The yellowheaded spruce sawfly (YHSS), Pikonema alaskensis (Rohwer), (Hymenoptera: Tenthredinidae), defoliates spruce, Picea sp., throughout the Northern United States and Canada. YHSS defoliation can result in substantial growth reduction and tree mortality. Young, open-grown trees, 3 to 18 feet in height and 5 to 9 years old, are more vulnerable to YHSS damage than are under story trees, older trees, or trees in dense stands. Young plantations and naturally regenerated stands of spruce often sustain YHSS defoliation, particularly in the Great Lakes region. Many Christmas trees, nursery stock, roadside and windbreak trees, and ornamental spruce are also damaged. Susceptibility to YHSS drops sharply once trees reach 10 to 12 years of age and stands reach the stage of crown closure.

Many aspects of YHSS biology have been extensively studied and related to the occurrence, or degree of defoliation sustained during outbreaks. Large-scale outbreaks in spruce plantations, especially in Minnesota in the 1970's, stimulated increased research efforts on YHSS. However, much of this information is scattered in unpublished theses, reports, and other difficult to locate sources. Recent damage to plantations in southwestern Michigan and to roadside trees throughout the Lake States has renewed regional interest in YHSS (McCullough 1994, Minnesota Department of Natural Resources 1994).

Our objectives here are to review and summarize the literature pertaining to YHSS, identify research gaps, and use existing infor mation to develop long-term silvicultural management strategies for YHSS and perhaps similar defoliators. This research paper is divided into five major parts:

1. YHSS biology, distribution, hosts, and the signs and symptoms of damage;
2. Ecology and dynamics of YHSS populations;
3. Prevention and control tactics;
4. Survey procedures to assess YHSS populations and defoliation; and
5. Guidelines for selecting appropriate management strategies.