Bag Worms

Sunday, July 7, 2019 1:08 PM

It seems like every year about this time I get bagworms on my evergreen trees.  Why does that occur and what can I do about it?   Rob W., Broken Arrow

The first evidence of Oklahoma bagworm infestation appears in early June on our arborvitae, juniper, pine, spruce and red cedars. So, June and July are the months to scout out and remove those pesky bagworms that appear on our evergreens. Look for small cocoons that are decorated with organic material from the host tree and are attached to such with silk-like threads. While a mild infestation is mostly a cosmetic issue, a heavy infestation can actually defoliate and kill smaller plants. And, once a plant becomes infected, the bagworm then becomes a persistent problem unless controlled. Thus, breaking the annual cycle is critical for the health of our evergreens.

The Life Cycle

Although the small bags start to appear in June, the bagworm’s life cycle actually begins the previous fall when eggs are laid and overwintered within the bags of one-year-old females. The eggs hatch in April and the young larvae begin to feed and construct their personal summer palaces.

Bagworm caterpillars then feed for about six weeks, enlarging the bag as they grow and withdrawing into it when disturbed. When the larvae are mature, they fasten the bag to a plant stem or branch with a silk-like thread. Pupation occurs in the bag in late summer and, in the fall, the males emerge and start their search for wingless females who are immobilized in their bags. After mating, the females lay hundreds of white eggs and then evacuate the bag and die. The eggs remain protected within the bag until they hatch the following June. Fortunately, these bag decorators only produce one generation per year.

Bagworms are found in most states east of the Rocky Mountains and are common to all areas of Oklahoma. Although bagworms prefer evergreens, they can be found on bald cypress, maple, box elder, sycamore, willow, black locust, and oaks. Fortunately, activity by natural enemies such as wasps, birds and predatory insects help curb bagworm populations, which helps to explain population fluctuations from year to year.

Control Measures

Small infestations can be reduced by simply handpicking the bags anytime of the year. Once picked, be sure to burn or destroy the bags and their viable eggs.

Chemical controls are a more complete approach and are effective if applied when the larvae are small in early June in Oklahoma. Bt (Bacillus thuringiensis) kurstaki is a bacterial insecticide reported to provide good control of bagworms. Also effective are products that contain the active ingredient spinosad, another microbial agent. Be sure to read and follow all label directions.

Insecticides must be ingested by the caterpillars or larvae to achieve kill, so be patient as it will take some time to see results. Repeat application two weeks following initial application may be needed because not all eggs hatch at the same time or there may be wind-spread migration from other host trees.

Although it may be a little too late to go to the full pesticide route this year, you can still hand pick and destroy the bags, and now be armed with the needed information to get ahead of the situation next year.