(News article for June 19, 2020)
Around this time of the year, people sometimes start to notice webbing near the ends of branches on plants like pecans, persimmons, crape myrtles, bald cypresses, and even blueberries.
In spite of their name, fall webworms become active in the spring after overwintering in the soil or under leaf debris as pupae. In Louisiana, they have three to five generations (pupa to adult/moth to egg to larva/caterpillar to pupa) before returning to the ground in the fall.
While they’re not likely to kill a tree, fall webworms can negatively impact the health of the plant, since they eat leaves and reduce the plant’s ability to make its own food. In ornamental trees, they affect the appearance.
People often want to know what type of insecticide to spray for fall webworms. There are insecticides that will kill them, but in a home landscape situation, it’s often difficult for people to spray effectively for them. The webs may be too high in the tree for people 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.
When fall webworm webs are low enough to reach with loppers or a pole pruner, one option is to just cut out the webbed limbs. If you’re concerned about leaving a gap in the tree, you can just break up the web instead, so that natural predators like birds can get to the caterpillars more easily.
If you do 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 and spinosad can be used while fall webworms are small. 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.
Fall webworms are not the only caterpillars that make webs in trees. While fall webworms make webs near the ends of limbs, eastern tent caterpillars create tents near the junctions of branches and trunks.
Eastern tent caterpillars tend to affect trees in the rose family, like wild cherries, apples, crabapples, and hawthorns. They also affect maples.
Unlike fall webworms, eastern tent caterpillars only go through one generation each year. By this time of year, the caterpillars have most likely left the tents, so spraying insecticides on remaining webs would be useless.
Eastern tent caterpillars overwinter as eggs. These are laid in masses on small limbs and covered with a varnish-like substance. During the winter, these egg masses can be pruned out. If tents are observed in March, they can be removed with a stick, or an insecticide can be applied while the caterpillars are still small.
The final web-makers I’ll mention are small insects called barklice. These feed on organic materials on the surfaces of tree trunks and do not harm plants. Some barklice create webs that remain close to the surfaces of trunks and branches but can extend quite a distance within the tree. While these are not harmful to the tree, if you find the appearance objectionable, you can spray water to knock off the webbing.
Let me know if you have questions.
Contact Mary Helen Ferguson.
Fall webworms at the ends of pecan tree limbs. (Photo: G. Keith Douce, University of Georgia, Bugwood.org)
Eastern tent caterpillar webs are often found at branch-trunk junctions. (Photo: Robert L. Anderson, USDA Forest Service, Bugwood.org)
Eastern tent caterpillar egg mass. (Photo: Robert L. Anderson, USDA Forest Service, Bugwood.org)
Bark lice webbing spread out over the surfaces of limbs. (Photo: Chazz Hesselein, Alabama Cooperative Extension System, Bugwood.org)