Fitness consequences of cannibalism in the fall armyworm, Spodoptera frugiperda

We investigated the consequences of cannibalism for some correlates of fitness in the fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae). The benefits gained by cannibals were ascertained by comparing survival, development rate, and pupal weight of larvae that had the opportunity to cannibalize with those that did not, at two levels of food availability. Larvae in the cannibalism treatments were provided with a conspecific one instar younger than themselves on five dates throughout larval development. Cannibalism was frequent; given the opportunity all larvae predated at least one younger conspecific. The frequency of cannibalism was not affected by the sex of the cannibal or by the availability of alternative food. However, cannibals suffered a significant reduction in survival at both high and low food availability. Furthermore, cannibals had a lower pupal weight in the high food treatment and a reduced development rate in the low food treatment. The only detectable fitness benefit associated with cannibalism was a positive correlation between the number of victims consumed and development rate under conditions of low food availability. On balance, cannibalism appears to be costly; alternative explanations for its occurrence in this species are discussed.