Nitrogen Fertilization of Hemlock Increases Susceptibility to Hemlock Woolly Adelgid

Abstract

Fertilizing eastern hemlock (Tsuga canadensis) with nitrogen stimulated population growth of the hemlock woolly adelgid (Adelges tsugae) in a forest plantation in Connecticut. Percent survival of nymphs and the number of eggs produced per adult of this adelgid were more than twice as high on fertilized hemlocks than on unfertilized ones. Fertilized hemlocks had five times more adelgids, had inferior color, and produced 25% less new growth than unfertilized trees after a single adelgid generation. These trends did not differ between hemlocks which had been fertilized 6 months prior to infestation by A. tsugae and those which were fertilized at the same time that trees were infested. Therefore nitrogen fertilization of hemlock neither increased host resistance to the adelgid nor repressed adelgid population growth following establishment. These results may be generally applicable to piercing and sucking insects which feed on trees and shrubs.