Laughlin: Bagworms, webworms are hanging around
Bagworms and webworms are both common pests of our landscape plants.
Bagworms are caterpillars that make distinctive spindle-shape “bags” on a variety of trees and shrubs throughout central Oklahoma. They attack both deciduous trees and evergreens, but are especially damaging to juniper, arborvitae, spruce, pine and cedar. Large populations of bagworms can strip plants of their foliage and eventually cause them to die.
Bagworms pass the winter as eggs inside the bag that contained the previous year’s female. In mid- to late May the eggs hatch, and the tiny larvae crawl out from the end of the bag in search of food. Using silk and bits of plant material, they soon construct a small bag around themselves that looks like a tiny, upright ice cream cone.
As the larvae continue to feed and grow, they enlarge the bag. Older larvae strip evergreens of their needles and consume whole leaves of susceptible deciduous species, leaving only the larger veins. The bag is ornamented with bits of whatever type of vegetation they are feeding upon.
If only a few small trees or shrubs are infested with bagworms, you can pick the bags off by hand and dispose of them during fall, winter or early spring before the eggs have hatched. The best time to apply an insecticide is in mid- to late June when the larvae are still small. Young bagworms are harder to see, so look closely for the small, upright bags which have the appearance of tiny ice cream cones constructed of bits of plant material.
For homeowners, the organic insecticide Bacillus thuringiensis (BT) provides good results when sprayed in the area they are feeding. BT is a safe bacterial insecticide that is specific to caterpillar pests and only affects caterpillars that are feeding on the tissue you spray. It is also widely available.
Fall webworms create the “webbing” we often find on pecan trees, but they can feed on nearly 90 different species of plants. Fall webworms overwinter as pupae and emerge as adult moths in late spring. A female moth can then lay up to 500 eggs in masses on the underside of leaves. Larvae hatch in early June and immediately begin to form a silken tent. There also will be a second generation, which can cause a lot of damage in the fall, so controlling this first generation now is helpful.
Fall webworms usually don’t cause any long-term damage to mature tree hosts, but they can temporarily ruin the aesthetic appearance of the tree. Newly planted small trees may be defoliated.
You can assist with their natural control by tearing open the silken bags, which allows predators like paper wasps and birds access to the caterpillars. On small trees you can simply remove the nest, along with the caterpillars, and destroy. A high-pressure water spray can also be used to remove the webs and knock the caterpillars down.
Webworms also are susceptible to the organic BT products mentioned for bagworms. This product can be applied near the webbing area, and the caterpillars eventually will expand their feeding into the treatment zone. A conventional contact insecticide treatment would need sufficient spray pressure to reach and penetrate the webs of these caterpillars. As always, follow the label’s directions for any application of a pesticide product.
Email Julia Laughlin, Oklahoma County Extension horticulture educator, at firstname.lastname@example.org .