Zombie flies: Invasion of the carrion eaters


October 30, 2017

Why didn’t these green bottle flies realize they were invading the home of a pest management professional?

We moved to Archibald Ranch, a new, single-family residential community housing development, in November 1991. At that time, Archibald Ranch, which is located in south Ontario, Calif., was in the Chino Valley Agricultural Preserve. Before the development was built, the Chino Agricultural Preserve had more dairy cows per unit area than anywhere else on the planet. It’s little surprise that the No. 1 agricultural commodity production of the County of San Bernardino, in which the preserve is located, was cream.

In 1991, Archibald Ranch was bordered on three sides by dairies. It was common to hear cows mooing at all hours of the day and night from our house. Needless to say, the place was inundated with house flies (Musca domestica). I remember walking on the sidewalk of Turner Avenue, south of Chino Avenue, and scaring up clouds of house flies ahead of me.

Photo: Shutterstock

Complaints by homeowners and actions taken by West Valley Vector Control District resulted in better fly management on the surrounding dairies. The fly problem alleviated a little. We learned to live with the flies by modifying our habits and behavior.

Gradually, it became untenable to run a profitable dairy in our area of the Agricultural Preserve, and many of the dairies moved out. The house fly problem essentially subsided as the dairies relocated. Currently, large acreage of land previously occupied by dairies in the previous Agricultural Preserve is being rapidly covered by newly built houses.

Zombie Fly Case No. 1: Creepy Calliphorids

During the first week of July 2017, we began experiencing a fly problem in our home. This was no ordinary fly problem because the offending culprit was the common green bottle fly, Lucilia sericata (Meigen), formerly known as Phaenicia sericata (Diptera: Calliphoridae, the blow fly family). This fly has been of medical importance since 1826, when German entomologist Johann Wilhelm Meigen first described it from maggots he removed from the eyes and facial cavity of a human patient.

This hirsute creature is a regular visitor to carrion, feces, garbage, dog excrement, decaying fruit, rotting vegetable waste, putrefying fish, poorly managed compost piles, dead snails (including those killed by snail bait), and occasionally animal manure. Insects of the family Calliphoridae are commonly referred to as necrophilous flies, and the larvae of most species are saprophages (scavengers).

Knowing the origins of these flies exponentially elevates the “yuck factor,” and makes them extremely detestable especially when they enter structures and are flying about in the dining room and kitchen. Because of their large size (they are larger than house flies), endless flying about indoors, and their loud obnoxious buzzing, these flies are much more annoying and repugnant to homeowners than house flies — not to mention the repulsiveness of their reproduction sites and their potential to mechanically carry disease organisms.

Calliphorid flies are known to cause myiasis, which is basically the invasion of living human tissue by larvae of Diptera. Lucilia sericata in particular has been reported to cause urogenital (urinary tract) and nosocomial (nasal tract) infections in humans. This is another troubling reason why this fly should not be tolerated in structures. Blow flies must be prevented from laying eggs or larvipositing on meat and fish because if ingested, they can remain alive and cause acute gastric and enteric disorders (James, 1955).

At first, I thought that this was just a random occurrence and I resorted to non-chemical control by killing the intruders where possible with a fly swatter. This can be a very frustrating exercise. A fly swatter was not efficacious at dispatching specimens that rested in protected corners of glass doors and windows, etc. These were more efficiently rendered persona non grata by directing a one-second burst of a pyrethrin aerosol product directly on the target.

As days went by, more and more flies began appearing in our house, prompting my wife to refer to them as “zombie flies.” She believed the flies were coming back to life after they were killed. Heeding that concern and wanting to keep the peace, I took to sweeping/vacuuming up and disposing of the dead flies outdoors. That did not solve the problem, as flies kept appearing indoors.

Taking action

It is virtually impossible to totally fly-proof a structure, particularly for blow flies. These insects are extremely adept at entering homes through small cracks, crevices, tiny openings around plumbing and electrical protrusions through exterior walls, nooks and crannies that are not obvious to humans.

I carefully inspected the exterior of the structure in an attempt to locate all possible fly entry points. Those that were suspected to be of concern were caulked, sealed, screened and patched as appropriate. Despite my best efforts at exclusion, however, flies kept appearing in the house. The stove/hood exterior vent pipe was thought to be a point of access, so I screened it with window screen (see Fig. 1). Stove exhaust ducts are normally screened by builders with quarter-inch hardware cloth to exclude rodents and birds, but this eyelet size is too large to keep out flies. Decades of field experience informed me that blow flies sometimes enter homes by this route because of a buildup of animal grease and fats in the vent pipe.

