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 sourceof conflict within broods whenever offspring are able obtaindifferential access to the parental resource. Unlike the provisioningof food, parental antipredator behavior is usually considereda resource that benefits all offspring simultaneously. In thethornbug treehopper (Umbonia crassicornis), offspring formaggregations in exposed positions on host-plant stems. Theyare subject to intense predation, and maternal defense is theirprimary means of protection. I examined the distribution ofrisk within these offspring groups, using natural variationin the outcome of more than 500 predation attempts (324 recordedon videotape) by vespid wasps (Pseudopolybia compressa) on18 U. crassicornis aggregations. I found three influences onan individual offspring's risk of predation. The first wasthe presence of a defending female: as expected, offspringwere much more likely to survive contact with a wasp if thefemale was present than if the female had disappeared. Thesecond influence was position relative to other offspring:when wasps were successful in removing an individual, they almostalways removed it from the edge of the group. The third influencewas distance from the female: the closer an offspring was tothe female at the time it was contacted by a wasp, the higherits likelihood of survival. The distribution of risk is determinedlargely by the behavior of defending females and the prey-searchingbehavior of wasps. The nature of risk within these aggregationssets the stage for two forms of sibling rivalry: selfish herdbehavior and competition for access to maternal defense. Italso raises the question of how a parent should allocate defenseamong offspring when it is unable to defend them all simultaneously.