Spruce sawflies
Spruce sawflies are various sawfly species found in North America that feed on spruce. There are multiple species of sawflies known as spruce sawflies, including species in the genera Gilpinia, Pikonema, Pristiphora, and Cephalcia. Each kind of sawfly attacks particular parts of the spruce as larvae during different times of the year.
Yellow-headed spruce sawfly, ''Pikonema alaskensis''
The yellow-headed spruce sawfly, Pikonema alaskensis, is widely known in the northern United States and Canada as a destructive pest of spruce. It attacks white, black, Norway, and Colorado blue spruces. The larvae at first prefer new foliage, but after becoming about half-grown, old needles are included in their diet too. Young plantations become susceptible a few growing seasons after establishment.The insect overwinters underground as a larva in a dark-brown papery cocoon encrusted with soil. In the spring, the cocoon changes into a pupa, from which the adult emerges, mainly in May or June at about the time the spruce bud scales are sloughed off. The spring-emerging females lay their eggs in shallow slits at the base of expanding needles, generally 1 per needle. Hatching takes place in 6 to 14 days, and the larvae feed in groups on the new needles until only short, brown stubs are left, after which the larvae move back on the twig to feed on the older needles until they are full-grown—usually in late July or early August. Larvae drop to the ground and spin overwintering cocoons. A single generation occurs per season.
Larvae are about 20 mm long when fully grown, and have chestnut-brown to reddish-brown heads. The body is yellowish-green above, lighter beneath, with a double row of broad, brown to olive-green stripes along the back, with another green stripe on each side. A small spot occurs near the legs on all but the rearmost section of the body.
In spite of the large numbers of parasites that attack this sawfly, planted spruce can be heavily damaged. Any insecticide would maximize larval mortality when applied about 10 days after bud caps have been shed, but when only few trees are infested, the larvae can be picked off by hand. Other than in young plantations, this sawfly generally causes little or no mortality of trees in spite of its widespread presence.