Image 1. These two races of Fall Webworms may occur in the same area. The redheaded race has an orange head and orange tubercle pairs. The black-headed race has a black head with black tubercles.
Battle of the Bugs: Fall Webworms by George Giltner
Fall webworms, Hyphantria cunea, are already causing problem defoliation problems in as many as 100 shrubs and trees in Louisiana. They usually first appear in late April and are not a problem until late fall when they are known for the large unsightly webs. This year they have an early start.
They overwinter in pupae that are found in cracks of tree bark or on the ground. Adults emerge as temps rise, mate, then female lay eggs in a mass of up to 600 eggs on a lower leaf surface. The entire life cycle can be completed within 50 days. 4 generations/year occur in our area.
Fall webworms are easily differentiated from tent caterpillars by their size, behavior, and web structure. Mature fall webworms (1 inch) are ½ the size of tent caterpillars. Fall webworm behavior is to twitch when threatened and they remain inside the web. Fall webworms spin webs at the terminal branches of their host trees, whereas tent caterpillars form tent in branch crotches. Fall webworms are hairy caterpillars with distinct paired dark spots on each segment of the back.
Internet control recommendations vary from Homer Simpson style, long pole, burning torches to some nasty chemical treatments. Since these offer Darwin Award opportunities, I will only recommend safer and natural means of control.
1) Damage the web – this will allow more access to 80+ species of parasites and predators. Just poke a stick or hose around their defensive web for entry by yellow jackets and paper nest wasps, birds, and predatory stink bugs. Parasitic flies and wasps are most important, therefore do not use contact chemical insecticides within the area on infection. Try to endure wasps until August when they change from insect predators to nectar/sugar feeders.
2) Apply Bacillus thuringiensis (Bt) or Spinosad – Applications are more effective on smaller feeding caterpillars. Spray next to the leaves not covered with webbing. UV protectants used with Bt offer more protection to the bacteria against UV solar light.
3) Trim out smaller infections of fall webworms.
4) As a last resort, Carbaryl sprays can be used to merely wet the web and cover nearby foliage. Caterpillars will contact the insecticide by walking on the web or by eating new leaves. There is no need to blast the web out of the tree!
5) Do nothing. Trees may look bad, but they will recover even if defoliated completely. Strong storms may be a physical factor that open webs for natural control.