Control of Bagworms on Trees and Shrubs

Control of Bagworms on Trees and Shrubs

Allen Robinson: Ask a Master Gardener

Sunday, September 2, 2018

Q: I am seeing lots of bagworms on my cedar trees and other trees right now. What can I do to get rid of them? Charlotte S., Tulsa

A: Bagworms are common pests on eastern red cedar, other junipers, arbor vitae and sometimes on bald cypress, elms, pines, willows, maples and others. They are unique in that once they form their protective bag later in the summer, insecticides are not helpful. Treatment should begin soon after the eggs hatch in late spring.

The cycle of worm production begins in the spring when eggs that have overwintered in bags hatch. Newly hatched larvae develop small, upright bags while feeding on the plant. Initially, the bags are less than ¼ inch but, when mature, they can reach up to 2 inches in length. Once mature, the larvae close off the bag and fix it to the tree. In mid-summer, the males emerge from bags, fly around and mate with females who never leave the bags. The females lay eggs in the bag and then die. The cycle begins anew in the following spring.

For smaller trees with small infestations, the easiest treatment is to simply pull the bags off and destroy them. This can be done at any time of the year. Be sure to burn them or place them in a well-sealed bag to destroy the bags and their viable eggs. Trees that have heavy infestations yearly should be treated with an insecticide because large numbers can completely defoliate and kill smaller trees.

Insecticide treatment must be done soon after the larvae hatch in late May or early June. No treatment is considered effective once the bag is closed. Be patient as most insecticides will require repeat applications every seven to 10 days for two to three treatments because not all eggs hatch at the same time or there may be migration (wind dispersal of small larvae during June) from other host trees.

There are two relatively safe organic insecticides. The safest is Bacillus thuringiensis kurstaki, or “Bt,” sold as Thuricide and other brands. The good news about this herbicide is that it is not harmful to people, pets or fish. It is a bacterium that infects the bagworm and causes it to starve. It must be sprayed directly on young larvae.

Another biological insecticide derived from a bacterium is Spinosad, a microbial agent that is sold in several brands including Fertilome Borer, Bagworm, Tent Caterpillar and Leaf Miner Spray. Spinosad has contact and systemic activity on target insects. It, too, has low toxicity and a good environmental profile. Be sure to always read all label directions.

Other nonorganic manufactured insecticides are labeled for bagworms and are effective in controlling young worms. However, these insecticides also kill the parasites and predators that normally keep bagworms under control.

So, while the most viable way to rid your trees of bagworms at this time of year is to pick them by hand and destroy them, consider keeping this information handy so that next year the problem can be dealt with in late spring.

Garden tips

• If your tomatoes are too tall and gangly, now is a good time to prune the top of the plants by as much as ⅓ to ½, depending on the plant. This will stimulate new limb growth and new fruit production after it cools.
• Reseeding fescue is best done from mid-September through mid-October. If you plan on reseeding, begin scouting for good seed now. Purchase a fescue blend of three or more varieties, with or without Kentucky bluegrass. Read the label on the seed bag. A good blend will have 0.01 percent or less of undesirable “other” crop seeds.
• In the fall, strawberry plants build up food reserves and form fruit buds for the next year’s crop. They should be fertilized between mid-August and mid-September with a nitrogen fertilizer, such as ammonium sulfate, at a rate of 1½ pounds per 100-foot row. Apply 1 inch of water if no rain is expected.
• You have all of September to plant cool-season vegetables like spinach, leaf lettuce, mustard and radishes, and until the middle of September to plant rutabagas, Swiss chard, garlic and turnips.
• The last nitrogen fertilizer application of the year on warm-season grasses should be applied no later than Sept. 15.