Blow flies are blue-black to green scavenger flies that are common across Kentucky. Females are attracted to carcasses, manure, or decaying organic matter. Some species will lay eggs on the unhealed umbilical wound or accumulated manure around the anus of calves. These flies also may place their eggs on necrotic tissue around wounds. A single female may lay as many as 200 eggs on an attractive site.
A tiny maggot hatches from each egg within 24 hours and crawls into soiled or decaying tissue to feed. Maggots usually are full-grown in 6 to 12 days. Mature maggots drop from the animal and pupate in the soil, emerging as flies in 7 to 10 days. Without treatment, areas where maggots developed become inflamed and exude puss, attracting more flies. The size of the wound increases and may become infected.
Prevention is the best treatment for fly strike.
- Regularly clean wounds and manure accumulations to reduce attraction of flies.
- An insecticide treatment may be used, but it does not take the place of regular, thorough cleaning. An aerosol product such as Catron IV (0.05% permethrin) be used on beef cattle, dairy cattle, sheep, goats, swine, and horses. Follow label directions.
Consult your veterinarian if wounds heal slowly or persist.
By Lee Townsend, Extension Entomologist