Midseason Insecticide Treatment of Balsam Twig Aphids (Homoptera: Aphididae) and their Aphidophagous Predators in a Wisconsin Christmas Tree Plantation

Abstract

The balsam twig aphid, Mindarus abietinus Koch, is a pest of balsam fir, Abies balsamea (L.) Mill., and Fraser fir, Abies fraseri (Pursh) Poir., Christmas trees. This study evaluated the impact of a growers' application of an insecticide (Phosphamidon-8) upon populations of aphids and their arthropod predators (Coccinellidae larvae and adults, Syrphidae larvae, Chrysopidae and Hemerobiidae larvae) in a plantation in western Wisconsin. Spraying occurred on 27 May 1994 as a result of aphids (fundatrigenia and sexuparae) visibly damaging Christmas trees targeted for December 1994 sales. Aphids and predators were sampled by visual counts of infested shoots before and after insecticide treatment. Percentages of treated aphid-infested shoots significantly decreased from 86% prespray to 3% postspray, whereas percentages of untreated infested shoots increased from 75-88%. Two weeks postspray, percentages of both sprayed and unsprayed infested shoots were 5%. Numbers of aphidophagous predators significantly decreased on sprayed trees and significantly increased on unsprayed trees. Spiders increased on all trees. Assessment of needle damage 9 mo after treatment revealed no significant differences in the mean number of damaged shoots between sprayed and unsprayed trees. However, during the following growing season, treated trees contained significantly less numbers of aphids and predators than did unsprayed trees. This indicates that insecticide treatment of the shoot-feeding stages (fundatrigenia and sexuparae) can reduce numbers of aphids and their predators in the current and subsequent year, but will not necessarily reduce needle damage.