Naoyuki Fujiyama

Comparison of genetic variation in growth performance on normal and novel host plants in a local population of a herbivorous ladybird beetle, Epilachna vigintioctomaculata

Local populations of herbivore species that display variation in host plant use are subject to natural selection for improved ability to use their own host species. Since natural selection changes the frequency of alleles that control host use, genetic variation in growth performance on host plants may change with each generation of selection. Therefore, within-population variations in the ability to use different hosts may reflect past selective forces. The present study reports the genetic variation in growth performance in a population of the herbivorous ladybird beetle, Epilachna vigintioctomaculata Motschulsky (Coccinellidae, Epilachninae) on both the normal host and a novel host. The present study found higher heritabilities for growth performance on the novel host than the normal host. The difference in heritabilities was caused by smaller among-family variance components on the normal host compared to those on the novel host. The results are compatible with the view that natural selection, which improved the ability of the population to use a particular host plant, has reduced genetic variation within that population in the ability to use that host plant. The pattern of reduced genetic variation in a population reared on the normal host observed in the present study is compared with that in another population, which showed local adaptation in the use of its own host plant. The effect of natural selection on differing host plant use on within-population genetic variation is discussed.

Genetic architecture for normal and novel host‐plant use in two local populations of the herbivorous ladybird beetle, Epilachna pustulosa

Trade‐offs in host‐plant use are thought to promote the evolution of host specificity. However, usually either positive or no genetic correlations have been found. Whereas factors enhancing variation in overall viability have been claimed to mask negative genetic correlations, alternative hypotheses emphasize the sequential changes in genetic correlation in the course of host‐range evolution. In this study, the genetic architectures of performances on different hosts were compared in two populations of the herbivorous ladybird beetle, Epilachna pustulosa, using three host plants, one being normal for both, one novel for only one population, and the other novel for both populations. The genetic correlations between larval periods on normal hosts were significantly positive whereas those between normal and novel hosts were not different from zero. There was no evidence for reduced genetic variation on the normal host‐plants. These results suggest that the host‐range is not restricted by the antagonistic genetic associations among exploitation abilities on different plant species, but rather that selection of different host‐plants may improve the coordination between genes responsible for the use of different plants.

Host‐plant specificity limits the geographic distribution of thistle feeding ladybird beetles

The relationships between two phytophagous ladybird beetle species, Epilachna pustulosa Kôno and E. niponica Lewis (Coleoptera, Coccinellidae), and their main host plants, thistles (Cirsium spp., Asteraceae) were investigated in Oshima Peninsula, southern Hokkaido, northern Japan. Epilachna pustulosa was found feeding on Cirsium kamtschaticum in the northernmost part of the peninsula, whereas E. niponica was confined to the Ohno Plain and adjacent areas in the southernmost part, and occurred mainly on C. alpicola. No thistle feeding epilachnines were found in the middle part of the peninsula despite the abundance of another thistle species, C. grayanum. Both beetle species showed lower adult preference and reduced growth performance on C. grayanum compared to their respective host plants under laboratory conditions. We concluded that the distribution of thistle feeding epilachnines in Oshima Peninsula was principally determined by the availability of appropriate host plants.

Genetic basis for established and novel host plant use in a herbivorous ladybird beetle, Epilachna vigintioctomaculata

Genetic trade‐offs in host plant use are thought to promote the evolution of host specificity. Experiments on a range of herbivorous insects, however, have found negative genetic correlation in host plant use in only a limited number of species. To account for the general lack of negative genetic correlation, recent hypotheses advocate that different stages in evolution of host use must be distinguished: initial performance on a novel host in comparison with the established host, and performance on both hosts after the insect population has interacted with both hosts for a long time. The hypotheses suggest that genetic correlation may not necessarily be negative at the initial stage. The present study examines growth performance on both the established and a novel host in a herbivorous ladybird beetle, Epilachna vigintioctomaculata Motschulsky (Coccinellidae, Epilachninae). The results show that traits of growth performance across hosts were positively or neutrally correlated, but there was no evidence of a negative genetic correlation. In addition, significant genetic variance of growth performance on each host was detected, suggesting that E. vigintioctomaculata can potentially respond to selection for increased performance on both plant species. These results and similar results from experiments on other herbivores suggest that host expansion may not be constrained genetically, at least at the initial stage of host range evolution.

Genetic basis for different host use in Epilachna pustulosa , a herbivorous ladybird beetle

The genetic basis for different host plant use was studied in a herbivorous ladybird beetle, Epilachna pustulosa, that exhibits interpopulational variation in host plant utilization. It usually depends on thistle, but one of the local forms occurs on both thistle and blue cohosh, which differ at the infraclass taxonomic level. In this local population, genetic association between the developmental performance on the two plants was neutral, suggesting genetic independence across the host plants. The form of reaction norms indicated some changes in relative rank position. Genetic variation among individuals on each host plant was detected. These results suggest that different genotypes are selected on different host plants and that a substantial proportion of the overall phenotypic plasticity is contributed by genotype-dependent environmental effects.

