A helicopter sprays gypsy moth caterpillars with Bacillus thuringiensis , an insecticide that affects only the gypsy moth caterpillars and other species of butterflies. The spraying will last five to six weeks and encompass 30,000 acres. Tuesday's spraying will cover about 2,000 acres in Davis County. The gypsy moth caterpillars have few natural enemies and devour hundreds of species of plants.