Win, Lose, or Draw: Effects of Residency, Size, Sex, and Kinship on High‐Stakes Larval Contests in a Moth

Many factors can affect the outcome of a competitive interaction. One such factor is the relatedness between competitors as competitive intensity may decrease between kin. Because adult females lay eggs in clusters, larvae of the moth Utetheisa ornatrix are likely to be found in high densities of their siblings. Larval U. ornatrix actively compete for access into seedpods of their host plant (Crotalaria spp.), and successful competitors will reap numerous reproductive benefits during adulthood. The objective of this study was to determine whether residency, size, sex, and relatedness affect competition over seedpods in U. ornatrix larvae. In one-on-one trials for access into artificial seedpods, we monitored occupancy and weight change of larvae varying in residency, size, sex, and relatedness. We found that larger larvae have a competitive advantage over smaller larvae. This finding has consequences for the mating system of U. ornatrix in that females, by selecting males based on pheromone levels that are correlated with body size, can rear larger offspring that will have an advantage in competition over seedpods. These data did not support our hypothesis that males would outcompete females, perhaps because the rewards of acquiring a seedpod are substantial for both sexes. Finally, our data show that resident larvae are more likely to maintain control of seedpods over sibling than non-siblings intruders, which suggests that relatedness affects competitive interactions.