Now is the time for eggs of the European pine sawfly, Neodiprion sertifer, to hatch into young larvae (not caterpillars) that are 1/4 inch long, olive green, with a distinct black head. The older larvae are over 1 inch long, with green stripes. European pine sawfly larvae are gregarious, tending to feed in large groups. When disturbed, the larvae arch back on their hind legs, almost forming an S shape. This behavior is a defensive response to avoid predation. The larvae feed on needles of a wide range of pines; however, they are particularly fond of Scotch, red, and mugo pine. Larvae strip the needles of mature foliage, leaving only a central core, which is white and then turns brown. These damaged needles eventually fall off. Larvae typically are done feeding by the time needles emerge from the candelabra. As a result, emerging needles will not be damaged. Sawflies are not a major concern, as there is minimal threat of branch or tree death due to their feeding. However, the loss of second- and third-year needles may be a concern, as this injury can ruin the aesthetic appearance of landscape and Christmas trees. Later in the spring, the larvae fall to the ground and pupate in brown, leathery cocoons near the base of trees. The adults, which resemble wasps, emerge in the fall and deposit eggs in the needles before winter. Females create yellow scars in the needles with their ovipositor when laying eggs. In Illinois, there is one generation per year.

European pine sawfly infestations can be controlled by hand removal (if feasible) or by washing larvae off plants with a hard stream of water. If necessary, a number of insecticides may be applied to infested trees, including acephate (Orthene), azadirachtin (Azatin/Ornazin), carbaryl (Sevin), or spinosad (Conserve).

Although European pine sawfly larvae resemble caterpillars (Order: Lepidoptera), they are actually larvae of insects related to wasps (Order: Hymenoptera). So what does this mean? It means that the bacterial insecticide Bacillus thuringiensis var. kurstaki (Dipel and Thuricide) is not effective in controlling European pine sawfly.