More than the normal numbers of fall webworm (FWW) tents have covered the ends of tree branches in some parts of Kentucky during the last few weeks. The sight of silk tents occupied by hairy caterpillars is unnerving to horse owners who remember the eastern tent caterpillar (ETC) Mare Reproductive Loss Syndrome outbreaks from 15 years ago. This raises the question: Do fall webworms pose a threat to pregnant mares or pastured horses?
Foal losses to MRLS resulted from ingestion of hairy ETC by pregnant mares. Enough of the hairs penetrated the digestive tract so that gut bacteria were able to move into the mare’s blood stream. The resulting septicemia moved into the foal’s blood resulting in toxemia and abortion.
There is no research data indicating whether ingestion of FWW caterpillars constitutes a danger to pregnant mares or horses. However, it is reasonable to assume that pregnant mares should not be exposed to large numbers of these caterpillars. FWW caterpillars do not appear to disperse as widely as ETC so FWW are more likely to remain near host trees and pupate on or in the soil.
A small percentage of open mares and other horses of both sexes that were naturally affected by MRLS developed an inflammation of the uterine wall, pericardium, or an eye irritation. Consequently, it is prudent to reduce their exposure, as well.
Fall webworm tents occur in trees along forest margins or gaps. Infestations tend to be around edges of woods, along roadways, and in fence line trees. When practical, remove tents containing live caterpillars. Minimize exposure by moving horses away from large concentrations, when possible.
Birds, spiders, beneficial insects, diseases, and weather are major factors that affect FWW populations.
By Lee Townsend, Extension Entomologist