Managing Eastern tent caterpillars Malacosoma americanum (F) on horse farms to reduce risk of mare reproductive loss syndrome

An equine disease now known as mare reproductive loss syndrome (MRLS) struck the Ohio Valley in 2001-2002 causing thousands of foal abortions and enormous economic loss. Evidence that pregnant mares' exposure to Eastern tent caterpillars Malacosoma americanum (F) induces MRLS created an urgent call for control measures suitable for use on horse farms. We surveyed egg mass distribution and monitored emergence in wild cherry trees, and evaluated reduced-risk treatment strategies including foliage sprays, trunk injections, winter egg mass treatments and barrier sprays to intercept larvae entering pastures. Egg masses were concentrated in the lower canopy, on exposed sides of trees. Larval emergence began in mid-March. Emergence was prolonged (3-4 weeks) in 2002, a typically cool spring, but more synchronized in warmer 2003. Winter treatment of egg masses with bifenthrin or permethrin in a penetrating solvent prevented emergence, but 3% horticultural oil was ineffective for that purpose. Insecticidal soap or oil sprayed directly on neonates gave relatively poor control. Bifenthrin and spinosad were effective as foliage sprays against all instars, their field-weathered residues active for at least 7 days. Bacillus thuringiensis Berliner var kurstaki controlled neonates within 3 days but was less active against late instars, with shorter residual action than bifenthrin or spinosad. Larvae did not avoid insecticide-treated leaves. Trunk microinjection of cherry trees with dicrotophos was effective against all instars, whereas microinjection with milbemectin or abamectin gave poor or inconsistent control. Trunk injection with emamectin benzoate also was effective. Dry permethrin residues controlled late instars crawling in pasture grass for at least 7 days. Factors complicating M americanum management on horse farms are discussed.