Question: I noticed these caterpillars eating leaves on one of our River Birch trees. Can you please identify the caterpillar and let me know if they will damage the tree? If necessary what pesticide should I use to get rid of the pest? Thanks in advance for your help. Montgomery County, Texas.
Answer: This is probably the Elm or Dusky Birch Sawfly. The Elm Sawfly adults strongly resemble a wasp. The larvae strongly resemble caterpillars, but they have six or more pairs of pro-legs on their abdomen whereas caterpillars have five or fewer.
Sawflies are related to bees and wasps and look much like them in their adult stage but without the stinger. Their larvae resemble caterpillars with green bodies, black dots and black heads. The dusky birch sawfly invades the river birch. The larvae feed heavily on the leaves for four to five weeks in the spring, and then spin cocoons where they spend the rest of the year. Adults emerge the following spring and lay eggs on the tree. While they can eat many leaves in their feeding period, river birches quickly recover, and treatment is unnecessary unless the tree is unhealthy or young and suffers complete defoliation. In this case, pick the larvae off and drop them in soapy water or apply an insecticidal soap intended for leaf eaters.
Control varies depending on numbers. If there are only a few I’d suggest either doing nothing or picking them off if possible. If they are causing defoliation then you will want to spray with an organic insecticide like spinosad or a conventional insecticides such as malathion. Bt (Bacillus thuringiensis), which is an effective natural control for true caterpillars, is ineffective on sawfly larvae.
Additional information: http://bugguide.net/node/view/37878