Bagworms

Sunday, September 2, 2018 8:58 AM

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

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 very helpful.  Treatment actually 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 in a well-sealed bag in order 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 ten 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.

Another biological insecticide derived from a bacterium is Spinosad, a microbial agent which is sold in several brands including Fertilome Borer, Bagworm, Tent Caterpillar and Leaf Miner Spray.  Spinosad has both 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 non-organic manufactured insecticides are labelled 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.