Sept. 14--There are nests of webbing at the ends of some of the branches in my crab apple trees. The trees are mature and have two to three of these nests in each of them. What is this and is there something that I should be doing?
-- Janet Jacobs, Evanston
Your description indicates that there are fall webworms on your trees, which become more noticeable in August and September and can be found throughout the state. This insect has a large host range and can be found on almost any deciduous tree and some shrubs. It is commonly found on crab apple, redbud, black walnut, hickory, pecan, maple and oak trees.
The caterpillars are pale green to yellow in color and are covered with long, white hairs. They may have black spots. The fall webworm caterpillars build large, protective nests that are generally located on the ends of branches. The webs help protect the caterpillars from birds and increase in size as the caterpillars feed. When the leaves inside the web are eaten, the silk webbing is expanded to include more leaves with webs of mature caterpillars reaching 2 to 3 feet in length. The caterpillars will make jerking motions when disturbed.
The loss of leaves from caterpillar feeding at this time of year will likely have little impact on your trees' health as long as their loss does not trigger new bud break. Since the damage in your trees is limited to a few branches, new leaf production is unlikely to occur. If the damage was more extensive earlier in the year and the caterpillars were still feeding, then a more aggressive control measure might need to be considered. Control at this time of year is primarily recommended to reduce aesthetic damage -- thus, fall webworm colonies at the tops of tall trees and in other less obvious locations can be ignored, depending on a gardener's preference.
You can simply tear up the nests if you can reach them. They can also be pruned out, but be careful not to ruin the form of the tree if you are pruning out the nests. There are many insecticides that will effectively control fall webworm if applied correctly and at the proper time. The webbing of the nest is waterproof, which makes it resistant to sprays. You will need enough spray pressure to break into the web and get the insecticide onto leaves and caterpillars within the nest. Check to make sure there are caterpillars present before spraying.
However, you should not need to spray the small number of nests that you have on your crab apples. If you do decide to spray, consider using an organic product such as Bacillus thuringiensis kurstaki (DiPel and Thuricide are two brand names).
Tim Johnson is senior director of horticulture for the Chicago Botanic Garden in Glencoe; ctc-realestate@tribune.com.
