Evergreen trees may have creepy crawlers

Evergreen trees that had bagworms last fall are now at risk of 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, called crawlers. Crawlers 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 one to one and a half inches long. If you are seeing these bags, then assume you have crawlers 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. The 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. The 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/2152. Any larger gray bags seen at this time are the abandoned homes left after egg hatch. Most of the emerging larvae will feed on the 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-three weeks after the first bagworms begin to hatch. This ensures that all 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 one 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 300-1,000 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. For more information on bagworms, visit the following websites: http://ohioline.osu.edu/factsheet/HYG-2149-10 Lentz is agriculture and natural resources educator with Ohio State Extension — Hancock County.