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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