| The yellowheaded spruce sawfly, Pikonema alaskensis Rohwer (Hymenoptera: Tenthredinidae), is a destructive defoliator of young spruce trees. Early-instar larvae feed predominantly in the lower crown, while late-instar larvae disperse upwards to complete feeding and development in the upper crown. The role of larval parasitism with respect to this dispersal behaviour was investigated. Both early and late-instar larvae had reduced parasitism in the upper crown as opposed to the lower crown, indicating that this region was enemy-free space. Despite parasitism costs in the lower crown, early-instar larvae had better overall survival in this crown level due to better quality resources. The role of parasitism and resources changed as larvae aged and dispersed within the tree crown, therefore, stressing the need to examine both bottom-up (i.e. resources) and topdown (i.e. natural enemies) factors when exploring their influence on the behaviour of insect herbivores. |
