Yellow-headed spruce sawfly
Larvae on June 17th were approximately ½ inch long in Grand Rapids and of an inch in Duluth on June 24th. When larvae finish feeding they drop to the ground and overwinter. In the spring adults lay eggs over a period of a couple weeks. Because of this you will find larvae of different sizes on individual trees. By the time the average larva is ½ to inches long it is likely some of the older larvae have already dropped to the ground to overwinter.
Spraying at this time, when the larvae are still active, is still effective in protecting the remaining foliage. However, because some larvae have already finished feeding and dropped to the ground there will likely be a population of sawflies present on the trees again next year. Spraying to get rid of the population needs to be done earlier.
First generation larvae are present on white pine at this time. They are likely to become more apparent and more abundant when the second generation larvae are active in August.