What is oak slugworm?
Oak slugworms are the larvae of a sawfly. The adult sawflies have black bodies about 7-8mm long and two pairs of blackish transparent wings, they emerge during spring.
Eggs are laid on the foliage and they hatch into larvae that are pale green with a slimy slug-like appearance. Their bodies are translucent and the gut contents can often be seen as a dark green line inside the body. The larvae grow to a length of about 12mm and are somewhat club-shaped with the head end being swollen. When fully fed the larvae go into the soil to pupate.
There are two generations of larvae a year between May and August. The second generation in mid to late summer can be the most damaging. In some years there is a third generation of caterpillars in the autumn. Larvae that complete their feeding late in the year overwinter in cocoons in the soil and adults emerge in spring the following year.