On May 21, the common bagworm (Thyridopteryx ephemeraeformis) eggs hatched in Wood County, Ohio. Often when you hear the term bagworm you may think about the large web of caterpillars hanging off branches of hardwood trees in late summer. Those insects are actually fall webworms (Hyphantria cunea).

Bagworms are the insects that look like upside down conical or spindle-shaped bags normally found feeding on conifers most notably on Arborvitae (Thuja spp) and Colorado Spruce (Picea pungens). Though in some years they may be found on deciduous trees mainly Norway maples (Acer platanoides).

Did you know the adult bagworm is a moth?

Bagworms are in the Lepidoptera order that is comprised of butterflies and moths. The larvae of the Lepidoptera order are caterpillars. When the eggs hatch of this order and produce larva singular or larvae plural the stages of growth are known as instars. The first instar of the bagworm larvae is very small with their bags measuring around 1/8” in length. Unfortunately, with conifers bagworm larvae cause damage to trees by eating the needles. A significant loss of needles results in growth loss, increased susceptibility to attack by other insects and pathogens, and sometimes tree mortality.

Egg hatch of bagworms is based on growing degree days or Phenology. Bagworms hatch when we reach 630 growing degree days. For reference points, the Washington Hawthorne tree, and the American Holly bush blooms at 642 growing degree days, while the Northern Catalpa tree is in full bloom at 816 growing degree days. Memorial Day we were at 770 growing degree days.

As soon as the eggs hatch, young larvae appear, and they immediately spin a small 1/8-inch-long cocoon-like bag to which are attached pieces of leaves from the plants they feed upon. They also release a fine spider-like thread that wind currents pick up. This allows the bagworms to travel from where they were hatched to new trees to feed upon. This type of movement among insects is known as ballooning. Once the young larvae land on a tree they prefer, they begin feeding. Sometimes it is the same tree they launched from. The caterpillars crawl part way out of the bags to feed. If disturbed, they retreat safely inside, and it is almost impossible to pull them out.

Bagworms mature in late August or early September. At this time, the bags are about 2 inches long and can no longer be killed by pesticides. The larvae then attach the bags firmly to branches or other objects and change into the adult stage. The wingless female moth never leaves the bag and is fertilized by the winged male moth. The eggs are laid in the bag where they pass the winter. Each female bag can produce over 1,000 bagworms eggs. Both male and female moths die during the winter leaving the eggs to hatch the following year.

Bagworms are difficult to control because they are often unnoticed until mature. Though there are a few known parasites and predators, they are often not adequate for control. Cultural control is achieved by hand picking the bags. This works well for smaller trees; however, for larger trees this is impractical.

Excellent biological control is achieved by spraying with a bacterial spray Bacillus thuringiensis var. kurstaki (Btk) (with trade names Dipel, Thuricide, etc.). BTK is considered an organic pesticide as it only targets young larvae of the Lepidoptera order of insects. The other benefit of using BTK products is they do not kill bio-allies such as predators and parasitoids that help provide natural control of bagworm populations.

However, Btk products have two limitations. The active ingredient must be consumed to kill the larvae of the Lepidoptera order of insects, and products have relatively short residual activity. Thus, timing is critical; products should not be applied prior to egg hatch. Even with proper timing, two applications may be required to cover the extended egg hatch. The other limitation is larvae are much less susceptible to BTK control once bags surpass 2/3” in length. Applications should be made two weeks after the first application.

Once the bags surpass 2/3” in length, control is achieved by switching from Btk products to using stomach insecticide sprays. Stomach insecticides are very useful for control of bagworms. Generally, pyrethroid insecticides are selected (these generally have names that end in “–thrin”) for larval control as they provide quick knock-down of small to large larvae. Once we reach mid -late August, pesticide sprays are not effective.

The phenology tool for plants and insects was developed by researchers with Ohio State University Agriculture Research and Development Centers (OARDC). This tool is available online: https://weather.cfaes.osu.edu/gdd/default.asp.