Connecticut Garden Journal: Fall Webworms
Most insect populations in our gardens and forests are on the decline with the fall weather. But one insect is in its glory. The fall webworm is creating white webbed nests on branches of a variety of deciduous trees such as crabapples, plums, elm, oak, and ash. They're obvious this time of year from their large webs on the branch ends. Unlike the tent caterpillars that form webs in branch crotches in spring, web worms cover the ends of branches in fall and feed safely from predators inside the web mesh.
Although they look horrendous, the fall webworm yellow caterpillars only feed on the old leaves that are beginning to die anyway. They don't attack the leaf buds for next year. The caterpillars will feed on the leaves, often defoliating the branch then drop to the ground to pupate and overwinter in the soil.
Because they are attacking old leaves, usually you don't have to do anything to control them. Fall webworms have more than 50 predators and parasites that feed on them, so Nature usually brings them back into balance. However, if they're defoliating whole trees, especially a young tree, then it might be smart to stop their feeding.
The simplest way to protect your trees is to use a pole and puncture the nest in many places, breaking it up. This allows predators a chance to enter and attack. For larger infestations you can spray Bacillus thuriengensis or Bt organic pesticide into the nest. Once the caterpillars eat Bt, they will die in a few days. Spray on a cloudy day to be most effective.