Antipredator defense as a limited resource: unequal predation risk in broods of an insect with maternal care

The allocation of parental investment is a potential source of conflict within broods whenever offspring are able obtain differential access to the parental resource. Unlike the provisioning of food, parental antipredator behavior is usually considered a resource that benefits all offspring simultaneously. In the thornbug treehopper (Umbonia crassicornis), offspring form aggregations in exposed positions on host-plant stems. They are subject to intense predation, and maternal defense is their primary means of protection. I examined the distribution of risk within these offspring groups, using natural variation in the outcome of more than 500 predation attempts (324 recorded on videotape) by vespid wasps (Pseudopolybia compressa) on 18 U. crassicornis aggregations. I found three influences on an individual offspring’s risk of predation. The first was the presence of a defending female: as expected, offspring were much more likely to survive contact with a wasp if the female was present than if the female had disappeared. The second influence was position relative to other offspring: when wasps were successful in removing an individual, they almost always removed it from the edge of the group. The third influence was distance from the female: the closer an offspring was to the female at the time it was contacted by a wasp, the higher its likelihood of survival. The distribution of risk is determined largely by the behavior of defending females and the prey-searching behavior of wasps. The nature of risk within these aggregations sets the stage for two forms of sibling rivalry: selfish herd behavior and competition for access to maternal defense. It also raises the question of how a parent should allocate defense among offspring when it is unable to defend them all simultaneously.