Search

In this study, we focused on the direct (i.e., predation) and indirect (i.e., potential threat from coexisting with a larger individual) effects of cannibalism and intraguild predation (IGP) during larval stages of two sibling ladybird beetles. These effects play an important role in the coexistence of the generalist–common Harmonia axyridis and specialist–rare H. yedoensis (Coleoptera: Coccinellidae). Direct predation effect of cannibalism and IGP was asymmetric in the two sibling ladybird beetles; the fourth instar larvae of H. axyridis were better intraguild predators than cannibals, while the reverse was true in the larvae of H. yedoensis. Neither cannibalism nor IGP significantly affected female body weight in either species. Larval H. axyridis surviving exposure to cannibalism or IGP had a reduced number of ovarioles as adults, whereas adult H. yedoensis ovarioles were not affected. For the indirect effects, longer developmental times in males and females and a lower total number of ovarioles in females were detected in H. axyridis. In H. yedoensis, shorter developmental time of males, lighter adult weight and fewer total ovarioles in females were observed. Olfactometer choice experiments clarified that the fourth instar larvae of H. axyridis avoided the first instar conspecific larvae, while those of H. yedoensis were attracted to the odors from H. axyridis and conspecifics. Thus, H. axyridis has an avoidance mechanism only for cannibalism but not for IGP, whereas H. yedoensis does not have any avoidance mechanism. These different behaviors in the direct and indirect effects of cannibalism and IGP observed in the laboratory may play important roles in the coexistence of generalist–common H. axyridis and specialist–rare H. yedoensis in natural conditions, compensating for the large handicap of H. yedoensis at reproductive interference from H. axyridis.

Predaceous ladybird beetles are known to consume alternative foods from flowers, especially when prey is scarce. Flower-rich semi-natural habitats in agroecosystems generally host a diversity of natural enemies, including predaceous ladybird beetles, suggesting that the availability of flowers may have a positive role in their fitness traits. In this study, we test whether feeding on flowers of Perilla frutescens (Lamiaceae) increases longevity and fecundity in Harmonia axyridis Pallas (Coleoptera: Coccinellidae). The longevity of H. axyridis females and males fed with five flowers was significantly greater than those fed with one flower and in the control group (no food), although the provision of flowers had no positive effect on the increase of body weight in males and females. The number of eggs and oviposition frequency in H. axyridis fed with flowers plus prey, as well as with prey only, were significantly larger than those fed with only flowers, whereas no significant difference was observed between individuals fed with the mixed diet and those with only prey. However, on the first day of the diet assignment, the number of eggs from individuals with the mixed diet was significantly higher than of those with only prey and those with only flowers. The results show that flowers of P. frutescens, mixed with prey, have a positive effect on H. axyridis survival and early reproduction, suggesting that flowers may play an important role in increasing fitness in H. axyridis. The possibility of P. frutescens sown in fields to support populations of the predator toward conservation biological control is discussed.

This paper presents a method with accuracy to estimate instars for insects which have large number of instars, especially at long growth period. We clarified the colony composition of the wood-feeding cockroach Panesthia angustipennis spadica Shiraki (Blaberidae) in a field, estimating the instars of field-collected individuals through a cluster analysis using field and laboratory data. Overall, 84.8% sampled units contained multiple individuals, and the largest colony consisted of 65 individuals. The colonies were composed of adults and nymphs (43.5%), or nymphs without adults (52.2%). The number of instars in this species was estimated as ca. ten. Furthermore, some colonies contained an adult pair and nymphs at multiple developmental stages, whereas other colonies contained only small nymphs (estimated instars 1–3). These results demonstrate that the social structure of P. angustipennis spadica varies widely, and that colonies containing both adults and nymphs may indicate the origin of subsociality in cockroaches.

