Yoshibumi Sato

Food Plant Choice of Pieris Butterflies as a Trade‐Off between Parasitoid Avoidance and Quality of Plants

This report assesses the role of specialist parasitoids in providing a major selection pressure for food plant preference in herbivorous insects. Three Pieris butterflies, P. rapae crucivora, P. melete, and P. napi japonica, use different sets of cruciferous larval food plants. P. rape is oligophagous and uses ephemeral plants. P. melete is polyphagous and uses persistent plants, as well as all of the ephemeral plants used by P. rape. On the other hand, P. napi is locally monophagous, using persistent Arabis. We assessed the intrinsic suitability of these crucifers by measuring survival rates, development times, and pupal mass of larvae growing on them at a constant temperature. All of the food plants of P. rape and P. melete are suitable for larvae of the three Pieris species. On the other hand, food plants of P. napi are the least suitable for all three species. Pieris larvae have two specialist parasitoids, the braconid wasp Cotesia glomerata (formerly Apanteles glomeratus) and the tachinid fly Epicampocera succincta. In newly established habitats, P. rapae can avoid both parasitoids. In long-lasting habitats, however, P. rape is heavily parasitized by both parasitoids. P. melete and P. napi, by contrast, live only in long-lasting habitats, where the parasitic pressure is potentially high. However, P. melete can partially avoid parasitism by killing the eggs of C. glomerata by encapsulation, though parasitized by E. succincta. On the other hand, P. napi seems to have evolved behavioral avoidance of parasitoids by specializing on Arabis plants. The different food plant preferences of the three Pieris species can be interpreted as resulting from differences in the balance of a trade-off between parasitoid avoidance and the intrinsic quality of potential food plants to Pieris species.

Cotesia glomerata Female Wasps Use Fatty Acids from Plant–Herbivore Complex in Host Searching

Cotesia glomerata parasitizes early instars of the cabbage butterfly,Pieris rapae, in Japan. Female wasps antennatedRorippa indica leaves damaged by feeding ofP. rapae larva, but ignored artificially damaged leaves. Females also antennated filter paper containingR. indica leaf juice plusP. rapae regurgitant. Chemical analysis revealed five compounds in higher amounts in the infested edges of leaves than in artificially damaged edges. Among them, we identified palmitic acid, oleic acid, and stearic acid. Female wasps antennated filter paper containing each of these three acids. We discuss the function of these acids in the tritrophic context.

The role of parasitoids in evolution of habitat and larval food plant preference by three Pieris butterflies

This article attempts to explain that parasitoids provide the evolutionary pressure responsible for relationships between habitat use and larval food plant use in herbivorous insects. Three species of butterflies of the genus Pieris, P. rapae, P. melete, and P. napi use different sets of cruciferous plants. They prefer different habitats composed of similar sets of cruciferous plants. In our study, P. rapae used temporary habitats with ephemeral plants, P. melete used permanent habitat with persistent plants, although they also used temporary habitats, and P. napi used only permanent habitat. The choice experiment in the field cages indicated that each of the three butterfly species avoided oviposition on plants usually unused in its own habitat, but accepted the unused plants which grew outside its own habitat. Their habitat use and plant use were not explained by intrinsic plant quality examined in terms of larval performance. Pieris larvae collected from persistent plants or more long lasting habitats were more heavily parasitized by two specialist parasitoids, the braconid wasp Cotesia glomerata and the tachinid fly Epicampocera succincta. The results suggest that Pieris habitat and larval food plant use patterns can be explained by two principles. The evolution of habitat preference may have been driven by various factors including escape from parasitism. Once habitat preference has evolved, selection favors the evolution of larval food plant preferences by discriminating against unsuitable plants, including those which are associated with high parasitism pressures.

Response of the wasp (Cotesia glomerata) to larvae of the large white butterfly (Pieris brassicae)

The large white butterfly (Pieris brassicae L) first invaded northernmost Japan from Siberia around 1994, and after a few years, began to expand its range. The wasp, Cotesia glomerata (L) parasitizes larvae of the small white butterfly (Pieris rapae crucivora Boisduval), a usual host in the same geographic area. Some Pieris brassicae larvae in Hokkaido have been parasitized by Cotesia glomerata, but the parasitism rate of Pieris brassicae larvae tends to be lower than that of Pieris rapae. To examine the process of parasitizing Pieris brassicae larvae, we observed how the parasitoid wasp responded to the host larvae on damaged leaves. Cotesia glomerata females tended to avoid Pieris brassicae larvae, and even when female wasps inserted their ovipositors into Pieris brassicae larvae, none laid eggs. The parasitoids obtained from Pieris rapae larvae failed to parasitize Pieris brassicae during the host-acceptance step.