterans indirectly andpositively affected red scale. Levels of parasitism decreased with increasing antrecruitment, and red scale density increased with both increasing ant recruitment and increasing honeydew-producing hemipteran abundance. Moreover, abundances of each of three honeydew-producing hemipteran species emerged as positive predictors of redscale density in different analyses; this finding suggests that individual hemipteran taxa exhibit distinct and spatially localised effects on ants.4. Evidence of positive, indirect effects between two focal species of honeydew-producing Hemiptera was also detected. Guilds of honeydew-producing hemipterans may provide ants with honeydew more consistently or for longer than that produced by any single species. These results argue for an appreciation of how mutualist diversity affects the ecological consequences of mutualisms.

Author(s): Yoo, Ho Jung S; Holway, David A | Abstract: 1. In ant–hemipteran mutualisms, ants receive carbohydrates in the form of honeydew, while hemipterans receive protection from natural enemies. In the absence of natural enemies, however, the direct effects of tending are generally less well known. We hypothesised that with increasing tending intensity (ant to aphid ratio), aphid performance would increase initially, then decrease at high tending levels due to the metabolic cost of producing high quality honeydew.2. We tested our hypothesis in a greenhouse experiment by manipulating Argentine ant (Linepithema humile Mayr) colony size while holding constant the initial size of aphid (Chaitophorus populicola Thomas) aggregations. The two parameters associated with survival, aphid survivorship to maturity and longevity, declined with increasing tending intensity, whereas per capita birth rate and time to first reproduction showed no relationship to attendance. The intrinsic rate of increase declined only at relatively high tending levels, suggesting a nonlinearity in the effect of tending intensity.3. Tending intensity measured in the experiment was similar to that observed in free-living aggregations of C. populicola. Furthermore, the per capita recruitment rate of ants to free-living aphid aggregations was negatively density-dependent, indicating that small aggregations tend to experience the highest levels of tending intensity. This finding suggests that the aphid's intrinsic rate of increase may be positively density-dependent, mediated by the aphid's mutualistic interaction with the ant.4. In the Argentine ant–C. populicola interaction, experimental manipulation of colony size revealed a direct cost of ant attendance that was conditional upon tending intensity. Experiments that manipulate only ant presence or absence may yield an incomplete understanding of the mutualistic interaction if underlying nonlinearities exist.

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), alternates between a primary overwintering host (buckthorn, Rhamnus sp.) and a secondary summer host (soybean, Glycine max). Selection of soybean cultivars with different maturity groups may provide growers with a management tool for A. glycines, either directly through its effect on summer populations that cause economic damage or indirectly through its effect on the production of migrants that disperse to the primary host in fall. This study investigated the abundance and seasonality of A. glycines on soybean cultivars with different maturity rates in central Indiana. The abscission of soybean foliage occurred earlier for early maturing than late maturing cultivars, but no other consistent difference in development or yield was detected among the cultivars tested in this study. The abundance of aphids did not vary consistently among cultivars when soybean was most susceptible to economic damage. A laboratory assay evaluating the larviposition preference of A. glycines alate females, combined with a 7-yr survey documenting the colonization of buckthorn by winged aphids, indicated that the production of gynoparae on soybean began in mid-September and continued until leaf abscission. The abundance of aphids during this period was higher on late maturing cultivars than on early maturing cultivars in both 2006 and 2008, whereas no significant effect was detected in 2007. Altogether, these results suggest that planting early maturing soybean cultivars has little effect on damage by aphids on the current season crop but may reduce the number of fall migrants to the primary host.

