Fall webworm webs are a common sight in pecan trees throughout Mississippi. Their distinctive webs are easy to spot and become quite unsightly on heavily infested trees. Although these caterpillars feed on many other hardwood trees, pecan and persimmon, are their preferred species here. Sweetgum is another favorite and they occasionally occur in fruit trees. Caterpillars feed inside a communal web, which protects them from birds as well as insect predators and spiders. As caterpillars grow, they expand the web to encompass more leaves so they can continue to feed without leaving the protection of the web. Although there are several generations per year, populations are highest in the fall.
Fall webworms are present every year, but populations are heavier some years than others, and based on early season reports and observations, this could be one of those heavy years. During heavy outbreak years they can completely defoliate large pecan trees in home landscapes. Such severe defoliation adversely affects nut fill on the current year’s pecan crop and reduces yield potential for the following year. Commercial pecan producers can easily control webworms by spraying appropriate insecticides with high volume air blast sprayers, but it is seldom safe or practical to apply foliar sprays to large trees in home and urban settings. Damaging infestations are also more likely to occur on other species during outbreak years. Fortunately, although the webs are unsightly, mature hardwood trees can survive heavy, or even complete, defoliation without suffering long-term adverse effects, other than to nut production.
Control: Doing nothing is the most common, and usually the most practical, approach to dealing with fall webworms on large landscape trees. One mechanical control method that is somewhat helpful is to use a hook, such as a bent clothes hanger, fastened to a long pole to tear the webs apart, thus exposing the caterpillars to mortality by predation.
Trees that are still small enough to be safely and properly sprayed with some type of hand-held sprayer, or a “tree-and-shrub” hose-end sprayer can be sprayed with spinosad (Fertilome, Bonide, Monterey, and Greenlight sell products containing spinosad), taking care to get good coverage of the web and to penetrate the web with a forceful spray. Pyrethroid insecticides such as bifenthrin, permethrin, cyfluthrin, and zeta-cypermethrin also work well on webworms, and provide faster kill, but it is still important for the spray to penetrate the web. Commercial applicators may use Acelepryn (chlorantraniliprole), which is highly effective against caterpillars, but has low mammalian toxicity.
See pages 20-21 of MSU-ES Publication 2369, Insect Pests of Ornamental Plants in the Home Landscape for more information. See pages 39-41, for more information on specific insecticides.
Also see Bug's Eye View No. 30 of 2016 for a past article that contains additional details about fall webworms.
Blake Layton, Extension Entomology Specialist, Mississippi State University Extension Service.
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