Time to control bagworms

Evergreen trees that had bagworms last fall are now at risk for severe feeding this summer. Eggs in the bags hatched several weeks ago and their larvae are now feeding on the foliage. Each bag may release between 500 and 1000 larvae. Larvae may be hard to see, but last year's hanging bags are easily detected. They are gray to beige in color, conical in shape, and about 1 to 1.5 inches long. If you are seeing these bags, then assume you have larvae feeding on the tree. 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 1 to 1.5 inches long. Larvae are still small at this time but will vary in size depending on the hatch date. Close examination will be required to see them. They can be spotted because the bags will look like small brown caps (https://bygl.osu.edu/index.php/node/1795). Any larger gray bags seen at this time are the abandoned homes left after eggs hatch. 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. The most effective control of bagworms is to apply insecticides about two to three weeks after the first bagworms begin to hatch. This ensures that all of the eggs have hatched from overwintering bags on the tree and the insects are in the crawler stage. Now is the time that spraying should begin on infected trees and shrubs in our area. Spraying insecticide is an effective control until the larvae have made bags about 1 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 (Btk) — sold as Dipel, Thuricide and other trade names. Btk products are more environmentally friendly since they are selective for larvae of many moths, such as bagworms, without harming beneficial insects. However, Btk products have two limitations: the insecticide must be ingested to work; and it has a short residual activity, which may require more than one application for control. Btk products should work well at this time. However, once bagworms reach three-quarters of an inch, about mid-July, one should 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. Once attached, hand removal becomes the only effective method of control. 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 in late May and early June depending upon air temperature and accumulating heat units. Bagworm feeding will be more severe on trees and shrubs under stress, such as extended dry weather. The sooner an insecticide is applied, the less chance that bagworms can cause damage. If not controlled, bagworms can eventually kill a row of large trees in windbreaks, evergreen borders and valuable landscape plants. An example of their damage can be seen on the evergreens along the east side of the lane entering Litter Landing. For more information on bagworms, visit http://ohioline.osu.edu/factsheet/HYG-2149-10 Lentz is Extension educator for agriculture and natural resources for Ohio State University Extension-Hancock County.