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), has recently invaded North America from Asia and has become a major pest in soybean. Using field surveys and cage exclusion techniques, we identified the effect of natural enemies and abiotic factors on the growth of soybean aphid populations in 2004 and 2005. The soybean aphid population was significantly limited by natural enemies in the field. Generalist predators dominated the natural enemy community. One species, Orius insidiosus Say (Hemiptera: Anthocoridae) represented 85–90% of predators found. There was a significant negative relationship between aphid population growth and O. insidiosus abundance. For other predators, there were no relationships between abundance and aphid population growth. The spatial distribution of aphids among plants affected the impact of O. insidiosus on aphid population growth. When aphids were distributed in a clumped manner, increases in O. insidiosus numbers resulted in lower aphid growth rates. For randomly distributed aphids, there was no effect of O. insidiosus abundance on aphid population growth. Finally, we found no relationship between aphid population growth and degree-day accumulations. The potential of O. insidiosus to suppress soybean aphid population growth at low aphid numbers and the importance of the predator to soybean aphid integrated pest management are discussed.

We tested host plant suitability of 10 taxa in the Rhamnaceae for successful preoverwintering development of soybean aphid, Aphis glycines Matsumura (Homoptera: Aphididae). In the laboratory, we offered each gynopara and ovipara a single member of each putative host plant and measured survivorship and fecundity. Rhamnus cathartica L. and Rhamnus alnifolia L’Heritier were confirmed as suitable host plants, and eight other taxa were confirmed as unsuitable. This study supports results from a companion study in which soybean aphids were offered multiple members of the Rhamnaceae in field cages. Of the two suitable host plants, R. cathartica supported greater longevity and oviposition rates in oviparae. The importance of R. cathartica as an overwintering host to soybean aphid ecology and management is discussed.

Species and cultivars of the genus Rhamnus and related genera in the Rhamnaceae were tested for their suitability as overwintering hosts of the soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae). The tests were carried out in outdoor cages during the fall through spring of 2002–2003 and 2003–2004. Response of the aphid to the hosts varied from successful overwintering on three Rhamnus hosts to complete rejection of all species in other genera. Fall migrants (gynoparae), egg layers (oviparae), males and eggs were found on the exotic Rhamnus cathartica (L.), and native Rhamnus alnifolia L’Heritier and Rhamnus lanceolata Pursh. In the spring eggs hatched, colonies developed and spring migrants were produced on these hosts. Other Rhamnus spp. were accepted by fall migrants and some level of colony development occurred, but no overwintering eggs were deposited on them. The phenology of the production of the various morphs, egg deposition, and egg hatch are documented on the suitable hosts. The implications of our findings on soybean aphid ecology and management are discussed.

1. Visual surveys for small organisms on complex substrates often yield serious underestimates of true counts. When both visual counts (relative estimates of abundance) and absolute counts can be obtained from the same sample, however, the visual counts can be calibrated such that absolute estimates can be obtained in the future from visual surveys alone. 2. A method is presented for converting quick, timed, visual counts of a sedentary insect on a shrub into absolute estimates of abundance. 3. Analogies were drawn from simple, well-known predation theories to develop a two-parameter non-linear model. Parameter estimates were obtained by both inverse prediction and direct estimation methods; the latter were found to yield more accurate predictions of absolute abundance. 4. The calibration model is mechanistic in its approach, and thus has potential for application in other systems in which all individuals are visible, but able to be missed during timed counts.

1 The present study evaluated the population dynamics of the heteroecious soybean aphid Aphis glycines Matsumura (Hemiptera: Aphididae) during an 8-year period in Indiana, shortly after its detection in North America. Sampling conducted at multiple locations revealed that A. glycines exhibited a 2-year oscillation cycle that repeated itself four times between 2001 and 2008: years of low aphid abundance were consistently followed by years of high aphid abundance. 2 Similar patterns of abundance of A. glycines and coccinellids (Coleoptera: Coccinellidae) in soybean fields, both within and between-years, suggest that late season predation by coccinellids plays a role in the oscillatory cycle of aphids. Insidious flower bugs Orius insidiosus (Say) (Hemiptera: Anthocoridae) were numerically more abundant than coccinellids, although the lack of synchrony between aphids and predatory bugs suggests that O. insidiosus has a limited influence on between-year variations in aphid density. 3 The inverse relationship between aphid densities before and after the start of the autumn migratory period changes direction in alternate years. High aphid density on soybean in the summer is associated with a reduced number of alate migrants produced in the autumn. Conversely, years with low density aphids on soybean in the summer are characterized by high numbers of alates that migrate to the primary host in the autumn. 4 From a pest management perspective, the 2-year oscillation cycle of A. glycines is a desirable attribute with respect to population dynamics because it implies that aphids cause significant economic damage only in alternate years (as opposed to every year). Cultural practices enhancing the conservation biological control of Coccinellidae may help to preserve the periodicity of aphid infestation and restrict the pest status of A. glycines.

