A Scattershot Approach to Host Location: Uncovering the Unique Life History of the Trigonalid Hyperparasitoid Orthogonalys pulchella (Cresson)

Abstract

American Entomologist • Summer 2009 Trigonalid wasps are a remarkable group of hyperparasitoids with a curious strategy for locating their hosts. While most parasitoids and hyperparasitoids lay their eggs directly on or in their host, trigonalids lay their eggs on foliage, where these eggs are incidentally ingested by herbivorous insect hosts. Other parasitoids have adopted this “scattershot” approach to host location, but their larvae consume their herbivorous host after hatching in the herbivore’s digestive tract. Trigonalids, however, require a third party; larval development requires that the host herbivore simultaneously host a (primary) parasitoid larva of another species. Although the framework for this unusual life history has been described previously, the identities of the herbivorous and parasitoid hosts of trigonalids have remained elusive. Here we present the first host records for the temperate trigonalid hyperparasitoid Orthogonalys pulchella and discuss the evolution of this unusual life history strategy. Trigonalid wasps are hyperparasitoids that are intimately associated with three other organisms over the course of their life cycle: host plant, host caterpillar, and host primary-parasitoid larva. Female trigonalids lay their numerous eggs on foliage (host plant) almost at random in what we call a “scattershot” approach. These eggs are incidentally consumed by herbivorous caterpillars (host caterpillar) as they feed on the host plants. For the trigonalid larva to survive, the host caterpillar has to be parasitized by another parasitoid species (host primary-parasitoid); the trigonalid larva then completes its development by feeding on the host primary-parasitoid (Fig. 1). Hyperparasitoids have not been well studied (Hawkins 1994, Brodeur 2000) and the life histories of trigonalids in particular are almost completely unknown (Carmean and Kimsey 1998). For most trigonalids, it is a mystery as to which hosts (plant, herbivore, and primary-parasitoid) they use in nature. In this article, we present the results of natural history fieldwork in which we were able to elucidate, for the first time, what host plants, host caterpillars, and host primaryparasitoid the trigonalid Orthogonalys pulchella uses to complete its development.