Blow flies (Figure 1) are non-biting flies that are distributed throughout the world. They primarily feed and lay their eggs on decomposing animal remains. Because of the condition of their food resource, they often come in contact with human and animal pathogens. Therefore, suppressing their populations is very important for reducing associated disease risks. The purpose of this publication is to provide basic information on the biology of blow flies and appropriate methods for their control.
Blow flies are metallic flies measuring approximately one quarter of an inch in length. Based on their appearance, they can be separated into two groups. The green bottle flies are metallic green but can in some instances have a bronze tint. These flies are active during the warm months of the year. The blue bottle flies are metallic blue to black or purple and are active during the cooler months of the year. Both groups of blow flies have world-wide distributions and can be found in most habitats ranging from urban to rural settings.
The blow fly life cycle has four stages of development. A single blow fly can lay up to 2,000 eggs during its life. Eggs are deposited on decomposing animal remains and typically hatch after approximately 12 hours. The resulting larvae (Figure 2) will aggregate and feed on the decomposing remains. At approximately 80oF ambient temperature, larval development will take 11 to 20 days, and during that time they will pass through three larval stages referred to as instars. During the later stage, the larva will leave the food resource and form a pupa (i.e. cocoon). Blow fly pupae typically are football-shaped. Pupation can account for a significant amount of the development time with this stage lasting from six to 12 days.
Two types of control methods are used to suppress blow flies: cultural and chemical. It is recommended that both methods be used. By using both tactics, blow fly populations can be suppressed with the least expense.
Cultural methods rely on modifying the environment so that blow flies can no longer develop and maintain their population. The best cultural method is to properly dispose of any decomposing animal remains that might be used by blow flies as an environment to rear their offspring. Dead animals make the best breeding-habitats for blow flies. Carcasses should be buried at a depth of 12 inches or placed in garbage bags that are properly tied in order to prevent access by blow flies. Additionally, cleaning and removing all decomposing materials from garbage cans on a weekly basis will reduce their level of attraction to blow flies. Keeping windows screened and doors closed will reduce blow fly access into homes. Businesses can install air curtain systems above their exterior doors in order to reduce fly access. Accordingly, mechanical doors can be installed in order to reduce fly access. Several types of traps are available to control blow flies that have gained access into homes or retail stores. Sticky traps and ultra-violet light traps do not use toxic chemicals and are a good alternative to the use of insecticides. Traps need to be placed at least 5 feet away from food processing areas to avoid contamination risks and away from windows to avoid attracting more flies into the building. Sticky traps need to be replaced frequently as they loose their effectiveness with time, especially in high dust situations.
Chemical methods when necessary can be used to suppress blow fly populations. There are many spray pyrethroid-based insecticides labeled for suppressing blow flies around and in homes. Those labeled for home use can be purchased at grocery and hardware stores. Fly baits, such as QuickBayt® and Golden Malrin®, are not recommended for controlling blow flies. These baits are usually sugar-based, and they contain a compound that attracts only adult house flies. Regardless of the insecticide being used, it is recommended that the user read and follow the directions listed in the product label.
The information given herein is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by the Cooperative Extension Service is implied.
Figure 1. Adult blow fly
Figure 2. Blow fly larva