Gypsy moth traps are set out to determine where gypsy moth is located in the state. In most of the state, these are triangular green or reddish orange cardboard traps about 10 inches long and about 4 inches per side. In areas where gypsy moths are more numerous, such as Lake, DuPage, Cook, McHenry, and Will counties in northeastern Illinois, the milk carton trap may be used. This trap is the general size and shape of a half-gallon milk carton and typically green. Its larger size allows the trapping of more moths. Both traps have a synthetic pheromone similar to the pheromone, or scent, that the female gypsy moth releases to attract the male for mating. Males can detect either pheromone from a considerable distance and follow the pheromone upwind.

These traps are set a mile apart on a grid system in most areas of the state where the Illinois Department of Agriculture in cooperation with the USDA APHIS is trying to detect this insect. In areas where the gypsy moth is known to occur, traps are commonly placed closer together to determine the size and shape of the infested area, as well as obtain an idea of the size of the infestation. This information is useful in determining the type and extent of control operations for next year.

The traps are government property and should not be tampered with. If one must be moved due to painting, tree removal, or other activities, contact information is on the trap. Agencies putting out these traps have right of trespass under federal and state laws. The traps will be removed in July or August.