Occasionally, one encounters shrubs or small trees, such as crape myrtles, that contain large numbers of tightly constructed spider webs. Usually these webs are located at the ends of limbs and are about 6 to 8 inches across (see photo). They look a bit like the webs fall webworms build in pecan and persimmon trees but are considerably smaller. Close examination will reveal the web contains large numbers of small spiders and the leaves inside the web have not been eaten, though they may be dead and brown. These are webs of southeastern social cobweb spiders, Anelosimus studiosus. These are tiny spiders, but they can occur in high numbers. Mature adults are only about 3/16 inch long; spiderlings are smaller. Often the plants they are on have a heavy infestation of some insect, such as crape myrtle aphids, which provides a ready source of food. As the name suggests, these are “social” or communal spiders, with females sharing communal webs and cooperating in the capture of prey and even in the feeding of young spiders.
On rare occasions entire trees will be so heavily infested with these webs as to be very unsightly (see photo). If this occurs, the problem will continue through the winter because the spider webs remain on the tree with large numbers of dead, dried leaves and other debris trapped inside. In such cases, control is necessary to improve the appearance of the tree/shrub and to prevent recurrence of the situation next year. Control may involve a two-step approach.
If aphids are present, they can be controlled by applying a soil drench of imidacloprid (Merit 2F) as described in the publication on Crape Myrtle Bark Scale: https://extension.msstate.edu/sites/default/files/publications/publicati...
Be sure to treat aphid-prone trees again the following year.
To control the spiders, it will be necessary to apply a foliar spray, or hire a commercial applicator to spray using a pyrethroid insecticide such as bifenthrin, cyfluthrin, permethrin or zeta-cypermethrin. Two homeowner products that would work for this are Ortho Bug-B-Gon (bifenthrin) and Sevin Insect Control Concentrate (zeta-cypermethrin).
This needs to be applied as a forceful spray that will wet and penetrate the webs. Apply a second application 7 to 10 days later to improve control. Read labels and consider off-target drift potential before treating. Although the webs will remain on the tree and have to “weather off” this should allow the trees to recover and look better next year.
Prepared by Blake Layton, Extension Entomology Specialist
