Landscape - More on Fall Webworm

The following is more information on fall webworm, a common pest that is seen in late summer.

This year is another “epic” performance by Fall Webworm (Hyphantria cunea) for best defoliation and webbing by a caterpillar. The characteristic “ghostly webs” are on branch terminals everywhere hickory, pecan, black cherry, but especially mulberry and mountain ash. Over 88 tree species have been noted in the USA as being susceptible. There are two generations per year. The white adults emerged and laid eggs earlier this year after overwintering as a cocoon in soil or debris. It usually takes approximately 6 weeks for full larval development, and the maturity of the second generation is definitely upon us. The larvae are yellow with white hairs when young. Once more mature, their hairs will be longer and come from orange or black spots so larval appearance will vary. These caterpillars have either a black or red head. There are actually two “races” of this pest in our region thus the difference in larval appearance. The caterpillars usually feed together in the same tent, but wander once older. Scouting for and pruning out small webs is a good option. At this point, it may be best to prune out this year's infestations and avoid spraying for it entirely. Damage from fall webworm is cyclical and reaches its peak once every four years or so. Several birds, wasps, yellowjackets, and other parasites will feed on them. Spraying through the large webs has limited effectiveness even with broad spectrum contact pesticides and coverage will be an issue. B.t. is not a good option against mature caterpillars and has had limited effectiveness against fall webworm for us. Plus, you'll still have the webs to deal with anyway. The damage is
mainly aesthetic.

Information from Casey Sclar, IPM Coordinator, Longwood Gardens