by David Moore
Manager of Technical Services and Board Certified Entomologist
with contributions by Eric Smith, PhD, BCE
Flies are typically separated into 2 major groups, the large flies and the small flies. The larger or filth flies (those we’ll cover in the blog) are typically about 1/8-3/8″ long, breed outdoors, and come from the outside (e.g., house and blow/bottle), while small flies (covered in the April blog) are typically about 1/16-3/16″ long, breed indoors, and are found indoors (e.g., small fruit, drain, humpbacked/phorid flies, and fungus gnats).
WHY THEY ARE OF CONCERN. Yes, their presence is a nuisance, but there are more important reasons why you don’t want them inside. Consider that they carry and transmit disease-causing pathogens/organisms. They are pests because they affect human health and food safety. For example, consider the following:
- Houseflies have been shown to harbor over 100 different kinds of disease-causing pathogens, many of which are associated with filth. Such pathogens include those causing typhoid fever, cholera, diarrhea, dysentery, tuberculosis, anthrax, ophthalmia (affects eyes), polio, and salmonellosis (food poisoning), as well as parasitic worms, including tapeworms and pinworms.
- They transmit pathogens via their vomit, feces, and contaminated external body parts. For instance, since adult house, blow, and bottle flies have sponging mouthparts and therefore cannot eat solid foods, they must first regurgitate a digestive liquid onto any solid food material to liquefy it before they can slurp it up.
- Many have a relatively short development time (egg to adult) and can therefore build into large numbers within a short time. For example, house fly developmental time can be as short as 8 days.
CORRECT IDENTIFICATION IS THE KEY. To access information about where the fly that’s causing the problem is coming from, we must correctly identify the fly at least to group. So, let’s get to it.
The common large flies. The blow flies, the bottle flies, and the house fly are the most commonly encountered of the filth or larger flies.
- House fly (Musca domestica)
- Adult house flies are about 1/8 to 1/4 inch long. Their bodies are dull gray, with 4 narrow black lengthwise stripes on the top of the thorax. The sides of the abdomen are often pale. Adult flies have sponging mouthparts. They are more abundant in the summer months but may be found throughout the year.
- Adults will live 15-25 days. The adult female mates and begins laying eggs only a few days after emerging from the pupal case. During her lifetime, she will deposit about 7 to 50 clusters of 20 to 50 small white eggs, or about 350-900 total eggs. Females lay eggs on moist food sources, such as garbage, animal excrement, and other decaying organic matter. The eggs hatch in about 8 to 20 hours, and the first-stage maggots begin feeding.
- Usually, the maggots will complete their development in about 3 to 7 days. Depending on conditions, development may take as few as 6 days. There may be 10 to 12 generations during the summer months. Mature larvae seek out cool, dry places to pupate. The pupa is enclosed in a small oval case (puparium). The case is yellowish at first but gradually changes to dark brown
- House flies are general feeders, consuming materials from excrement to human foods. Because of their sponging mouthparts, adults can only feed on liquids. Through regurgitation, they are able to liquefy solids. House flies typically excrete and regurgitate every time they come to rest. This habit, plus their many body hairs and sticky tarsal pads, makes them efficient transporters of disease organisms.
- Blow flies and bottle flies (family Calliphoridae)
- Blow and bottle flies are the commonly encountered medium-sized (1/8 to 5/8 inch long) flies that are metallic blue, green, or bronze, or are shiny black. Their wings have the fourth (third long) vein strongly angled forward and almost meeting the third (second long) vein at the wing margin/edge.
- These flies are primarily scavengers and most develop in meat or animal carcasses, but also in animal excrement and garbage. Dead animals (rodents, or animals caught in the chimney) are the usual source of flies within a structure, while dog excrement and garbage are common outdoor sources.
- Black blow fly (Phormia regina)
- Adult black blow flies are about 1/4 to 7/16 inch long. Thorax and abdomen are dark and shiny, dark blue to black and sometimes with a metallic luster.
- Females lay eggs in glued masses. At optimal temperature of 99° F, eggs hatch in 8.1 hours. The three larval instars require 4 to 15 days. The pupal stage lasts 3 to 13 days. Developmental time (egg to adult) requires about 10-25 days.
