ISSUE 8 June 22, 2006
YELLOWHEADED SPRUCE SAWFLY UPDATE
The Yellowheaded spruce sawfly is a damaging defoliating insect of spruce plantings in North Dakota. The insect is found throughout the northern half of the state and is most damaging in the northern tier counties and counties of the north central region. Infrequently the insect is found in the southern half of the state however severe damage is rarely observed (Figure 1).
Fig. 1. Yellowheaded spruce sawfly risk map for ND
Generally, small and medium sized spruces growing in full sunlight appear to be more susceptible to the yellowheaded spruce sawfly because female sawflies prefer to lay eggs on spruce growing in open areas. Damage to the tree is caused by the feeding larvae (figure 2) that may consume needles for up to 4 weeks. Larvae stop feeding in mid to late summer, drop to the ground and spin cocoons.
Fig. 2. Yellowheaded spruce sawfly larvae
The damage to spruce trees caused by the sawfly can be substantial. Repeated defoliation causes reduced growth and at times tree mortality. Needles consumed by the sawfly are not re-grown.
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.
On June 13th, the first signs of defoliation were observed on the currents year’s shoots of spruce in the north half of the state. The larvae will continue to feed for several weeks thereby causing additional damage. Control measures must be incorporated within the following weeks to minimize sawfly damage. Handpicking larvae may be sufficient to reduce damage on small isolated trees. Additionally, a strong jet of water will often dislodge feeding larvae.
Chemical controls may need to be incorporated if many trees are infested and the potential for damage is high. Insecticides can effectively reduce populations if applied properly. 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 such as those that are feeding at this time of year. Early larval stages show high survival, whereas older larvae experience substantial mortality. Acephate and carbaryl are labeled for use to control sawflies for homeowners. Always read and follow label instructions!
Yellowheaded spruce sawfly life cycle
Forest Health Specialist
North Dakota Forest Service