The wood-feeding cockroach Panesthia angustipennis spadica Shiraki (Blaberidae) is a gregarious species, and its groups containing both nymphs and adults have often been observed in the field. To clarify the effects of aggregation with a parent and siblings on nymphal development in P. angustipennis spadica, we raised nymphs of this species in the laboratory under the following three experimental categories: (a) aggregate with a female adult; (b) aggregate without a female adult; and (c) solitary. Survival rates did not significantly differ among the three categories. Unexpectedly, our results clearly showed that nymphs raised in aggregates with a female adult were smaller, lighter, and reached a lower instar than those raised without a female adult. These results indicate that the presence of a female adult does not positively affect nymphal development; i.e. there may be no direct parental care in this species as indicated by previous studies. Moreover, solitary nymphs grew faster and larger than nymphs in aggregates, showing that the aggregation with siblings also has no positive effect on nymphal development. Thus, it seems likely that external factors, e.g. predation and environmental conditions, may drive aggregating behavior in P. angustipennis spadica.

In predatory ladybirds male embryos in clutches of eggs infected with male-killing bacterial endosymbionts do not hatch and are consumed by female hatchlings. Moreover, it is predicted that infection with male-killing bacteria should be prevalent in a ladybird population if the fitness advantage to female ladybirds due to the resulting reallocation of resources is high. We compared male-killer bacterial prevalence in two sibling species of the ladybird genus Harmonia that use different host ranges. Harmonia yedoensis feeds mostly on the highly elusive pine aphid and its hatchlings can greatly enhance their ability to capture prey and thus their survival by consuming un-hatched eggs in the clutch. In contrast, Harmonia axyridis feeds on a wide range of prey and consumption of un-hatched eggs by the hatchlings does not necessarily increase their foraging success. In the study area where these two species of ladybird occur sympatrically 14 of the 22 females of H. yedoensis (64%) laid clutches of eggs of which approximately only a half hatched and did not produce male offspring, whereas only one of 12 of the females of H. axyridis (8%) exhibited symptoms indicating infection with male-killing bacteria. In the H. yedoensis that exhibited symptoms of being infected with male-killing bacteria, the total allocation of resources to each hatchling, which was provided by laying larger eggs and eating the un-hatched eggs in the clutch, was considerable. These results suggest that the difference in the prevalence of infection with male-killing bacteria in these two species of ladybird may reflect differences in the quality of their prey and their foraging behaviour.

In Hymenoptera and Heteroptera, the absence of micropyles is one criterion for categorizing an egg as trophic. Undevel- oped eggs are observed in more than 90% of the egg clusters of the ladybird beetle Harmonia axyridis Pallas. Traditionally, these undeveloped eggs are regarded as "trophic eggs." The surfaces of the eggs of H. axyridis were examined using scanning electron microscopy and the presence of micropyles in the shells of developing and undeveloped eggs determined. Micropyles are circularly distributed around the top of eggs and present in both developing and undeveloped eggs. The number of micropyles in the shells of developing and undeveloped eggs did not differ significantly. Our results indicate that the undeveloped eggs of H. axyridis have micropyles, suggesting that the mechanisms regulating the production of undeveloped eggs in H. axyridis differ from those resulting in the production of trophic eggs by Hymenoptera and Heteroptera.

The sibling species H. yedoensis Takizawa coexists sympatrically and simultaneously with H. axyridis only on pine trees in Japan. To elucidate the mechanisms enabling coexistence of these two sympatric sibling species, a laboratory experiment was per- formed that focused on differences in their maternal investment through eggs and the role of sibling cannibalism. The egg size (vol- ume) of H. yedoensis was 24.91% larger than that of H. axyridis. Cluster size in H. axyridis was significantly larger than that in H. yedoensis; however, the total number of eggs and oviposition cost (by volume) per female in H. yedoensis were not significantly dif- ferent from those in H. axyridis, although total number of clusters tended to be slightly higher in H. yedoensis than in H. axyridis. The percentage of undeveloped eggs per cluster in H. yedoensis was not significantly different from that in H. axyridis, whereas the percentage of developed eggs with delayed hatching per cluster was significantly larger in H. yedoensis than in H. axyridis. Moreo- ver, the cost of sibling cannibalism per hatched larval cluster in H. yedoensis (worth 4.43 sibling eggs) was 3.36 times larger than that in H. axyridis.Therefore, maternal investment through egg and sibling cannibalism in developed eggs with delayed hatching are more intense in H. yedoensis than in H. axyridis, implying a higher larval survival rate through higher ability of prey capturing at the first instar. The results in this study suggest that the higher survival rate and accelerated development in H. yedoensis by the two maternal investments, i.e., a large egg and intense sibling cannibalism of developed eggs with delayed hatching, may play an impor- tant role in sympatric coexistence with the aggressive aphidophagous ladybird beetle H. axyridis.

