Fall webworm is a caterpillar, the immature stage of a non-descript white moth. The adult moths emerge from coccoons in early spring and lay their eggs on the undersides of leaves of preferred host plants. As soon as the young larvae hatch, they begin spinning a loose silk web that soon covers the tips of the branches on which they feed. Inside the webbing, dozens of hairy caterpillars will be found feeding on the leaves inside. Fall webworm caterpillars are variable in color but have a double row of black dots down their backs and grow to about one inch in length.
If you are sharp-eyed enough to catch fall webworm nests before they spread, you may be able to prune or knock the infestation out of the tree. Place a garbage bag under the web and use a rake to pull off the webbing and knock down the caterpillars. If you cannot reach the web, or there are too many to remove by hand, insecticide sprays can eliminate the infestation. Low impact pesticides for tree-feeding caterpillars include insecticide soap, horticultural oil, Bacillus thuringiensis, or spinosad insecticide sprays. Pyrethroid insecticides will also provide fast control of most caterpillars.
Unless infestations cover a tree, they are usually not that damaging to tree health. An otherwise healthy tree will withstand up to 40% defoliation during the summer months, and will often re-leaf after being stripped by caterpillars. So if you don’t treat in time, don’t worry. Chances are your tree will survive, although you may not be happy with its “webby” appearance.
Other common, summer-feeding tree caterpillars include the eastern tent caterpillar (makes webs in branch crotches and lives in the eastern half of Texas) and the genista caterpillar (commonly found on Texas mountain laurel). These caterpillars can be controlled in much the same way as the fall webworm.