A WAR OF ATTRITION BETWEEN LARVAE ON THE SAME HOST-PLANT - STAY AND STARVE OR LEAVE AND BE EATEN

Many insect species lay their eggs according to a clumped distribution , which causes food shortage among the larvae. To avoid starvation, at least some larvae have to migrate to another plant at some stage. Eve n though this migration involves a high mortality risk, larvae (of, fo r example, the cinnabar moth) often leave before their host plant is d efoliated, thereby enabling others to remain safely on the plant. To u nderstand why this remarkable behaviour has evolved, we modelled the s ituation of two larvae on a small plant as a war of attrition. Our mai n conclusions are that, in the case where the larvae are unrelated, th ey should only migrate prior to defoliation in certain time periods, c alled migration periods, when the pay-off of leaving decreases. Moreov er, the optimal migration time is a random variable. When the pay-off of leaving fluctuates, there can be several migration periods. In the case where the larvae are related, it can also be optimal to migrate w hen the pay-off of leaving increases. Relatedness increases the length and the number of the migration periods, as well as the leaving tende ncy during these periods. Furthermore, relatedness gives rise to a par ent-offspring conflict over the optimal migration strategy.