June 26, 2024

LSU AgCenter's Weekly Message

What Insects Make Webs in Trees?

Around this time of year, people sometimes notice webs near the ends of branches on plants like pecans, persimmons, crape myrtles, bald cypresses, and even blueberries. At the Hammond Research Station, I’m seeing them on buttonbush, which is a native plant that tends to grow in wet areas.

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, 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 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 subspecies 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).

Since webworms are often hard to reach, people sometimes ask about insecticides that can be applied to the soil around the bases of trees. There are insecticides with the active ingredient imidacloprid that are applied in this way, because this chemical can move from roots to shoots. However, imidacloprid is generally not effective against caterpillars and so is not recommended for managing fall webworms.

Make sure any insecticide you use is labeled for use on the type of plant on which you plan to use it, and 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.

Let me know if you have questions.

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

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