Effect of parasitoids on food-plant choice by Pieris butterflies

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Pieris napi and P.melete adult females collected from forest cores in national parks where R.sylvestris, naturalised plant, did not occur naturally laid eggs on both C.leucantha and R.sylvestris in field cages. In contrast, P.napi and P.melete adult females collected from areas that contained C.leucantha and R.sylvestris in adjacent patches, laid eggs on only R.sylvestris and only C.leucantha, respectively. That is, the butterflies avoided laying eggs on the plants that grew in the habitats they normally used. These results suggest that after around 1960,when R.sylvestris invaded Hokkaido Island, P.napi lost its preference for the original food plant C.leucantha, and P.melete has lost its preference for the potential food plant R. sylvestris in areas where both butterflies usually encounter both plants.To explain this phenomenon, we assessed the intrinsic quality of each plant and the parasitism rates of Pieris larvae by a parasitoid wasp and tachinid flies, and quantitatively analysed their relative importance using a mathematical model. The model suggests that the primary factors are natural enemies and interspecific competition.Rorippa sylvestris was an intrinsically superior food plant for both P.napi and P.melete as assessed by larval survival and development time and pupal mass and the mean parasitism rate of P.napi on R.sylvestris in areas was significantly lower than both that on C.leucantha in areas. It is suggested, therefore, that there was the interspecific competition on enemy-free space, R.sylvestris plants, between P.napi and P.melete, which appears to be very strong, possibly resulting in competitive exclusion by reproductive interference.