Blow flies are an assorted group of flies that range in size from approximately 6 to 14 mm in length. In general, this group has a metallic sheen to the bodies in varied colors depending on the species ranging from green to orange to copper. Blow flies occur throughout the lower 48 states and many other areas throughout the world. Adults have blunt mouth parts and do not bite. Adult blow flies are attracted to nectar, carrion, waste and debris. This frequently becomes an issue for animals with bloody or soiled hair, fur or wool. Adults deposit eggs into castration and dehorning wounds of livestock and wounds or soiled areas of fur on other animals. Larvae begin feeding on decaying flesh and matted hair upon hatching. White to yellowish in color, the larvae or maggots of blow flies develop through three instars before entering their pupa stage to transition into an adult. Feeding by blow fly larvae results in inflammation, hair loss and at times blood poisoning. With the exception of green-bottle fly larvae, most blow fly larvae do not attack healthy tissue. Green-bottle fly larvae on the other hand are known for spending early developmental stages in superficial wounds before burrowing deeper into healthy tissues.
In general, blow flies typically develop from egg to adult in 10 to 25 days and complete an average of 4 to 8 generations each year. Black blow flies are cool weather flies, which correlates with higher populations in the spring and fall. This species overwinters in the adult stage. Other species of blow flies are common during warmer weather and overwinter as larvae or pupae. In the short 2 to 8 week life span of a female blow fly, she deposits thousands of eggs in masses of 40 to over 1,000 eggs. Larger masses are typically the result of multiple females utilizing a single site for depositing eggs. Incubation typically lasts less than 24 hours, but may take as long as 4.5 days depending on environmental conditions.
The most successful control of blow flies is typically sanitation related. Removal and proper disposal of garbage, carcasses and other potential breeding materials is the best method of limiting blow fly populations. Livestock, especially sheep, require special procedures and precautions to limit complications associated with blow flies.