Gypsy moth larvae are hatching in northeastern Illinois. By April 23 and 24, 30 to 50% of the egg masses had hatched in Addison and Geneva in DuPage County; but on April 27, eggs were just starting to hatch in McHenry County. Newly hatched gypsy moth larvae sit on or near the egg mass for a few days before dispersing into the tree canopy to start feeding on leaves; and at the above dates and locations, the caterpillars were still on and around the egg masses.
A complicating factor in treating for gypsy moth caterpillars at this time of year is that there needs to be enough leaf surface for the insecticide spray to impinge upon. Oaks, one of the preferred hosts of gypsy moth, tend to leaf out late in the spring, so one has to wait for enough leaf expansion to occur to make spraying practical. This typically occurs when bridal wreath spirea, Spiraea x vanhouttei, is in bloom, which will probably occur in 2 to 3 weeks in northern Illinois.
Recommended insecticides include Bacillus thuringiensis ‘kurstaki’ (Dipel, Thuricide), spinosad (Conserve), deflubenzuron (Dimilin), and tebufenozide (Mimic). Btk is a bacteria that produces encapsulated toxins that release only in the guts of caterpillars, making it very low to nontoxic to other insects and other animals. Spinosad is a group of toxins produced by a fungus that is very effective against caterpillars but with a very low toxicity to animals other than insects. Deflubenzuron and tebufenozide are insect-growth regulators, mimicking insect hormones that do not occur in most animals, making them low in toxicity to noninsect animals as well. All of these substances must to be ingested by the gypsy moth caterpillars to be effective, so good leaf coverage is essential.
You can keep up-to-date on gypsy moth in Illinois by visiting our University of Illinois Extension Gypsy Moth Website at http://www.urbanext.uiuc.edu/gypsymoth/default.cfm. Timely information about overall infestations in Illinois and other parts of the United States can be found at http://www.fs.fed.us/ne/morgantown/4557/gmoth/.