Fig. 1 The stove exhaust vent is a suspected fly entry point.
Photo: Dr. Hanif Gulmahamad, BCE-Emeritus, PCA

This still did not solve the problem. We became very conscious of entering and exiting our home, and took extraordinary precautions to do so very quickly so as to prevent flies from entering. The garage door entry to the house, as well as the main front double door access, did not have insect screens on them. Yet careful observations did not reveal the presence of flies resting, loafing or hovering about entry doorways.

Blow flies are swift fliers, and they are not easily seen when in flight outdoors. They have been recorded to fly as much as 12 miles from their source of origin in search of a suitable carcass on which to lay their eggs. I was growing frustrated.

Meticulous sanitation is always practiced on this property to minimize pest problems. Still, I did a thorough inspection of my property for sanitation issues and breeding sites. I found none. Nor did I detect any odor of an animal carcass indoors.

Tracking down the culprit

On July 22, we noticed a dramatic reduction in the number of green bottle flies in our home. On July 23, only a single fly was found indoors by the glass patio door. Since that date, not a single fly has been found indoors.

As it turns out, July 22 was a notable date. You see, on July 7, a dumpster was delivered to my next-door neighbor’s house. It sat in the street for several days before it was rolled onto the property and pushed up against a low concrete block wall that separates our adjoining properties (see Fig. 2). The dumpster was situated 10 ft. from my automatic garage door. It was about this time that the fly problem started.

Fig. 2 Dumpsters in residential neighborhoods are potential generators of blow flies.
Photo: Dr. Hanif Gulmahamad, BCE-Emeritus, PCA

I had been preoccupied with a number of matters and did not pay much attention to the dumpster. I did notice an odor emanating from the dumpster when I walked by it, but again, had not put two and two together.

The neighbor had parties on July 15 and 16, and party trash, food scraps, food debris and other materials were simply tossed into the dumpster, not confined in sealed plastic trash bags. This was the peak period of our “zombie fly” problem.

The dumpster remained on the property until July 20, when it was pushed out to the curb for removal. It was removed July 21. What a world of difference it was to have no flies in our home less than 24 hours later!

In his 1975 landmark book Urban Entomology, PMP Hall of Famer Dr. Walter Ebeling (Class of 2003) reported that blow flies commonly breed in dumpsters and outdoor trash containers in California. Years of personal experience inform me that blow flies usually dominate the Dipteran fauna in and around dumpsters in southern California. While employed by the Los Angeles Unified School District as its integrated pest management (IPM) coordinator, I encountered a number of incidences where dirty, odoriferous dumpsters were acting as fly incubators, generating thousands of green bottle flies that quickly became a health and safety hazard near children’s lunch areas.

To combat this, I required all food trash and waste to be placed in sealed trash bags before being disposed in dumpsters. We instituted a program to have problematic dumpsters power-washed and steam-cleaned as appropriate with a non-toxic, biodegradable dumpster wash and deodorizer.

The arrival of a dumpster next door and its subsequent removal was too closely correlated with a high population of green bottle flies in the vicinity to be attributed to happenstance. During the period in question, high ambient daytime temperatures baked the area, sometimes reaching 106°F and accompanied by high humidity. Low outdoor nighttime temperatures were in the mid-70s. These conditions were ideal for the accelerated reproduction of the green bottle fly, and its life cycle was significantly shortened. Blow flies have a very high biotic potential, and a single female of some species can lay up to 2,000 eggs in batches during her lifetime. As many as 5,645 eggs were counted on a 150-gram piece of meat after five hours of exposure. In 1994, PMP Hall of Famer Stoy Hedges (Class of 2013) reared more than 130 blow flies from a single mouse carcass.

Given the environmental circumstances involved, it became almost impossible to keep these flies out of the house — especially when there was a high population of them in the immediate vicinity. The excessive daytime heat forced adult flies to seek out the cool, air-conditioned interiors of buildings. Adult green bottle flies are cryptic and very adept at finding small, hidden entry points into structures.

Zombie Fly Case No. 2: When in doubt, look over the fence

An acquaintance we’ll call Ms. Smith recently moved into an exclusive neighborhood in Corona, Calif. After a few weeks, she was plagued by an invasion of green bottle flies. She turned to me for help.

She admitted to me that she had first gone to a home improvement store and bought more than $100 worth of fly control chemicals, devices and equipment, including two over-the-counter insect light traps (ILTs) — one of which burned out shortly after purchase. After examining the cache she bought, it was easy to conclude that most of it was useless against green bottle flies because their biology, ecology and behavior are different from that of the house fly.

But I would never criticize her, nor would I be critical of any homeowner who is in the stressful situation of dealing with these flies. After all, I — a professional — was in the same situation just a few weeks before! Sensitivity, empathy, careful listening, and good customer relation skills on the part of a pest management professional (PMP) are essential when dealing with these matters.

