No definite cause of mare reproductive loss syndrome (MRLS) has been identified, but a recent discovery that Eastern tent caterpillars (ETC) can cause early fetal losses (and likely contributed to MRLS) has scientists at the University of Kentucky focusing on eliminating the insects from horse farms. On May 2, 2002, during the optimal ETC eradication period (when the larvae are still in trees), Lee Townsend, PhD, extension entomologist at the University of Kentucky (UK) recommended a list of insecticides for horse owners and farm managers to use to keep ETC off their pastures. These recommendations had individuals wondering which insecticides are safe to use, and which should be avoided.

Petra Ann Volmer, DVM, MS, Dipl. ABVT, Dipl. ABT, is an assistant professor of toxicology, and the director of the veterinary toxicology residency program at the University of Illinois. She says that most of the insecticides on UK’s list are from the pyrethrin family. These are some of the safest insecticides, in her opinion, because of their short impact on the environment. Some insecticides included in this family are Bifenthrin, Lambda-cyhalothrin, and Cyfluthrin. These insecticides are used commonly across the country for many different jobs (such as fly repellent and roach control), but are not registered for application to pastures.  If used properly on fence rows or tree bases, they should not be a threat to mares or foals grazing on nearby pastures.

Carbaryl (a carbamate insecticide) is also safe in Volmer’s opinion. “Carbaryl is the material used in flea powder that is applied directly to cats and dogs,” says Volmer. Carbaryl is labeled for application to pastures for insect control.

 “The insecticide family you want to avoid is organophosphorus,” says Volmer. “They tend to be more toxic to mammals than the pyrethrin family.” There were no insecticides recommended on UK’s list from this family