Ferguson: Fall webworms have been quite abundant this season

Published 12:50 pm Friday, October 22, 2021

While driving between Tangipahoa and Washington parishes during the past few weeks, I’ve noticed a great deal of webbing in trees along the roadsides. Calls and emails from clients suggest that some of you have noticed these webs, too.

Fall webworms are present every year, but they seem to be especially abundant this year. They feed on the leaves of many plants, with pecans, persimmons, bald cypresses, and sweetgums among the trees on which they’re most commonly observed. Webs formed by fall webworms typically start near the ends of limbs but may progress towards the trunk as the caterpillars continue to feed.

In spite of their name, fall webworms become active in the spring, after overwintering as pupae under leaf debris and in bark crevices. In Louisiana, they have three to five generations (pupa to adult/moth to egg to larva/caterpillar to pupa) between spring and fall.

Fall webworms can negatively impact the health of a plant, since they eat leaves and reduce the plant’s ability to make its own food. However, it’s now late in the growing season, and trees have had a number of months to make food for themselves. Deciduous trees will be dropping their leaves soon, anyway, and fall webworms will enter the non-feeding pupal stage for the winter.

In some of the webs I’ve observed recently, I’ve found living caterpillars. In others, I’ve only seen frass and shed skins left behind by molting caterpillars. Webs are likely to remain on plants for some time, even after caterpillars abandon them.

When fall webworm webs are low enough to reach with loppers or a pole pruner, and only one or a few limbs are affected, one option is to just cut out the webbed limbs. Another option is to break up the webs, so that natural predators like birds can get to the caterpillars more easily.

There are insecticides that will kill fall webworms, but it’s often difficult for people to spray effectively for them. The webs may be too high to reach. Even if they’re not, the pressure provided by home spray equipment may not be adequate to penetrate the webs and reach the caterpillars.

If you find fall webworms on your plants earlier in the growing season in future years and decide to spray an insecticide, it’s still best to break up the webs so that the spray will be more likely to reach the webworms. Quite a few insecticides are effective on caterpillars. To minimize damage to beneficial insects, insecticides with active ingredients like Bacillus thuringiensis subsp. kurstaki and spinosad can be used while fall webworms are small. Other active ingredients with efficacy against caterpillars include acephate, carbaryl, and a number of the pyrethroids (names ending in “-thrin,” plus esfenvalerate). Make sure any insecticide you use is labeled for use on the type of plant on which you plan to use it, and be sure to read and follow label directions.

Let me know if you have questions.

Dr. Mary Helen Ferguson is an Associate Extension Agent with the LSU AgCenter, with horticulture responsibilities in Washington and Tangipahoa Parishes. Contact Mary Helen at mhferguson@agcenter.lsu.edu, 985-839-7855 (Franklinton) or 985-277-1850 (Hammond).