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Joseph F. Boggs, Ohio State University Extension, Hamilton County/Southwest District; Curtis E. Young, David J. Shetlar, James A. Chatfield, Erik A. Draper, Pamela J. Bennett, Gary Y. Gao, |
Nest-Making Caterpillars
Populations of eastern tent caterpillar (Malacosoma americanum) were heavy across southern Ohio, southern Indiana, and throughout much of Kentucky. Significant defoliation was observed. The 2001 season was very similar to the 2000 season regarding population distribution and densities (8). However, reports from Kentucky indicated populations were much heavier there, particularly in the Lexington area. High eastern tent caterpillar populations in the "horse region" of Kentucky, coupled with the widespread loss of Thoroughbred foals, produced a tenuous connection between the two events that captured national attention.
The equine problem became known as Mare Reproductive Loss Syndrome (MRLS) and was characterized by extremely high fetal mortality rates during the spring of 2001. More than 5,100 foals of six breeds died in Kentucky's spring epidemic. The economic impact of the foal deaths was estimated to be some $336 million through 2003, according to a study released by the Kentucky governor's office, with most of the impact more than $300 million hitting the Thoroughbred industry around the Bluegrass. The horse industry is Kentucky's leading agriculture cash crop.
But the problem, including loss of foals, was not confined solely to Kentucky. Reports came in of mysterious foal deaths in neighboring states that shared Kentucky's weather patterns. Similar foal deaths were reported in Ohio, including Morgan and Highland Counties in the southern part of Ohio. More foal deaths were reported in eastern Ohio, western Pennsylvania, and Tennessee as well.
Early speculation focused attention on a possible role played by the eastern tent caterpillar. While the caterpillars feed on a wide range of hosts, cherry trees are preferred, and black cherry is very common in central Kentucky. It was speculated that the caterpillars somehow served as a vehicle to carry the cyanogenic compounds contained in the cherry leaves to the horses. Observations that some symptoms appeared to be consistent with cyanide poisoning seemed to support this theory. Consequently, some horse farms implemented a black cherry removal program.
However, research conducted later in the season by the University of Kentucky, Department of Entomology indicated that it was highly unlikely the caterpillars delivered cyanide to the horses. The caterpillars are apparently very good at detoxifying the cherry leaves, and virtually all the cyanide was removed in the caterpillars' foregut. The compound was not found in the midgut, the hindgut, or the frass (excreta). It was also not incorporated into the body of the caterpillars. Although cyanide was present in the foregut, it was calculated that a horse would have to consume more than 2,000 lbs. of caterpillars in order to experience a toxic reaction.
The search for a causal agent of MRLS continues to be pursued, but as with many puzzles encountered in nature, guideposts illuminated by solid scientific evidence remain elusive. The entire story thus far provides a fascinating glimpse into the practical application of the scientific process.
As their name implies, eastern tent caterpillars produce very obvious silk nests at limb and branch forks. The caterpillars are dark with a white stripe down their backs, and they are covered with short, grayish-white hairs. They prefer to feed on trees in the family Rosaceae, particularly those in the genus Prunus such as cherries, although they will occasionally feed on other plants, such as ash, birch, maple, and oaks.
Eggs first hatched in late March in southern Ohio, and by mid-May BYGLers reported that mass migrations had commenced. While it is unusual for these gregarious caterpillars to leave their tents, they may take to the road in search of food after they have totally plundered their host. Consequently, these marauding hordes may suddenly appear on surrounding vegetation causing damage to unusual hosts. But, more often than not, they fail to find usable plant material. Late instars are capable of pupating, but wandering early instars often run out of luck and die before finding food.
Fall webworm (Hyphantria cunea) was virtually a "no show" this season. The same was true for mimosa webworm (Homadaula anisocentra) in most areas of the state, with the exception of northeastern Ohio. From 1995 thru 1998, the number of damaging populations of this insect seemed to be on the rise across the state, with each season worse than the season before (3, 4, 5, 6). However, during the 1999 and 2000 seasons, this web-producing pest of honeylocust failed to appear in significant numbers (7, 8), and that trend continued this past season. Significant populations did occur in northeastern Ohio, with heavy damage observed; however, these were extremely localized and not widespread.
As with the 2000 season, common bagworm (Thyridopteryx ephemeraeformis) populations were again spotty with small pockets of significant defoliation observed (8). This cosmopolitan woody ornamental pest overwinters as eggs inside a dead female's body which is housed in the female's bag constructed during the previous season. In the spring, eggs hatch and first instar larvae either begin feeding on the readily available host that the old female bag is attached to, or they spin threads of silk, catch the wind, and "balloon" to new hosts. The caterpillars surround themselves with bags constructed of silk and pieces of their host. The bags are enlarged as the caterpillars mature.
Once the bagworm larvae complete their development, only the male caterpillars pupate and emerge as winged adult moths. The late instar female caterpillars produce mature sex organs. They then release a sex pheromone that attracts the male moths. Mating occurs inside the female's bag, and after mating, the female's body rapidly fills with 300 to 1,000 fertilized eggs. There is one generation per year.
Eggs hatched in southern Ohio during the last week of May, and late instar caterpillars were still being found in late September in central and southwestern Ohio. Compared with previous seasons, this seemed to be somewhat of an extended tour. For example, in 1997 and 1998, eggs hatched in late May, and male moths were flying in early September (5, 6).
Applications to control this general defoliator should target early instar larvae shortly after egg hatch. Applications of insecticides made to late instar larvae may only stimulate early completion of larval development. This insect does have a number of predators and parasitoids that may play a significant role in reducing population densities. For example, during a Christmas tree growers clinic held this past season in southwestern Ohio, participants observed a significant number of bags with gaping holes ripped in their sides indicating that a hornet, yellowjacket, or paper wasp had extracted the caterpillar to feed to its young. Caterpillars with their bodies filled with a single, grub-like parasitic wasp larva were also found in high numbers.