COSTS AND BENEFITS OF EGG CLUSTERING IN PIERIS-BRASSICAE

1. Pieris brassicae lays large batches of eggs (up to 150) and the cat erpillars that hatch frequently have to migrate in search of new plant s before completing development. Some of the costs and benefits of thi s behaviour for the caterpillars are investigated. 2. A model is devel oped to show how the degree of inhibition of plant growth due to larva l feeding can affect the extent to which more food is available to a g roup of larvae feeding on two plants sequentially than to two groups, each of half the size, feeding on two plants simultaneously. 3. In a f ield experiment, brussels sprouts plants were infested with 0, 5, 40 o r 100 first instar P. brassicae caterpillars. The presence of caterpil lars was found to have an inhibitory effect on plant growth. No effect s of caterpillar group size on mortality during the first three instar s or on parasitism by Cotesia glomerata were found. 4. In a second fie ld experiment, fourth and fifth instar caterpillars were released eith er 0.5 m or 2.5 m away from host plants. After 72 h, 81% of those rele ased 0.5 m away had located a plant, but only 16% of those released 2. 5 m away had done so. The results were not significantly affected by t he age of the caterpillars used or by the density of background vegeta tion. 5. It is suggested that laying more eggs than can be supported b y a single host plant may be beneficial to female butterflies if the l ocal density of host plants is high. After exhausting their natal host plant, caterpillars can then exploit the uninhibited growth of neighb ouring and previously uninfested plants with only a small cost to migr ation. Under such circumstances, butterflies might be expected to trea t clumps of host plants, rather than individuals, as the unit of resou rce over which oviposition decisions should be made.