[Annual Fly Control Issue] Fly, Fly Away

Flies belonging to the insect families Muscidae (in this case, house flies), Calliphoridae (blow flies), and Sarcophagidae (flesh flies) are domestic non-biting flies commonly seen in/around human dwellings. They are often collectively called "filth flies." These flies do not bite, but are medically important in the mechanical transmission of disease agents from feces or dead animals to foodstuffs or food-preparation areas.

Throughout the world, these flies serve as carriers of organisms causing diseases such as typhoid, diarrhea, amoebic dysentery, cholera, giardiasis, pinworm and tapeworm. They may spread these agents via their mouthparts, body hairs, or sticky pads of their feet, as well as through their vomit or feces. In addition, the egg-laying habits of these insects may lead to another human malady, myiasis, which is infestation of people by the maggots of flies. Filth flies often lay eggs on dead animals or decaying organic matter, therefore it is understandable that they might occasionally mistake a neglected wound on a person as a "dead thing" and thus lay eggs on it. This type of myiasis is opportunistic (sometimes referred to as facultative), and rarely, if ever, leads to infestation of healthy tissue. Nonetheless, no one wants a patient in a nursing home or hospital to get maggots in their open sores. A lawsuit against both the health-care facility and pest control company is sure to follow such an event.

MORPHOLOGICAL DIFFERENCES. House flies are about 5 to 8 mm long, with a dull gray thorax and abdomen (not shiny). The thorax has four longitudinal dark stripes. Mature house fly larvae are 10 to 13 mm long and usually creamy white in color. Overall, the larvae have a conical shape very much like a carrot with two dark-colored mouth hooks at the narrow end and two oval spiracular plates at the broad posterior end. The three sinuous slits in the spiracular plate are diagnostic features for this species.

Flesh flies look like house flies but are generally larger (11 to 13 mm long); they have three dark longitudinal stripes on their thorax, a checkerboard pattern of gray on the abdomen, and sometimes a reddish-brown tip on the abdomen. The larvae of flesh flies are similar to those of house flies, except they have straight spiracular slits and often an incomplete ring around the spiracular plate.

Blow flies (also known as green or bluebottle flies) are about the same size as flesh flies, although some of the bluebottle flies (genus Calliphora) are larger and more robust. Blow flies, with the exception of the cluster fly, are metallic bronze, green, black, purplish or blue colored. They are the commonly encountered "green flies" seen on flowers, dead animals or feces, or occasionally indoors. Blow fly maggots resemble both house fly and flesh fly maggots but have straight spiracular slits and most often a complete sclerotized ring around the spiracular plate.

GEOGRAPHIC DISTRIBUTION. House flies, Musca domestica, occur worldwide in association with human dwellings. There are numerous blow fly species occurring over most regions of the world. One common blow fly, Phaenicia sericata, is cosmopolitan in distribution. Calliphora vicina is one of the most common bluebottle species in Europe and North America. Various species of flesh flies also occur worldwide. One of the most common is Sarcophaga hemorrhoidalis, which is virtually worldwide in distribution and common in the United States. This species has a red-tipped abdomen. (No wonder it is named hemorroidalis!)

BIOLOGY/BEHAVIOR. Like all Diptera, these flies exhibit a complete life cycle, having egg, larva, pupa and adult stages. All three filth fly groups discussed in this section have similar biologies. Females lay their eggs, singly or in clusters, on or adjacent to an appropriate medium for larval development. However, flesh fly females deposit living larvae rather than eggs. One to a few days later the eggs hatch and the first-stage larvae emerge and enter the food source where they feed and develop.

Since the outer skin of a larva is nonliving and cannot grow, this skin (exoskeleton) must be periodically shed as the larva grows. Molting occurs three times in these filth fly families. Eventually, larval growth is completed and a pupa is formed inside the last larval skin. The larval growth period ranges from two days to three weeks depending upon temperature, and the pupal stage takes a similar length of time.

The entire house fly life cycle can occur in eight to 10 days under summer conditions. Emerging adults are active, moving from one attractant to another throughout most of the daylight hours. House flies are strongly attracted to feces, decaying organic material and foodstuffs. Garbage and pet feces are almost always the important breeding sources of house fly problems in urban settings. Flesh flies breed in decaying meat or animal excreta. Most blow flies lay their eggs on animal carcasses, and the developing maggots quickly dispose of the carrion. However, some blow fly species breed in dog manure or decaying organic matter. The entire blow fly life cycle requires nine to 25 days or more.

CONTROL. Filth flies are best controlled by a combination of good sanitation, mechanical exclusion, ultraviolet light traps, and chemical control. Good sanitation includes emptying and steam-cleaning Dump-sters® on a regular schedule and otherwise eliminating breeding sites. Cracks and crevices under Dumpsters and compactors should be inspected for larvae developing in organic debris accumulated there. Mechanical exclusion methods include air curtains and properly fitted doors and screens. Ultraviolet light traps (there are a whole array of these traps that work in different ways) work well indoors and are generally quite safe. There are several baits registered for fly control in and around buildings. For temporary abatement, fogging a facility with pyrethrins will lead to an immediate reduction in adult flies.

The author is a medical entomologist for the Mississippi Department of Health and clinical assistant professor of preventive medicine at the University of Mississippi Medical Center, Jackson, Miss. Goddard can be reached via e-mail at jgoddard@pctonline.com.