Ecological studies on evolution of food plant choice by Pieris butterflies

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Three Pieris butterflies, P.rapaecrucivora, P.melete and P.napijaponica, used different sets of cruciferous larval food plants. When a female butterfly laid eggs on actual food plants ordinarily used, it passed through two successive steps : (1)plant habitat location and (2)plant location. However, it stoped its ovipositional behaviors at either of these steps to the potential cruciferous plants not used. The food plant choice by the Pieris butterflies were govemed by inferior plant quality and/or parasitoid pressure as the ultimate factors. However, actual response of egg-laying females is not directly affected by the ultimate factors but by the proximate ones. This study clarifies that which step females of each Pieris butterflies stop their ovipositional behaviors to each potential food plant.The habitat of each Pieris butterfly corresponded the collective habitats of its actual food plants. Therefore, when the potential food plants were in within-habitat of the Pieris species, the butterflies could pass through the step of the plant habitat location and stoped at the step of plant location, presumably because they recognized some chemical deterrents in the plant. On the other hand, the butterflies stoped their ovipositional behaviors to the potential food plants in without-habitat at the step of plant habitat location, even though those plants grew at the closely adjacent places, presumably because they recognized the intrinsic barriers as the proximate factors. However, they have not evolved avoiding behaviors to those plants at the step of plant location. When potential plants seem to be in without-habitat, it is usually considered that they are not used because of being in without-habitat. However, sometimes without-habitats were the result of the butterflies not using the potential food plants.