The current study evaluated the potential of using counts of winged adults captured in suction traps to forecast the local abundance of soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), in soybean, Glycine max (L.) Merr., fields. The abundance of aphids was evaluated weekly by sampling plants in four to 11 soybean fields and recording the number of aphids in suction traps between 2006 and 2008 in four counties in Indiana and Illinois. Fields in each county were located within 10 km of their respective suction trap, which allowed us to evaluate the relation between aphid abundance on soybean plants and in suction traps at the county level. Migrant soybean aphids caught in suction traps exhibited distinct seasonal trends each year: in 2006, trapped migrants consisted predominantly of individuals dispersing from soybean to buckthorn (Rhamnus sp.); in 2007, in contrast, the majority of trapped migrants were apparently individuals dispersing among soybean fields. The cumulative number of aphids captured in suction traps was positively related to aphid densities on soybean plants. However, the utility of suction traps as a monitoring tool may be limited by the variation in temporal patterns observed in suction traps and on soybean plants each year, and the spatial variation in aphid abundance among soybean fields within a county.

The soybean aphid, Aphis glycines (Hemiptera: Aphididae), is a pest of soybeans in Asia, and in recent years has caused extensive damage to soybeans in North America. Within these agroecosystems, generalist predators form an important component of the assemblage of natural enemies, and can exert significant pressure on prey populations. These food webs are complex and molecular gut-content analyses offer nondisruptive approaches for examining trophic linkages in the field. We describe the development of a molecular detection system to examine the feeding behaviour of Orius insidiosus (Hemiptera: Anthocoridae) upon soybean aphids, an alternative prey item, Neohydatothrips variabilis (Thysanoptera: Thripidae), and an intraguild prey species, Harmonia axyridis (Coleoptera: Coccinellidae). Specific primer pairs were designed to target prey and were used to examine key trophic connections within this soybean food web. In total, 32% of O. insidiosus were found to have preyed upon A. glycines, but disproportionately high consumption occurred early in the season, when aphid densities were low. The intensity of early season predation indicates that O. insidiosus are important biological control agents of A. glycines, although data suggest that N. variabilis constitute a significant proportion of the diet of these generalist predators. No Orius were found to contain DNA of H. axyridis, suggesting intraguild predation upon these important late-season predators during 2005 was low. In their entirety, these results implicate O. insidiosus as a valuable natural enemy of A. glycines in this soybean agroecosystem.

1. We investigate the potential effects of parasitoid egg limitation on host-parasitoid population dynamics. We define a parasitoid as egg-limited if a shortage of eggs means that at some time she cannot utilize an opportunity to oviposit. 2. We develop models which consider the differing physiologies of pro-ovigenic and synovigenic parasitoids to investigate the potential consequences of egg limitation. Pro-ovigenic parasitoids emerge with their full complement of mature eggs. Synovigenic parasitoids also emerge with some eggs, but can mature more later in life. 3. The models indicate that egg limitation in pro-ovigenic parasitoids has no effect on stability. This result is independent of the egg load distribution in newly emerged parasitoids, and of the maximum number of eggs with which a female emerges. In synovigenic models, however, egg limitation is found to be destabilizing, though the reduction in stability is decreased by (i) an increase in egg load at emergence, and (ii) a decrease in the 'latent' time required for egg maturation and gut emptying following a host meal. The latent period, during which time the full gut precludes feeding on hosts, differs from a conventional handling time in that host attacks (via oviposition) are still possible if the female has mature eggs. The maximum ovary storage capacity of females in a synovigenic parasitoid population has no effect on stability.