This is the time of year when the eggs of European pine sawfly, Neodiprion sertifer, hatch into larvae (NOT CATERPILLARS) that are 1/4 inch long, olive green, with a distinct black head capsule. Older larvae are at least 1 inch long, with green stripes. The larvae of European pine sawfly are gregarious—tending to feed in large groups. When disturbed, the larvae en masse arch back on their hind legs, almost forming an “S” shape. This behavior is a defensive response to avoid predation—primarily by birds. The larvae feed on the needles of a wide range of pines; however, they tend to prefer Scotch, red, and mugo pine. Larvae strip the needles of mature foliage, leaving only a central core, which appears white and then turns brown. Any damaged needles eventually fall off. The larvae have normally completed feeding by the time needles emerge from the candelabra. As a result, newly emerging needles are not fed upon. In general, European pine sawfly is not a major concern because there is minimal threat of branch or tree death due to feeding—unless the tree is very young (1 year old or younger) and a large population of larvae is feeding. Additionally, 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 inside brown, leathery cocoons around the base of trees. The adults resemble wasps, and emerge in the fall; the females deposit eggs in the needles prior to winter. Females create yellowish scars or wounds in the needles with their ovipositor when laying eggs. In Illinois, there is one generation per year.
A very quick (and environmentally friendly) way to deal with an infestation of European pine sawflies is either by hand removal (if feasible) or by washing the larvae off plants with a hard stream of water (this doesn’t include the use of a fire hose). If warranted, a number of insecticides may be applied to infested trees, including acephate (Orthene), azadirachtin (Ornazin/Azatin), carbaryl (Sevin), or spinosad (Conserve).
Although European pine sawfly larvae resemble caterpillars (Order: Lepidoptera), they are actually larvae of insects closely related to wasps (Order: Hymenoptera). So what does this mean? Well, it means that the bacterial-based insecticide Bacillus thuringiensis var. kurstaki (Dipel or Thuricide), which is recommended for control of young caterpillars, does not control European pine sawfly.