It doesn’t take long to collect a ½ pound of bagworms if one knows where to look. Bagworms are caterpillars that construct bags on trees and shrubs throughout central Oklahoma. They can be found on deciduous trees and evergreens but are especially damaging to juniper, arborvitae, spruce and cedar. Bagworms can strip plants of their foliage and eventually cause them to die. Infestations often go unnoticed because people mistake the bags for part of the plant, as the worm makes its bag from the plant it is consuming.
Bagworms are the caterpillar of a moth. Only the males develop into a flying moth. The adult female is grublike and remains inside the bag until just before she dies. Bagworms spend winter as eggs inside the bag that contained the previous year's female. In mid- to late-May, the eggs hatch, and the tiny larvae crawl out from the end of the bag in search of food. Using silk plant material, they build a small bag around their hind part. As the larvae grow, they enlarge the bag and can withdraw into it when disturbed. Older larvae strip evergreens of their needles and consume whole leaves of susceptible deciduous species, leaving only the larger veins.
By early fall, the bags reach their maximum size of 1 to 2 inches. At this time the larvae hang their bags (pointing downward) from twigs, and transform into the pupa or resting stage before becoming an adult.
Adults emerge in early fall. Males are black, furry, clear-winged moths with about a 1-inch wingspan. They fly in search of females that remain inside bags. The females produce a pheromone, that attracts males. The creamy white females lack wings and legs and appear to be no more than grubs. The male flies to the female bag, inserts his abdomen in the hole in the bottom of the bag and mates with the female. After the fertilized female has laid several hundred eggs inside her old pupal case in the bag, she drops from the bag and dies. The eggs stay in the bag until the following May when the cycle begins again. There is one generation per year.
Bagworms move around in two ways. Young larvae may spin strands of silk and be carried long distances by wind. Larger larvae go short distances by crawling.
If only a few small trees or shrubs are infested, one can pull bags off by hand and burn or place in a tightly sealed bag and put in trash. This is most effective in fall, winter or early spring before the eggs hatch.
When many small bagworms are on evergreens, an insecticide may be needed to prevent serious damage. The best time to apply an insecticide is while the larvae are very small. In Oklahoma, this is usually in June.
Carefully inspect susceptible landscape plants, especially evergreens, for last year's bags. Young bagworms are harder to see; look closely for the small, upright bags that look like tiny pine cones. Preventive treatment is a good idea for plants heavily infested with bagworms the previous year.
For homeowners, Bacillus thuringiensis (BT) provide satisfactory results but are only effective against younger larvae. If large bagworms are present, a conventional insecticide probably will provide better results. Foliage should be thoroughly wetted with the insecticide spray. Timing of treatment is critical; once worms are sealed inside bags, insecticides are ineffective. ALWAYS FOLLOW DIRECTIONS ON CONTAINER.