Fully grown webworms are pale green caterpillars with long white hairs growing from black and yellow warts all along their bodies. Adult moths vary from all white to white with black spots. As soon as larvae hatch they spin a web of silk over the foliage where they feed. As they grow, they enlarge the web to enclose more foliage. Once mature, they drop to the soil to pupate. These caterpillars can defoliate trees but they don't often kill them.
Eastern Tent Caterpillar
The eastern tent caterpillar is black with a white stripe along its back and rows of blue dots along its sides. The entire body is sparsely covered with hairs. As soon as caterpillars hatch, they spin silvery, conical tents of silk at branch crotches. The tents offer protection from the elements and from predators and as the caterpillars grow, their tents enlarge. Eastern tent caterpillars feed in colonies and can defoliate trees by early summer. As they move around the tree, they leave a trail of silk behind them. During large infestations, entire trees can be covered in silk from tents and from wandering caterpillars.
Bagworms also spin a protective shelter out of silk but theirs are somewhat different from the eastern tent caterpillar and webworms. Bagworms include bits of organic matter along with silk in the protective covering they spin on trees. Bagworms tend to feed on conifers and occasionally the bags they make are mistaken for pine cones. Older larvae are capable of defoliating conifers and deciduous trees. Successive infestations over many years can kill the host tree.
Web spinning caterpillars are easily controlled by mechanical methods. Find and remove egg masses in the spring to prevent infestations. Destroying webs with a long pole or hook can expose caterpillars to predators. Picking the bags of bagworms off trees can control them as well. Sprays containing the bacterium Bacillus thuringiensis var. kurstaki are effective and safe for people, animals and beneficial insects. If you feel insecticides are warranted, spray them in the spring when caterpillars are still small.