Found throughout the eastern United States wherever wild blueberries grow, the cranberry fruitworm, Acrobasis vaccinii (Riley), affects cultivated blueberries that are poorly maintained. This pest overwinters as a fully grown larva in the litter near the soil surface under the bushes. The small, brown, adult moths emerge when the berries of early varieties begin to form, and they begin inserting eggs along the rim of the calyx cup. After hatching from the egg, the larva enters the berry. It eventually webs several berries together with silk, feeding inside as many as four. One generation hatches each year. The cranberry fruitworm infestation is characterized by masses of brown frass (excrement) and silk.
Cranberry fruitworm infestations can be difficult to detect early. Look for a pin-sized entry hole near the stem of any small, shrunken berries that have turned blue, and then open adjacent berries to find the larva. The distinctive frass and webbing do not appear until later in the larva's lifetime when it begins to move between berries. One method of control for this pest (and for cherry fruitworm) is to pick and destroy infested berry clusters showing evidence of webbing. Repeated disking to eliminate weeds and trash also helps in its control. Pesticides used for plum curculio also usually result in control of cranberry fruitworm.