Effect of parasitoids on food-plant choice by Pieris butterflies

从国家公园的森林核心中收集的美洲粉蝶和美洲粉蝶成年雌性在野外笼子中在白花粉蝶和欧洲粉蝶上自然产卵。相比之下，从相邻斑块中包含白花杜鹃和欧洲杜鹃的区域收集的油菜和欧洲杜鹃成年雌性分别仅在欧洲杜鹃和白花杜鹃上产卵。也就是说，蝴蝶避免在它们通常使用的栖息地中生长的植物上产卵。这些结果表明，在1960年左右，当欧洲赤眼蜂入侵北海道岛后，欧洲油菜失去了对原始食物植物银合欢的偏好，而欧洲赤眼蜂也失去了对潜在食物植物欧洲赤眼蜂的偏好。为了解释这一现象，我们评估了每种植物的内在质量和粉蝶幼虫被寄生蜂和寄蝇的寄生率，并使用数学模型定量分析了它们的相对重要性。结果表明，天敌和种间竞争是影响小菜蛾和菜青虫寄生率的主要因素，从幼虫存活发育时间和蛹重两个指标来看，天然的菜青虫是菜青虫和菜青虫的上级食物源，菜青虫对当地菜青虫的平均寄生率显著低于菜青虫和菜青虫对当地菜青虫的寄生率。因此，油菜和欧洲油菜在无天敌空间--欧洲油菜植株上存在着种间竞争，这种竞争非常强烈，可能是通过生殖干扰而导致的竞争排斥。

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.