Trees are blooming, and so are the bagworms

The Northern Catalpa tree is beginning to bloom with its large white flowers. When people ask me when the best time is to treat for Bagworms (Thyridopteryx ephemeraeformis), my normal answer is when the Northern Catalpa tree begins to bloom. The next question is, what is a Northern Catalpa tree?

The Northern Catalpa tree is one of Ohio’s native trees, withlarge, attractive heart shaped leaves, showy white flowers, and decorative hanging pods in the winter.

Then I quickley add that the egg hatch of bagworms is based on growing degree days or Phenology. The development of both plants and insects is temperature dependent, plants can accurately track the environmental factors that determine when insects are active. For this reason, plant phenology can be used to predict insect emergence. To track these activities, we look at growing degree days.

Bagworms hatch when we reach 630 growing degree days. On Memorial Day we were at 530 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 begins to bloom at 675 growing degree days. The week after Memorial Day we had several days in the nineties degree Fahrenheit. This accelerated the growing degree days and a week later we are at 679 growing degree days.

As soon as the eggs hatch, young worms or caterpillars 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 caterpillars 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.

The favored plant in our area are conifers, most notably Arborvitae and Colorado Spruce. Unfortunately, with conifers defoliating, insects such as bagworms cause damage to trees by eating leaves or needles, and by removing the photosynthetic tissue critical for plant maintenance and growth. A significant loss of leaves or needles results in growth loss, increased susceptibility to attack by other insects and disease-causing pathogens, and sometimes tree mortality.

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 worms 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.

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 in urban habitats. 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 (worms) of the Lepidoptera order of Insects. Lepidoptera is an order of insects that includes butterflies and moths, and the Bagworm is considered a Moth. 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 caterpillars, 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 caterpillars are much less susceptible to BTK control once bags surpass 2/3” in length. Applications should be made two weeks after the first application. Phenology comes into play again since the best time to apply Btk products is at 816 growing degree days or when the Northern Catalpa tree is in Full Bloom.

Once the bags surpass 2/3 inches in length, control is achieved by switching from Btk products to using stomach insecticide sprays. Stomach insecticides are very useful for control of bagworms. Remember that the plant foliage is to be thoroughly covered because the larvae are protected from contact by being in the silk bag. 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. This tool is available online: