BYGLers observed numerous second generation fall webworm (Hyphantria cunea) nests on a wide variety of trees during their OSU ENLT Team retreat held just east of Columbus late last week.

Fall webworm caterpillars feed as skeletonizers on leaves enveloped by their silk; they only feed on leaves within their nest. As caterpillars grow in size, they enlarge their nests by casting silk over more leaves to accommodate their expanding appetites. First generation nests are seldom as numerous as second generation nests, and second generation nests are usually much larger because female moths often lay their eggs on or near the nests from which they developed. Thus, second generation caterpillars expand the nests once occupied by first generation caterpillars. Second generation nests typically reach their maximum size in the fall, which accounts for the common name.

"Red-headed race" webworm caterpillars have red to reddish-orange head capsules and two rows of reddish-orange tubercles running the length of their light to dark tan bodies. Adult moths usually emerge from overwintering cocoons 2-3 weeks after the black-headed race adults have emerged. Red-headed webworms remain together throughout their development to produce truly spectacular multilayered nests enveloping dozens of leaves at the ends of branches. Red-headed caterpillars were the dominate race observed by BYGLers last week.

BYGLers reported spotting nests of second generation mimosa webworms (Homadaula anisocentra) on honeylocusts in central Ohio. The general consensus among BYGLers is that populations are very spotty this season. Despite their common name, mimosa webworms are most often found on honeylocusts in Ohio. The caterpillars feed gregariously as skeletonizers within webs spun over the foliage. Attention is usually drawn to an infestation by clusters of orangish-brown "torched" leaves and leaflets that are tightly encased in webbing.

There are two to three overlapping generations per season in Ohio and female moths often lay eggs on nests from which they developed. Consequently, the nests are expanded by each new crop of caterpillars. Eventually, the nests become so dense that insecticides will fail to penetrate to kill the caterpillars. This means that effective management strategies should focus on targeting the current first generation caterpillars.

Mimosa webworms on honeylocust are generally considered an aesthetic and nuisance pest problem. The nests make trees unsightly and caterpillars will occasionally drop from infested trees to visit backyard gardeners and grillers. There is seldom enough leaf damage in a single season to cause long-term weakening of trees; however, defoliation over multiple successive seasons could present a challenge to tree health.

Standard insecticides labeled for use on honeylocusts are effective in controlling mimosa webworms. However, the numerous predators and parasitoids that naturally deplete the caterpillars can be preserved by using products based on the naturally occurring bacterium Bacillus thuringiensis (Bt). Two applications of Bt products spaced 7-10 days apart and targeting the first generation caterpillars is recommended. Nests on small trees may be removed by hand. Preventing first generation nests will reduce the attraction of trees to second and third generation females.

Although it was reported on earlier in BYGL that the bagworm (Thyridopteryx ephemeraeformis) larvae have been active and feeding for a couple of months, it is now time to remind readers that the bagworm has reached the phase in its development when it is devouring plants rapidly. In many cases, damage has reached levels where permanent damage has been done to the most susceptible plants. Arborvitae, junipers and blue spruce have been damaged to the point of no return. Curtis Young in the NW Ohio area reported that he is able to spot infested arborvitae, junipers and blue spruce from his car while driving down the highway at 50-60 mph. Portions of or the entire plants are thinning and/or turning brown and approaching the point of not being able to recover from the damage. Bagworms may not have a lot of time left in their feeding stage, however, there is still plenty of time for them to produce greater damage than what has already been done. Thus, get out there and check those thinning, browning plants before it is entirely too late. Some of the products that can be used to stop the devastation by bagworm include formulations of acephate, bifenthrin, carbaryl, cyfluthrin, lambda-cyhalothrin, malathion, permethrin, pyrethrins and piperonyl butoxide, and spinosad. Some products are only labeled for professional use and others only for nurseries and Christmas tree plantations. Read all insecticide labels completely and carefully before using the product