Dusky birch sawfly has been reported at The Morton Arboretum in northeastern Illinois. These insects attack not only birch trees, but also alders and willows. They can be present from now into September.
The larvae are caterpillarlike, yellowish insects with dull, dark orange heads and two rows of large black dots running down its back. On each side, there is another row of smaller black dots. Its mature size is about 1 inch long. Although caterpillarlike in appearance, it has at least seven pairs of prolegs rather than the five or fewer that caterpillars have. More important, they are related to the wasps, which makes them immune to Bacillus thuringiensis kurstaki.
If their numbers warrant control, use carbaryl (Sevin), diazinon, chlorpyrifos (Dursban), or a synthetic pyrethroid labeled for the host.