ISSUE 8 July 1, 2010
CHECK SPRUCE TREES IN SHELTERBELTS FOR YELLOWHEADED SPRUCE SAWFLY
The yellowheaded spruce sawfly, Pikonema alaskensis (Hymenoptera: Tenthredinidae) has been observed feeding on spruce in western North Dakota (Morton and McKenzie Counties). It is primarily a pest of shelterbelt and ornamental plantings.
Description/Biology: Adults are reddish-brown in color and about 8 mm long. Sawflies overwinter as pupae in cocoons. In late May to mid-June, adults emerge and mate and females begin to lay eggs. A single egg is deposited at the base of a needle. Eggs hatch in five to10 days. Larvae are about 4 mm long when they first emerge. Mature larvae are almost 2 cm long and dark glossy green with a light lateral stripe and reddish brown head (Fig. 1). Larvae feed for 30 to 40 days consuming new foliage first and then older needles. Mature larvae drop to the ground and spin cocoons for overwintering. There is only one generation per year.
Figure 1. Yellowheaded spruce sawfly larva
(E.B. Walker, VT Dept Forests, Park & Recreation, Bugwood.org)
Figure 2. Spruce tree damaged by yellowheaded
spruce sawfly (J. Knodel)
Damage/Symptoms: Defoliation is caused by larval feeding. Heavily infested trees appear ragged, especially near the top (Fig. 2), and can be completely stripped of foliage. Severe infestations over one to several years can kill trees directly or make trees susceptible to attack by other insects or adverse weather conditions.
Control: Open grown trees that are five to nine years old are more vulnerable to yellowheaded spruce sawfly damage than are older trees or trees in dense stands. Although rodents will feed on pupae, and birds on sawfly larvae and adults, these predators, in addition to various parasites, are not always effective in keeping yellowheaded spruce sawfly populations at acceptable levels. If infestations are light, adequate control may be achieved by simply removing young larvae by hand. When an isolated ornamental tree is infested, spraying young larvae off of the tree with a strong jet of water will often be effective in reducing insect numbers. Yellowheaded spruce sawfly tends to attack the same trees repeatedly; therefore, chemical control often becomes necessary as a sawfly population increases. Acephate, carbaryl, imidacloprid, malathion and several pyrethroids (like tempo) are labeled for use against sawflies. Since most yellowheaded spruce sawflies are believed to overwinter very near the soil surface, removing the duff beneath infested trees may reduce the impact of this insect. Biorational treatments include azadirachtin, horticultural oil, insecticidal soap, pyrethrin and spinosad.