How well a fly spray kills certain insects can depend on the species of the insect and how long the insecticide has been in place, according to a new study by Greek and British researchers. The insect mortality rate can also vary from one part of the horse’s body to another.

Hairs extracted from horses treated with cypermethrin (found in many fly products available on the market) were found to kill biting midges and two varieties of mosquitoes at different rates according to body location and time since treatment. Hairs were removed from the back, belly, and lower legs on the day of treatment and at one-week intervals up to five weeks (without retreatment), said Elias Papadopoulos, DVM, MSc, PhD, Dipl. EVPC, assistant professor at the Faculty of Veterinary Medicine in Thessaloniki, and primary author of the study.

Researchers used a sponge to apply insecticide to the study horses’ backs. The midges continued to die from exposure to hair taken from the back even five weeks after cypermethrin treatment, at rates as high as 60%. However, efficacy was significantly lower and shorter-lived with hair from the belly and legs, Papadopoulos said. The results were similar for the yellow fever mosquitoes (Aedes Aegypti), he added.

"It appears that there might have been run-off from the legs following treatment, or that the product was slow to spread to other body areas," he said