Illinois' Got a Brand-new Bagworm Parasite

The goal of a biological control program is to control a pest while having a minimal effect on nontarget species. Past biological programs based on this principle have proven to be successful and safe alternatives to conventional control practices. There is a growing concern in both scientific and public forums, however, about the impact of introduced biological control agents on nontarget hosts and native natural enemies.

An insect pest against which many natural enemies have been released is the gypsy moth, Lymantria dispar. Accidentally introduced into the eastern United States from Europe in 1869, the caterpillar stage of the gypsy moth can cause damage to 500 species of trees and shrubs, most notably oaks and aspen. The parasitic wasp Pimpla disparis was first introduced for control of gypsy moth in the 1970s, and continues to be released in some northeastern states. Female P. disparis lay their eggs into the pupae of moths. The larval stage of the parasites kills the developing moth and adult wasps eventually emerge from the pupal case.

In hindsight, it is not clear why P. disparis was considered a good biological control agent suitable for release. For instance, a parasite being considered for biological control should not have a very broad host range, but P. disparis is a generalist and attacks members from at least 13 moth families. Also, parasitism rates of gypsy moth by P. disparis are frequently minimal, sometimes as low as 1-2%, which further argues against the release of this species.

Researchers Jodie Ellis and Lawrence Hanks from the entomology department at UIUC discovered that P. disparis must have steadily expanded its geographical range well beyond areas where the gypsy moth has been detected. Clearly, P. disparis is attacking and surviving in nontarget hosts. The effects of P. disparis on nontarget moth populations have not been documented and little is known about the life history of this introduced parasite in Illinois. Although the gypsy moth is not established in central Illinois, P. disparis is present and is frequently recovered from the evergreen bagworm, Thyridopteryx ephemeraeformis(Ellis and Hanks, submitted for publication). The evergreen bagworm is an important native pest that feeds on foliage of arborvitae and juniper, as well as a variety of other woody plant species. In Champaign County (IL) at least three wasp species are known as parasites of bagworms (Ellis and Hanks, submitted for publication). All three species are members of the Ichneumonidae wasp family. Two of the species, Itoplectis conquisitor and Gambrus ultimus, are native to the area. The most abundant parasite, however, is the introduced P. disparis.

The parasites attacking bagworms go through as many as five generations per year. However, the evergreen bagworm is suitable as a host only when it pupates in autumn. The parasites must therefore utilize alternative hosts at other times of the year. Potential alternate hosts of P. disparis may include such introduced pests as the European corn borer and the codling moth in apples. This behavior would actually be deemed a beneficial side effect of the introduction of P. disparis. However, if P. disparis also attacks native moths that are not considered pests, the secondary effects of this introduction may not be particularly desirable. In addition, we do not know the impact of the exotic P. disparis on the established native parasites in central Illinois.

Researchers from the Department of Entomology at UIUC (L. Hanks) and the Illinois Natural History Survey (M. Alleyne and R. N. Wiedenmann) have joined forces to study the parasitoid-bagworm complex. A retrospective examination of the impact of P. disparis on the bagworm-natural enemy complex in central Illinois plans to investigate the behavioral and physiological traits that allow competition between an introduced parasite and its native counterparts. The ability of a parasite species to find and utilize alternative hosts determines the abundance of a particular parasite species and its ability to compete with other parasites for the same host. It may be that P. disparis is more common in the fall when bagworm pupae are available because it outcompetes native parasites earlier in the season for other potential hosts, or it may be better at utilizing overwintering hosts. Our study may provide valuable information that can help improve the safety and efficacy of biological control programs by minimizing negative effects on nontarget species and native natural enemies.

Marianne Alleyne, Center for Economic Entomology

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