Yellow-headed spruce sawfly

Taxonomic classification and description
The yellow-headed spruce sawfly (Pikonema alaskensis) is a member of the Hymenoptera order and the Tenthredinidae family. It is recognized as a primary defoliator of spruce trees, often causing severe damage to both nursery-grown seedlings and ornamental landscape trees.

Host plants
The insect primarily attacks trees of the genus Picea. Common targets include white spruce (Picea glauca), Colorado blue spruce (Picea pungens), and Sitka spruce (Picea sitchensis). The larvae show a strong preference for the new growth of the current season.

Biology and life cycle
This species typically completes one generation per year. The insect overwinters as a prepupa within a silken cocoon in the soil or duff layer. In spring, adult sawflies emerge, mate, and females deposit eggs into individual slits made in the needles of the newly developing spruce foliage.

Damage and economic impact
The larvae feed gregariously or individually on the needles. Early instars typically eat only the surface of the needles, leaving them skeletonized and brown. Older larvae consume entire needles. Heavy infestations result in severe defoliation of the tree, which can significantly reduce the tree's vigor and lead to branch dieback or mortality if the tree is repeatedly defoliated.

Control measures
Effective management strategies include the following steps:

• Regular monitoring of new foliage during the spring for signs of larval feeding.
• Manual removal of infested branches when local populations are low.
• Application of biological insecticides containing Bacillus thuringiensis (Bt) for effective control.
• Use of chemical insecticides registered for conifer pests when larvae are small and actively feeding.