Rate of defoliation of balsam fir in relation to spruce budworm attack and timing of spray application

Abstract

Studies were conducted in 1976 and 1977 to determine feeding behaviour and progressive damage to balsam fir (Abiesbalsamea (L.) Mill.) throughout the larval feeding period of the spruce budworm (Choristoneurafumiferana Clem.).Four plots had high populations of approximately 30 larvae and four had very high populations of over 100 larvae per 45-cm branch tip at peak L2. In early spring emerging second-instar larvae mined needles of old foliage; few buds were attacked prior to third instar. Defoliation of current year's growth was between 70 and 80% in plots with high populations and close to 100% in those with very high populations. In the high-population plots the 25% level of foliage destruction occurred close to peak fifth instar and the 50% level just before peak sixth instar. In plots with very high populations 25% defoliation occurred close to peak third instar and 50% defoliation close to peak fourth instar. The relationship between defoliation patterns and best timing of insecticide application is discussed.