- Bluebottle flies (e.g., Calliphora vicina).
- Adults are about 3/8 inch long. The thorax is dull, but the abdomen is shiny metallic blue.
- This bluebottle fly female lays up to about 180 eggs at one time, for a lifetime total of up to 720. At 25-35° F and 40% relative humidity, eggs hatch in about 11 hours. The 3 larval instars require about 4 to 7 days. The pupal stage lasts about 8-12 days. Developmental time (egg to adult) requires about 15-25 days.
- Bronzebottle fly (Phaenicia pallescens)
- Adults are about 3/16 to 3/8 inch long. The thorax and abdomen are shiny bronze and the thorax is without stripes on its upper surface.
- Eggs are deposited in batches of about 100. At 99° F, eggs hatch in about 7.7 hours. The 3 larval instars require 72 hours under favorable conditions. The pupal stage lasts 6 to 7 days. Developmental time (egg to adult) may be as short as about 10 days.
- Flesh flies are larger (longer and robust) than house flies (1/4-7/16″ long), often have reddish eyes, have 3 black longitudinal stripes on the top of their thorax, the abdomen often with a checkerboard of black and gray spots, and the tip of the abdomen is usually red or pink. Adults have sponging mouthparts.
- Females give birth to larvae/maggots instead of laying eggs. Depending on the species, the life cycle (adult to adult) ranges from 8-36 days. Many species develop in excrement or decaying flesh/carrion, some are parasitic, and others can develop in a variety of materials.
HABITS. The preferred breeding source(s) is in bold type.
- Food. Meat and fish, carrion, feces (dog), garbage, human and animal wounds. Comments: Note that larvae of many species will migrate just before pupation. Check for dead animals in chimney or voids.
- Bottle flies. Food: Carrion, confined garbage, garbage dumps. Comments: Check for dead animals in chimney or voids.
- Housefly. Food: Excrement (horse, pig, human, cow, dogs), bird droppings, fermenting vegetation, kitchen garbage. Comments: No garbage or other organic material can be allowed to remain exposed for more than 1 week (8-day developmental time).
- Flesh flies. Food: Meat scraps, feces (dog, human), and garbage. Comments: Pay particular attention to where dogs are kept.
MANAGEMENT. The application of IPM methods and tools.
Because large or filth flies breed primarily outside, control efforts center around decreasing attraction to the building, excluding their entry, and then harvesting those that do manage to get inside.
Due to limited space, we’ll review examples of IPM pest control strategy and procedures primarily as they can be used in single-family homes and apartments.
- Reducing attraction.
- Cooking odors. These are only produced periodically and little can be done to reduce them.
- Garbage odors.
- Keep garbage and trash receptacles as far away from doors as is possible.
- Keep garbage in tight closing containers that are emptied weekly.
- Trashcans. Keep insides clean and lined with a plastic garbage bag.
- Pet poop. Remove when deposited or at least daily.
- Proper grounds management. For decaying vegetation, leaves, and compost, its best to eliminate these or to keep them as far away from doors as is possible.
- Exterior lighting. Sodium vapor lights attract the fewest night-flying insects. If not practical, use yellow compact fluorescent or yellow incandescent bulbs.
- Preventing fly entry. It is usually far easier and more cost effective to prevent the entry of flies into a structure than it is to eliminate them once inside. This is typically done by using physical barriers.
- All holes in exterior walls sealed or vent holes screened with screens in good repair.
- All windows which open are screened, screens in good repair.
- All doors tight-fitting and fitted with self-closures.
- Screen doors with self-closures used where appropriate and screens in good repair.
- On decks and porches, the use of water vapor misting fans will keep flies away from people and doors if properly positioned.
- The use of outdoor odor-producing traps can be effective if located far away from the building itself and if serviced regularly.
- Harvesting flies that do gain entry.
- The use of Insect Light Traps (ILTs) is very effective if they are properly located and are installed at the proper height.
- Fly ribbons, attractant glue sticks, etc. work but most people find them unattractive.
- Fly swatters. Yes, these are rather primitive but can be very effective depending on the user’s agility.