Elm sawfly larvae are being found at this time in Illinois, particularly in the northern areas of the state. These yellowish to white larvae are thickbodied and approach 1-1/2 inches long when fully grown. They have yellowish heads and a black line running down the middle of their backs with a row of black spots on each side. They are usually curled up on a leaf when discovered. These insects feed on elm and willow, and we are finding most of them either on willow or walking across the ground towards willow trees. They can cause considerable defoliation on their hosts.
Although these insects look somewhat like caterpillars, the adults are wasplike. Not true caterpillars, they have more than five pairs of prolegs, false legs, on the abdomen. B.t.k. will not control them. Instead, use carbaryl (Sevin), a synthetic pyrethroid, or another labeled insecticide. Control may not be necessary because leaf loss this late in the growing season is unlikely to harm the health of the tree.