I have what appears to be bagworms on my juniper. Will they damage by shrub? LH

Bagworms are out in full force this year. I received an email from Dr. Erik Rebek - Extension Entomologist recently and he used the term “bumper crop” in reference to the quantity of bagworms being found in northern and eastern Oklahoma this year. I don’t know about you, but I would rather hear the term “bumper crop” used in reference to delicious peaches from Porter, Oklahoma rather than when talking about bagworms, but here we are.

In Oklahoma, bagworms are partial to eastern red cedar, arborvitae, and other junipers but they will also feed on true cedars, pine, spruce, bald cypress, maple, boxelder, sycamore, willow, black locust, and oak. There are 130 different plant species in various parts to the U.S. that can play host to bagworms. This year they are so plentiful, you can even find bagworms on roses.

Most of us are familiar with the bags the bagworm larvae construct from leaf tissue. These bags can look different depending on the host plant, but however they look, bagworms can be destructive if you have an outbreak of sufficient number.

The bagworm lifecycle is pretty interesting. The eggs for the bagworms causing us trouble right now were actually laid last year. The eggs overwinter in the bags and begin to hatch in late April or early May. Once hatched, the young larvae begin to feed almost immediately.

The first evidence of a bagworm infestation typically occurs when you begin to see the bags. These bags start out about 1/4 of an inch in length. As the larvae inside the bag grow, they increase the size of the bag to make room for this growth. Bags of fully grown larvae are usually 1.5 to 2 inches in length. Once they mature, they produce a kind of silk from their salivary glands to attach the bag to the host plant.

The mature larvae pupate inside of the bag and adult males emerge in late summer to early fall. These adult males are small moths with a black hairy body, clear wings, and a wingspan of about 1 inch. After emerging, these males immediately begin to search for a female.

The females are wingless and do not have any functional legs, eyes, or antennae and continue to look like larvae. The females never leave their bag. Once the male locates a female, they enter the bag. After mating, the female lays between 500 and 1000 eggs inside of her bag. These eggs overwinter in the bag to hatch the next spring and the cycle begins again.

Small concentrations of bagworms are usually not problematic, however larger infestations can completely defoliate some plants. One challenge with this defoliation is that evergreen plants do not produce new foliage every year so recovery from bagworm damage can take several years.

Broadleaf hosts do not typically suffer extreme damage from bagworms, but they can be weakened by infestations causing them to become more susceptible to disease or wood-boring insects.

One thing that makes bagworm control problematic is that most of their lives they are enclosed in their bags and those bags do a very good job of protecting the insect inside. Because of this, if you are going to want to spray your host plant with something to eliminate the bagworms, timing is critical. The best time to apply any chemical treatment is in early June when they are actively feeding. An organic product called Bacillus thuringiensis provides good control of bagworms, but we are past the point where an application of this pesticide would do any good this year. Spinsoad is another organic pesticide that can work, but again, the treatment ship has sailed for this season.

There are several parasitic and predatory wasps that adversely affect bagworm populations as well as a few fungal pathogens. It’s possible this explains the difference in bagworm populations we experience yearly.

No matter the season, you can always pick the bags off by hand and dispose of them. Do not place them in your compost or debris pile as you will have just relocated them rather than gotten rid of them. Granted, not everyone will be fine with hand-picking, but it is a good strategy unless the plant is on the larger side. If the infestation is on a large tree, it’s just not a good idea to get out there on a ladder and start stretching and picking. Instead, you can contact a local arborist to assist with bagworm infestation on larger trees or shrubs.

The bottom line is that smaller bagworm populations are not usually a problem, but it doesn’t take long for them to get out of control. Good luck.

You can get answers to all your gardening questions by calling the Tulsa Master Gardeners Help Line at 918-746-3701, dropping by our Diagnostic Center at 4116 E. 15th Street, or by emailing us at mg@tulsamastergardeners.org.