Variable correspondence of female host preference and larval performance in a phytophagous ladybird beetle Epilachna pustulosa (Coleoptera : Coccinellidae)

Genetic and environmental factors causing intraspecific variation of the thistle Cirsium kamtschaticum Ledeb. as a host plant of the phytophagous ladybird Epilachna pustulosa Kôno were investigated through simple food-choice tests and rearing of larvae. Two thistle clones (T1U1 and T4H2) were used, originally growing approximately 12 km apart. A previous study showed that adult female ladybirds preferred T1U1 to T4H2, and that larval performance was better on T1U1, when leaves from the clones in situ were examined. The two clones retained their characteristics with respect to beetle preference after transplantation into a common garden. However, the difference between T1U1 and T4H2 with respect to larval performance was reduced after the transplantation. When leaves from shoots of T1U1 exposed to different sunlight intensities were offered, adult female ladybirds did not show obvious preferences. Larval eclosion rates increased significantly with the increase in leaf sunlight intensity exposure. These results suggest strongly that both genetic and environmental factors are involved in interclonal variation of thistle quality in beetle preference and/or performance. It is suggested that the quality of thistle leaves for larval performance is largely affected by environmental factors, while leaf quality for beetle preference may be determined strictly by genetic factors. Under certain conditions, E. pustulosa females may behave maladaptively, preferring plants not appropriate for larval growth, or not choosing plants appropriate for the larval growth.

Individual variation in two host plants of the ladybird beetle, Epilachna pustulosa (Coleoptera : Coccinellidae)

Individual variation in two species of host plants (thistle,Cirsium kamtschaticum, and blue cohosh,Caulophyllum robustum) of the herbivorous ladybird beetleEpilachna pustulosa was examined under laboratory conditions for their acceptability to adult beetles as a food resource, for adult preference and for larval performance. When clones of these plants were subjected to non-choice tests using posthibernating female beetles, there was found to be significant intraspecific variation among clones in terms of their acceptability, but interspecific variation was not detected. Significant intraspecific as well as interspecific variation were frequently detected in the two host plants when clones of these plants were subjected to choice tests using posthibernating female beetles; the magnitude of interspecific plant variation for beetle preference is not necessarily larger than that of intraspecific plant variation. Individual variation across plant species with respect to beetle larval performance was also significant. A positive correlation between adult preference and larval performance is suggested across the two taxonomically remote host plant species, thistle and blue cohosh, although this needs further investigation.

Oviposition site selection by herbivorous beetles: a comparison of two thistle feeders, Cassida rubiginosa and Henosepilachna niponica

In insects that feed on plants in both adult and larval stages, it is often difficult to distinguish oviposition preference from adult feeding preference, because oviposition can occur at or in proximity to feeding sites. In the present study, characteristics of oviposition site selection of two beetle species, Cassida rubiginosa Müller (Coleoptera: Chrysomelidae) and Henosepilachna niponica (Lewis) (Coleoptera: Coccinellidae), were investigated in the field and laboratory, with particular attention to relationships with adult feeding sites. In the field, distances between adult feeding scars and egg masses differed for C. rubiginosa and H. niponica, with the former being very small and the latter averaging 24.6 cm. The same tendencies for the distances between adult feeding scars and egg masses of the two beetle species were confirmed in cages in which only female beetles were released. Cassida rubiginosa restricted egg laying to host plants in the field and to leaves in laboratory assays. On the other hand, H. niponica placed 8% of egg masses on plants adjacent to host plants in the field and often placed eggs on artificial substrates rather than leaf discs in laboratory assays. These results suggest that oviposition and female feeding sites are virtually inseparable in the case of C. rubiginosa, while H. niponica females do not necessarily keep to host plant leaves as oviposition substrates and they tend to oviposit at some distance from their feeding sites. Results are discussed in relation to proximate and ultimate causes of host selection behavior.

Conspecific thistle plant selection by a herbivorous ladybird beetle, Epilachna pustulosa

Host selection by Epilachna pustulosa Kôno (Coleoptera: Coccinellidae) was surveyed in an area of about 130 ares, focusing on the role of the spatial distribution pattern of the host plant, thistle Cirsium kamtschaticum Ledeb. (Asteraceae) and the environmental conditions of habitats where thistle plants were growing. A total of 198 thistle clones were found in the area studied, and approximately 40% showed some degree of E. pustulosa infestation by July. Eggs were only oviposited on thistle clones that were fed on by adults. Adult beetles and egg masses of E. pustulosa showed an aggregated distribution in the earlier season (June) among thistle clones. The distribution of adults became more random (but still aggregated) by the later season (July), along with an increase in the number of infested clones. Multiple logistic regression analyses revealed that clone size and soil moisture were consistently important for the beetles choice of clones to feed on. The other logistic regression analyses indicated that thistle‐clone size and sunlight condition influenced egg distribution. Thistle clone selection by E. pustulosa changed with season from a rather strict selection in June to a more obscure one in July, expanding the range of thistle clones used as feeding and oviposition substrate.

Differentiation in the ability to utilize Pterostyrax hispida (Ebenales: Styracaceae) among four local populations of the phytophagous ladybird beetle Henosepilachna yasutomii (Coleoptera: Coccinellidae)

The oligophagous ladybird beetle Henosepilachna yasutomii Katakura (Coleoptera: Coccinellidae) shows inter-population differences in its host-use. In this beetle, populations using the deciduous tree Pterostyrax hispida Sieb. et Zucc. (Styracaceae) were recently rediscovered in the Kanto districts of central Honshu, Japan. In the present study, the ability to utilize P. hispida and other host plants was compared among four populations of H. yasutomii occurring on P. hispida and Scopolia japonica Maxim. (Solanaceae), Chelidonium japonicum Thunb. (Papaveraceae), and Solanum tuberosum L. (Solanaceae). As regards the feeding habits of adult beetles, only the populations occurring on P. hispida accepted this plant’s leaves, which differed distinctly from the feeding habits of the other populations, although the differences among the four populations were not readily apparent with respect to the leaf amounts consumed. The larvae from the populations occurring on P. hispida showed significantly higher survivorship on this plant than did the larvae from the other populations. Considering the host use patterns and the life cycles of beetles under natural conditions, the large abundance of P. hispida leaves throughout the season may have played an important role in selection for the ability to utilize P. hispida observed in the H. yasutomii populations occurring on this woody host.