At this time of year, bagworms are small, two inch long, brown bags covered with dead brown needles and tightly attached to twigs. They hang down and are cocoon-like. Unless you look closely for bagworms, you may think your evergreen just has brown needles in it.
Bagworms are most often found on spruce, Juniper, Arborvitae, and pine trees and shrubs. The overwintering bags contain as many as 300 to 1000 eggs each. Removing them now can help reduce their summer populations.
Once the eggs hatch in early June, large bagworm populations can completely defoliate and kill evergreens during summer. Light infestations slow the growth of evergreens and lower their windbreak value and aesthetic appearance.
If you find bagworms, remove and destroy them from now to June. Crushing is one method that can be used to destroy the eggs. If bags containing larvae are discarded on the ground, the larvae may return to host plants after hatching.
Insect Lifecycle
Bagworm eggs hatch in late May and early June. At this time, inspect trees again for bagworms. Young bagworms are as small as one-eighth inch long and can be difficult to see. However, mid to late June is the best time to apply insecticides for bagworm control.
After hatching, the larvae emerge from a hole at the base of the bag and spin down a strand of silk. The tiny insect is often ballooned by the wind to nearby branches or plants.
Once a host is found, larvae begin to form a new bag around their body. The larva is a brown or tan caterpillar with black markings. Larvae remain in their protective bag, sticking their head out to feed.
Bagworm larvae feed up until late August. The mature larvae then attach their bag to a branch with a strong band of silk and begin to pupate. Adult males emerge in September. They are small, furry gray moths with clear wings. The adult female does not have wings and never leaves the bag.
Chemical Control
Feeding by mature caterpillars slows in August before pupation into adults, so chemical control in late summer and fall is often not effective. Insecticide control needs to be aimed at young larvae in mid to late June to be effective and to reduce damage.
Bacillus thuringiensis is available at nurseries and garden centers as Dipel or Thuricide. Other insecticides currently labeled for bagworm control include acephate, carbaryl, cyfluthrin, malathion and permethrin. Affected plants must be thoroughly covered with the insecticide in June so the insects ingest it as they are feeding. Be sure to read and follow all label directions.
Do not apply insecticides to evergreens at this time of year to control bagworms. The overwintering bags only contain eggs and insecticides will not have any affect on the eggs. Hand-picking the bags and destroying the eggs is the only control method to use now.
Bagworms may also feed on shade and ornamental trees, fruit trees, ornamental shrubs and perennial flowers, however, severe attacks are unusual. Since deciduous plants grow new leaves each year, the defoliation caused by bagworm feeding on otherwise healthy plants is usually not serious. The growth of small or newly planted trees, however, could be slowed by leaf feeding.
Source: Kelly Feehan, University of Nebraska–Lincoln