To elucidate the temporal response of parasitoids to their host density, we investigated the temporal relationship between the density of the leafroller moth, Eudemis gyrotis (Lepidoptera, Tortricidae), and larval and pupal parasitism in the Þeld. E. gyrotis had overlapping-generations, and the density of larvae and pupae of E. gyrotis varied seasonally. Analysis of the temporal relationship between the density of E. gyrotis and the percentage parasitism revealed a tendency toward positive density-dependent attack by the parasitoid Apanteles sp. (ater-group) (Braconidae) on early-stage E. gyrotis larvae, and inverse density-dependent attack by Goniozus japonicus (Bethylidae) in late-stage larvae. However, no temporal density-dependent attack was observed by the pupal parasitoids Brachymeria excarinata (Chalcididae), B. lasus, and Itoplectis alternans spectabilis (Ichneumonidae). These different responses to host density by the parasitoid species were discussed in terms of the mode of parasitism, the host stages attacked, host-searching behavior, and the effect on the host population.

Reproductive strategy affects population dynamics and genetic parameters that can, in turn, affect evolutionary processes during the course of biological invasion. Life-history traits associated with reproductive strategy are therefore potentially good candidates for rapid evolutionary shifts during invasions. In a series of mating trials, we examined mixed groups of four males from invasive and native populations of the harlequin ladybird Harmonia axyridis mating freely during 48 hours with one female of either type. We recorded the identity of the first male to copulate and after the 48 h-period, we examined female fecundity and share of paternity, using molecular markers. We found that invasive populations have a different profile of male and female reproductive output. Males from invasive populations are more likely to mate first and gain a higher proportion of offspring with both invasive and native females. Females from invasive populations reproduce sooner, lay more eggs, and have offspring sired by a larger number of fathers than females from native populations. We found no evidence of direct inbreeding avoidance behaviour in both invasive and native females. This study highlights the importance of investigating evolutionary changes in reproductive strategy and associated traits during biological invasions.

1) Life tables of the predatory ladybird beetleHarmonia axyridis were analysed by taking samples from twenty or twenty four sites colonized by eight species of aphids for two years. 2) Larval mortality was a key factor in theH. axyridis populations. Mortality of all the stages tended to be density-dependent when life cycles were divided into four developmental stages, except in the case of mortality at hatching due to sibling cannibalism. 3) Sibling and non-sibling cannibalism played an important role on population stability and persistence.

Seasonal variation in elytral colour polymorphism in Harmonia axyridis (the ladybird beetle): the role of non-random mating

Sibling (cannibalism among siblings) and non-sibling cannibalism (cannibalism among non-siblings) were studied in a natural population of a lady beetle,Harmonia axyridis Pallas. Of all the eggs laid (n=2269), 24.76% (n=562) were killed by sibling cannibalism and 36.10% (n=819) were killed by non-sibling cannibalism. Sibling cannibalism occurred constantly and intensively in most egg batches throughout the entire oviposition period. On the other hand, non-sibling cannibalism was more intense in the middle and late oviposition periods, and when the egg batches were close to an aphid colony. This may be due to the high density ofH. axyridis larvae relative to aphid density in the middle and late oviposition periods and also the larvae searching intensively near an aphid colony.