Bagworms are a type of moth larvae that build and live in a cone-shaped, bag-like nest. In small numbers, they are easily controlled and do little damage, but large infestations can destroy entire trees by devouring all the leaves. Unfortunately, most cases of bagworm are not discovered until the infestation is severe. This makes removal more difficult and time consuming. In areas where bagworms are common, it helps to check plants regularly for signs of their presence, which include brown bags that appear similar to small pine cones.
Remove bag cocoons from small trees during late fall, winter and early spring. The bags can be removed by plucking them by hand from the tree. During these months, the cocoons contain eggs that will hatch caterpillars. If the eggs can be destroyed, the life cycle is disrupted, and the trees will not be damaged in the spring.
Destroy the cocoons by sealing them in a plastic bag or burning them.
Spray infected trees with an appropriate insecticide in late spring or early summer. Insecticides are most effective against bagworms when applied while the caterpillars are young or newly hatched. Insecticides known to be effective against bagworm caterpillars are acephate, spinosad, bifenthrin and permethrin.
Treat late summer infestations with repeated applications of insecticide. Adult caterpillars have strong resistance to pesticide, so regular treatments are necessary to keep them under control. Spraying insecticide on plants infested with mature bagworms is unlikely to cure the problem, but may reduce it enough to allow full control the following winter and spring months.
Attract natural bagworm predators to help control the population. While predators alone will not eliminate bagworms, they can help keep the numbers down to manageable levels. The most common predators are sparrows and finches. Attract these birds with feeders and fruit- and seed-bearing plants, and by providing sheltering plants such as shrubs and hedges.