Yellow-headed Spruce Sawfly
The following article was written by John Luhman of the Minnesota Department of Agriculture and is an integrated pest management strategy for controlling yellow-headed spruce sawflies on young spruce trees.
Timed incorrectly, spraying insecticides is often ineffective at preventing long term tree damage and loss. This is because spraying is usually begun when most of the feeding damage has already occurred, too late to kill enough of the larvae and just at the time natural enemies (mostly insects) are at their peak. Spraying trees with mature 5th or 6th instar larvae kills many of the parasitic flies and wasps and other predators that are trying to naturally control these instars. This sets the stage for reinfestation the following year, since most of the natural enemies die from the insecticide treatment or die within killed larvae.
More than forty different insect parasites have been reared from yellowhead spruce sawfly larvae. Even small areas will have a half dozen different parasites. The strategy then is to avoid killing the local parasite populations so they can become established and reduce or eliminate the sawfly population. Since most of the parasites lay most of their eggs in late instar larvae, any insecticide application should be for early instars. For small clumps of trees, knocking the larvae off by striking the branch stem can be just as effective as insecticidal treatment. Sawfly larvae then fall prey to ground predators.
The life cycle of the spruce sawfly is well known. Females lay eggs in the new growth (candles), during late May or early June depending on the weather. The newly hatched larvae will be evident the first two weeks in June, so sites should be monitored for them. Since the larvae feed gregariously (large numbers feeding together), spray coverage would only need to be on the candles to kill large numbers of larvae. Look for defoliation and the reddish head and yellow and blackish green stripped body of the larvae. More than fifteen insecticidal products are labelled for use against these sawflies. Remember that Bt is not effective on sawflies. Spraying should cease by the third week of June or when larvae are more than ½ inch long. This is when most of the natural enemies, especially parasitic flies and wasps, are present.