By Ed Lentz, OSU Extension
The continuous rain this spring may cause us to forget that insects are still active. Often these insects may be controlled if insecticides are applied at the correct time. One of these insects that has reached that critical time for control are bagworms.
Bagworms can be a serious problem in town and on the farm. Bagworms can take out 20 foot tall trees in rural windbreaks, large evergreens in yards, and smaller shrubs around homes and businesses. .
Bagworms began to hatch from their protective cocoons several weeks ago. A few bagworms do little harm. However, many bagworms on a shrub or tree can cause excessive defoliation. A severe infestation may kill the plant within one or two seasons.
Bagworms do the most damage on arbor vitae and cedars, but will attack pines, junipers, spruce and at least 130 other trees and shrubs. They may not harm the deciduous trees, but they spread from these trees to more susceptible evergreens.
Larvae will begin feeding and start to build a camouflage bag with plant parts within a few weeks after hatching. They will continue to feed and eventually build a bag that is one to one and half inches long. Any dried and gray bags seen at this time will from last year. However, upon close examination, larvae can be seen with small new bags
Most of the emerging larvae will feed on same tree that contained their overwintering home. Others will form silk threads and allow the wind to carry them to adjacent trees. This is the most common way that bagworms spread from tree to tree in a windbreak planting. Rain events may have diminished the movement from tree to tree.
The most effective control of bagworms is to apply insecticides about two weeks after the first bagworms begin to hatch. This insures that all of the eggs have hatched from overwintering bags on the tree and the insects are in the crawler stage.
The Hancock County area has reached that two week point and spraying should begin on infected trees and shrubs. Spraying insecticide is an effective control until the larvae have made bags about ¾ inch in length, which generally occurs in late July.
Most foliar applied insecticides should provide effective bagworm control especially when applied to small larvae. One may want to consider the biological insecticide Bacillus thuringiensis var. kurstaki (Bt). Bt products are more environmentally friendly since they are selective for larvae of many moths, such as bagworms, without harming beneficial insects.
However, Bt products have short residual activity and may require more than one application for control. Also, complete vegetative coverage is important for Bt products since the worm has to actually ingest the insecticide while feeding to be effective.
Bt products would work well at this time. However, if spraying is delayed until mid-July, one may need to switch to more traditional insecticides. The non-Bt products generally are more effective since the product only has to come in contact with the larvae. Whatever product is selected make sure it is labeled for bagworms and the tree or shrub.
Control will become more difficult once the larvae stop feeding and attach their protected mobile home to the tree. Hand removal becomes the only effective method of elimination at this time.
Bagworms generally attach their protective home to a stem around mid-August and then pupate inside. About a month later male moths will emerge and mate with females in the bags.
Females never leave the bag. After mating a female will lay 300 to 1000 eggs in the bag, die, and form a mummified body around the egg mass for extra winter protection.
Eggs will hatch the following spring to start the next generation. Tiny emerging larvae (crawlers) will start to emerge late May and early June depending upon air temperature and accumulating heat units.
Bagworms have become more a problem in recent years for our area. It was thought that numbers had increased from milder winters and warmer springs. However populations have continued to increase even after severe winter conditions.
If not controlled, bagworms can eventually kill a row of large trees in windbreaks, evergreen borders and valuable landscape plants. For more information on bagworms, visit the following websites: http://ohioline.osu.edu/factsheet/HYG-2149-10 and https://extension.entm.purdue.edu/publications/E-27/E-27.html