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. rapae 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. rapae. 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. rapae, 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 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. rapae 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, through 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 of Pieris species.

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Abstract. 1. Experimental studies have shown that larvae of threePieris butterflies, P.rapae L., P.melete Mènètriés and P.napi L., are attacked by a parasitoid wasp, Apanteles glomeratus L. Although P.rapae larvae are parasitized heavily in the field, P.melete and P.napi are infrequently parasitized successfully because they possess mechanisms for encapsulating parasitoid larvae and for avoiding parasitism. 2. This study examines spatial and temporal variation in rates of parasitism of the three Pieris species by A.glomeratus in the field. We attempted to determine whether P.rapae possesses any means of avoiding parasitism by this wasp, and then to deduce why both P.melete and P.napi have more distinctive avoidance mechanisms than P.rapae. 3. Our results indicate that in temporary habitats, which are the main habitats of P.rapae, P.rapae is able to escape A.glomeratus in time and space by colonizing new habitats before the parasitoid arrives. In permanent habitats, however, such escape is not possible. P.rapae larvae lack physiological or behavioural avoidance mechanisms of reducing parasitism rates in permanent habitats. P.melete and P.napi, in contrast, live only in permanent habitats, where the parasitic pressure is potentially high, and have evolved active avoidance mechanisms.

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Summary 1.Batesian mimicry describes the situation in which a palatable mimic resembles an unpalatable model. In some species of butterflies, both sexes mimic, but in others only females do. Two mechanisms have been proposed to generate female-limited mimicry: sexual selection via female choice, and sexual selection via male-male competition. Each is not satisfactory because of too many exceptions. 2. I hypothesized that reductions in physiological life span because of mimicry constituted the costs, while extensions of ecological life span because of mimicry constituted the benefits. Mimicry would result from the balance of a costs/benefits relationship; when balances are favourable, mimicry occurs. 3. Non-mimetic females of female-limited mimicry butterfly Papilio polytes lived longer than its mimetic females in a butterfly farm greenhouse. Therefore, reduction in physiological life span must be a cost of mimicry. 4. Male-biased sex ratios were found in 11 of the 14 non-mimetic and non-model species netted in the Kakamega tropical rainforest in west Kenya, in other one species almost equivalent, and in remaining two species female-biased. For each species, the sexes netted less frequently had more beak marks than those netted more frequently. That is, the biased sex ratios could be responsible for the higher predation rates of the opposite sex. 5. Predators may have selectively attacked females with wider thoraxes, which flew at higher levels and flew more quickly. However, attack rates on males of some species may be high or low regardless of thorax width. 6. As the benefits accrue largely to females with wider thorax due to female-biased predation, balances of costs/benefits relationships divide participating species into three theoretical groups: those in which the adaptation is favourable for both sexes; favourable only for females; and unfavourable for both sexes. 7. In female-limited mimics of five species, thorax widths varied, whereas thoraxes tended to be wider in both-sex mimics of four species.Key-words : cost and benefit, ecological life span, female-biased beak mark ratio, flight velocity, male-biased sex ratio, physiological life span, predation pressure.

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Summary The adult populations of three Pieris butterflies, P. rapae, P. melete and P. napi, were studied in an area of their coexistence throughout the flight seasons by using the mark‐and‐recapture method. The study area, about 3×1.5 km, was set up in a farm village surrounded by the mountainous area in Inabu, Aichi Prefecture. The habitats were qualified by the four factors, i. e., oviposition plants, adult nector plants, roosting‐sites and light conditions. Between P. rapae and P. napi, there were sharp differences with regards to overall habitat preferences. P. melete had the widest preferences for all the habitat resources, which overlapped greately with requirements of P. rapae and P. napi. P. melete and P. rapae showed similar preferences for oviposition plants, but the former preferred shaded habitats while the latter preferred sunny places. P. melete and P. napi, having similar preferences for shaded situations, showed differences in the preferences for oviposition plants. Moreover, three species of Pieris were different in their preferences for adult nector plants. Thus, they were more likely to partition habitat resources rather than competing for them. The habitat structures of each species in respect of time, space and stability to weather changes were much different each other in the same area. The habitat of P. rapae was temporary, localized and unstable. While, that of P. melete was more permanent, widespread and stable than that of P. rapae. P. napi seemed to live in the intermediate habitat, i. e., permanent, localized and stable one.