Gypsy moth control measures are on hold in northeastern Illinois due to the cold weather. News from The Morton Arboretum is that the larvae are hatched but are just sitting on the egg masses waiting for warmer weather. Because the primary insecticide is Bacillus thuringiensis kurstaki, the caterpillars have to ingest it to be controlled. If they aren’t eating, application will not be very useful.

At least in one way, the cool weather should aid in gypsy moth control. Btk is most effective on smaller larvae, so application early in the season is best. However, gypsy moth hatches at about the time of budbreak on oak, one of the insect’s primary food plants. In most years, there is a dilemma between waiting long enough for oak leaves to expand to the point that there is enough leaf surface area to impinge the insecticide versus the gypsy moth caterpillars getting larger than is ideal for a high level of control.

Gypsy moth caterpillars do not develop at temperatures much below 50°F and develop slowly at temperatures in the 50s and 60s. However, oak leaves will continue expanding at temperatures above the 30s. The temperatures experienced over the last couple of weeks have kept the larvae small and very vulnerable to insecticides but have allowed the tree foliage to expand. Aerial application to the tops of the trees where gypsy moth larvae tend to feed should be more efficient due to the larger leaf surface area.