ISSUE 9 July 1, 2004
YELLOWHEADED SPRUCE SAWFLY
The yellowheaded spruce sawfly is out on evergreens in shelterbelts and yards. It is a native defoliating insect of spruce in Canada and the Northern United States. Although it feeds on all species of spruce, feeding preference varies geographically.
Eggs are laid on current years foliage during the last week of May through the first week of June. After about 10 days, larvae emerge from these eggs and begin feeding on the current years growth. Initially larvae are 1/8 inch long and have olive green bodies with red heads. Mature larvae may eventually reach a length of ¾ inch. The larvae feed on the current years needles first and then move to older foliage to continue feeding. Larvae stop feeding in mid to late summer, drop to the ground and spin cocoons. The sawfly will spend the winter in these cocoons and emerge the following spring as adults.
There are many parasites and predators that help to keep sawfly populations in check however localized outbreaks may occur periodically.
The damage to spruce trees caused by the sawfly can be substantial. Repeated defoliation causes reduced growth and at times tree mortality. Branches defoliated by the sawfly will not re_grow needles. Generally spruce trees require 5 to 7 years of needle retention for optimal growth and survival. Needle loss from sawfly defoliation reduces the trees ability to convert sunlight into energy. This stress may exacerbate other pest problems or if severe enough, can kill the tree outright.
Early detection and timing are the keys to successful yellowheaded spruce sawfly management. Chemical controls may need to be incorporated if many trees are infested and the potential for damage is high. Unfortunately, most people do not notice sawfly damage until mid to late summer when defoliation has already occurred, the larvae are absent, and insecticides are ineffective. Insecticides are most effective if used to target early larval stages. Early larval stages show high survival, whereas older larvae experience substantial mortality. Targeting the early larval stages in mid June can reduce the population before significant defoliation occurs.
(SOURCE: Michael Kangas, NDSU Extension)
Janet J. Knodel
Area Extension Specialist Crop Protection
North Central Research and Extension Center