PI(s): Bergh, J Christopher (Project Leader)

Abstract: The woolly apple aphid (WAA), Eriosoma lanigerum, is a cosmopolitan pest of apple. In the mid-Atlantic region, WAA can simultaneously infest the roots and aerial portions of apple trees and can have significant adverse effects on the establishment of young trees. The loss of certain broad-spectrum pesticides under the 1996 Food Quality Protection Act has created the potential for the resurgence of some secondary apple pests, including WAA. A specialist parasitic wasp, Aphelinus mali, has been considered the main biocontrol agent of WAA in the US and abroad. We have discovered that two generalist and one specialist species of hover fly are abundant and important predators of woolly apple aphid early in the growing season, before A. mali becomes abundant. Furthermore, larvae of the specialist hover fly, Heringia calcarata, are purported to prey on both root and shoot colonies of WAA. The objectives of this project are to understand the roles of these hover fly species in the suppression of WAA and the influence of biotic and abiotic factors on populations of WAA and its natural enemy guild. Documenting and quantifying the life history and predator/prey association between H. calcarata and WAA are key goals, as well as understanding the intraguild interactions among predators and the parasitoid.

Description:

Biological control of arthropod pests can be an important component of Intregrated Pest Management programs in many agricultural systems. Understanding the trophic relationships between a pest species and its natural enemies and those that exist within each natural enemy guild is key to successful exploitation and enhancement of the effects of predation and parasitism on pest suppression. The woolly apple aphid (WAA) occurs as a pest of apple in most production regions of the world. Although the specialist parasitoid, Aphelinus mali was widely exported from North America in the early and mid-1900's and has established on WAA in many locales, there remain annual problems with WAA management in numerous countries, including parts of the USA.While it has long been believed that predation on WAA early in the growing season is important to suppressing damaging outbreaks later, there remains a dirth of information on guilds of WAA predators and their roles, and this is particularly true for North America.

Using weekly deployments of potted sentinel apple trees supporting arboreal colonies of WAA, we have documented the relative and seasonal abundance of the three most common hover fly predators of WAA in Virginia over three consecutive seasons. Two species are generalist predators that also prey on other aphids, including other aphid pests of apple, while the third species, Heringia calcarata, is a specialist predator of WAA  in the apple ecosystem. We have documented distinct differences among these species in the sculpturing of the egg exochorion and have used those difference to differentiate among the unhatched eggs deposited in colonies on our sentinel trees. Using this sampling approach, we have shown that one of the generalist species, Eupeodes americanus, is abundant early in the growing season (April) but then becomes rather uncommon until later in the season (September), when it shows a second, minor peak. Unhatched eggs of H. calcarata first appear somewhat later (May), although the two species are temporally sympatric for at least several weeks. Unhatched H. calcarata eggs can be found in colonies throughout the growing season, but are most abundant during the first several weeks following their initial appearance. The other generalist predator, Syrphus rectus, has not shown these seasonal trends but has been found in relatively low numbers at various points throughout the sampling period.

Using potted sentinel apple trees infested with WAA in association with predator exclusion cages deployed in an apple orchard, we have shown pronounced effects of predation by hover flies on the suppression of both the number and the size of WAA colonies. Colonies on trees that were fully exposed to natural enemies and those that were partially enclosed within "semi-cages" were brought to near extinction within a period of less than two weeks whereas colonies on trees fully enclosed by cages showed a constant increase in number and many grew considerably in size over the 6-week test period.

Since these three species are sympatric with one another and with Aphelinus mali, elucidating the existance, extent and effects of intraguild predation among the hover fly species between them and A. mali will become a primary focus of this project.