The Search for Resources by Cabbage Butterflies (Pieris Rapae): Ecological Consequences and Adaptive Significance of Markovian Movements in a Patchy Environment

Unlike many insects, the caterpillars of the cabbage butterfly, Pieris rapae, are less abundant on host plants that grow in large dense stands than on hosts that are scattered sparsely in diverse vegetation. In this study we describe how the flight behavior of ovipositing P. rapae results in the placement of more eggs on isolated hosts. The flight paths of females were measured in terms of move lengths and tuning angles; this permitted us to analyze the flight as a sequence of movement vectors punctuated by landings and oviposition. A variety of tests revealed that the butterflies conformed with the assumptions required to describe their flight as a Markov process. Females ovipositing in potato—collard bicultures behave identically to females oviposition in collard monocultures. In gardens containing collards planted at different spacings, females make shorter moves and land more frequently when encountering dense stands of collars. Under all circumstances, however, ovipositing butterflies tend to follow linear routes of flight. As a result of this directionality, the number of eggs laid per plant declines with increases in host plant density, even when these increases in host concentration prompt shorter moves. Females foraging for nectar, on the other hand, readily abandon their linear flight paths. By engaging in tight turning behavior in clumps of flowers, nectar—feeding butterflies concentrate their activities in resource patches. The differences between the nectar—feeding and ovipositional flights highlight an "egg—spreading syndrome." In particular, three features of P. rapae oviposition behavior enhance each female's tendency to spread her eggs widely: (1) eggs are usually laid singly on plants, (2) females tend to follow linear flight paths, and (3) females typically pass over many suitable hosts. Such behavior cannot be the product of natural selection acting solely to increase energetic efficiency or competitive ability. We evaluate several adaptive explanations for this egg—spreading syndrome and exclude all but one. The remaining hypothesis is that P. rapae scatter their eggs widely to "spread the risk" and minimize the generation—to—generation variance in replacement rates. To estimate the potential advantages accrued by egg spreading, we use data on the spatial variability in survivorship to simulate the replacement rates that would be achieved by scattering eggs on different plants within a garden and in different gardens within a region. Although this simulation cannot prove the risk—spreading hypothesis, it does demonstrate the effectiveness of egg spreading in averaging out observed variations in larval survivorship. The variability in P. rapae survivorship appears to be associated with the highly unpredictable milieu (local herbivore load, plant condition, etc.) of a developing caterpillar. The implications of such unpredictability to evolution and community organization are discussed.