Hort Q&A: Treating bag worms

Carla Smith, horticulture educator
Pottawatomie County Extension Service
Bag worms

Bagworms were all over my trees last year. When do I need to begin to treat for them?

Many pests go in cycles of activity. Bagworms have been busy little critters and they may not give us a break anytime soon. We usually notice their houses or bags about mid to late summer, when they are large. Start to scout for them now, especially if you had a lot last year. Treating early is very helpful, so start keeping a eye out now. Here is a little more information.

Bagworms can be a real nuisance on many plants. In Oklahoma, the most common hosts are eastern redcedar, other junipers, and arborvitae. Other hosts sometimes damaged include pines, spruce, bald cypress, maple, boxelder, sycamore, willow, black locust, oaks, and roses. The bagworm has been recorded on 128 different plant species in various parts of the United States.

Symptoms: Bagworm larvae damage their hosts by feeding on the foliage. Heavy infestations can completely defoliate small plants. Defoliation usually kills hosts such as redcedar and other junipers. Broadleaf hosts are not killed but are weakened and become more susceptible to borers and diseases.

Life Cycle: The overwintered eggs (in the year-old female bags) begin to hatch in late April or early May and the young larvae begin to feed and construct bags immediately. The first evidence of an infestation is normally a small bag, about 1/4-inch long, standing almost on end. As larvae grow, silk and fragments of the host plant foliage are added to the bag until it reaches 1 1/2 or 2 inches long. When larvae are mature, they fasten the bag to a plant stem with silk. Pupation occurs in the bag in August and males emerge in late August and September. They engage in a mating flight in search of the wingless females still inside their bags. After mating the female lays several hundred white eggs inside her old pupal case, drops from the bag, and dies. There is one generation per year.

Description: Adult males are small, clear winged moths with a black, hairy body and a wingspread of about 1 inch. Adult females are wingless, have no functional legs, eyes, or antennae, and are almost maggot-like in appearance. The female’s body is soft, yellowish white, and practically naked except for a circle of woolly hairs at the posterior end of the abdomen. Mature larvae have a dark brown abdomen, and the head and thorax are white, spotted with black. They are about 1 inch long. Both larvae and adult females are found in silken bags on the host plants.

Cultural control: Infestations can be reduced by handpicking bags (and overwintering eggs within bags) during fall, winter, or spring before eggs hatch. Eggs remain viable within bags so be sure to destroy bags upon removal by crushing, burning them, or discarding in the trash (bagged or they will crawl out). When larvae become active, bagworms can still be removed by hand if the numbers are small and the affected host plants are small enough to reach the canopy. Again, take care to destroy the bags once they are removed.

Biological control: There are several naturally occurring parasitic wasps and predatory insects that attack bagworms. The activity of these natural enemies apparently explains the fluctuation in bagworm populations observed from year to year. 

Chemical control: Chemical controls are most effective if applied early when larvae are small. In Oklahoma, it is normally a good practice to make applications of insecticide by early June. Bacillus thuringiensis var. kurstaki, a bacterial insecticide, is reported to provide good control of bagworms. Some labels will clearly say BT for short.  Also effective are products that contain the active ingredient Spinosad, another microbial agent. These insecticides must be ingested by the caterpillars to achieve kill, so be patient as it will take some time to see results. Repeat applications may be needed later in the summer to keep susceptible plants free of bagworms. This is not due to the occurrence of multiple generations. Rather, not all eggs will hatch at the same time in some years and there may be migration of larvae between host plants. In most years, treatment in early June will catch most of the emerging larvae and provide good, season‐long control. The larger, older larvae can be controlled with products containing acephate (Orthene), carbaryl (Sevin), bifenthrin, cyfluthrin, and lambda‐cyhalothrin.

Oklahoma State University, as an equal opportunity employer, complies with all applicable federal and state laws regarding non-discrimination and affirmative action.  Oklahoma State University is committed to a policy of equal opportunity for all individuals and does not discriminate based on race, religion, age, sex, color, national origin, marital status, sexual orientation, gender identity/expression, disability, or veteran status with regard to employment, educational programs and activities, and/or admissions. For more information, visit https:///eeo.okstate.edu