An inspection revealed no sanitation issues or breeding sources on or in the affected property. All indoor and outdoor trash receptacles were examined, and found them to be free of fly breeding.

I asked all the usual questions:

  • Did you detect any obnoxious odor in or on the premises?
  • Did you deploy any rodent bait in or on the property?
  • Did you set out any rodent traps in or about the property?
  • Do you have companion animals? If yes, where is the pet food stored?
  • Did you detect an oniony smell or other “off” odor at any location in or near the property? (The reason for this question is that blow flies are not always associated with a carcass. Green bottle flies are also attracted to gas leaks, according to an article written by Aaron N. Fleischer in the November 1957 issue of PMP magazine).
  • Did you notice any bird activity on the structure?

The answers to the above questions were all negative.

Start with an inspection

The property in question is an expensive newer home that is fairly well pest-proofed. A few entry points were identified; these were caulked, sealed, patched and screened as appropriate. One suspected major point of entry (see Fig. 3) was taped up to immediately prevent entry. None of the exterior doors on this structure were equipped with screens, so normal entering and exiting this house allows speedy flying insects such as green bottle flies to gain entry. Food odors and escaping cool air from an air-conditioned structure, particularly on hot days, are very attractive stimuli for necrophilous flies.

Fig. 3 Flies were entering where the air-conditioning lines penetrated the wall. 
Photo: Dr. Hanif Gulmahamad, BCE-Emeritus, PCA

Fly entry points can be anywhere, from the foundation to the roof eaves of a two-story building, and these are not easy to locate. A pair of binoculars is sometimes helpful in trying to alleviate this problem, but in this case I didn’t find anything out of the ordinary.

When I returned to Ms. Smith to tell her about my inspection, she told me she found maggots that morning underneath a welcome mat outside her back door (see Fig. 4). This gave me pause, for it is common for mature blow fly maggots to crawl away from their breeding medium to find a suitable place to pupate. Pupation sites are limited on properties where the exterior is almost covered by hardscapes, and the underside of a doormat offers an alternative pupation location.

Fig. 4 A back-door welcome mat harbored maggots underneath it.
Photo: Dr. Hanif Gulmahamad, BCE-Emeritus, PCA

We quickly removed the mat and swept the area clean. I then treated the expansion joint and cold joint between the exterior concrete slab and building foundation with a crack-and-crevice treatment. The spot was treated and allowed to dry before the mat was put back in place.

In addition, I performed an exterior perimeter treatment using a wettable powder formulation of a pyrethroid up the sides of walls, around doors and windows, at suspected resting sites, and at all suspected entry points. Indoors, Ms. Smith did not want any chemicals used. Instead, I vacuumed all the flies that were on the glass doors and windows (see Fig. 5). This action helped alleviate the problem by immediately removing a constant visual reminder.

Fig. 5 Dead flies on window ledges should be vacuumed, then disposed of outside to prevent secondary pest infestations.
Photo: Dr. Hanif Gulmahamad, BCE-Emeritus, PCA

It should be noted that fly carcasses in patio door runners, along baseboards, and on window ledges should be vacuumed only using devices with high-efficiency particulate air (HEPA) filters and disposed of outdoors. If left in the home, dead flies become invitations to secondary pestiferous scavengers such as pharaoh ants and carpet beetles.

Besides, it is somewhat unprofessional for a PMP to let fly carcasses become dry, brittle, crumble and disintegrate — and possibly become airborne contaminants and respiratory hazards in the home.

Again with the neighbors!

It is easy to lose sight of the big picture when dealing with these kinds of fly issues. After still being somewhat baffled by the source of this problem, after I had finished my treatment, I glanced over the fence on the north side of the property.

Lo and behold, dog excrement was scattered all over the non-landscaped neighboring backyard. No wonder green bottle flies were on the adjacent property!

These are prickly situations to deal with without jeopardizing good neighborly relations. Luckily, our IPM strategy was enough to resolve Ms. Smith’s problem and keep the flies on the other side of the fence for the rest of the summer.

What have we learned?

The take-home message from both these incidents is: Be aware of extraneous factors, beyond the ordinary and the regular that could be associated with and/or conducive to a green bottle fly infestation in residential neighborhoods. And always check out the trash and yard-cleaning habits of the neighbors.


Ebeling, W. 1975. Urban Entomology. University of California. Division of Agricultural Sciences. Berkeley, California.
Fleischer, A.N. 1957. “Flue flies.” Pest Control. 25(11): p. 42.
Hedges, S. A. 1994. Field Guide for the Management of Structure-infesting flies. Franzak and Foster. Cleveland, Ohio.
James, M.T. 1955. “The blowflies of California (Diptera: Calliphoridae).” Bull. Calif. Insect Survey 4: 1-30.

Contributor Dr. Hanif Gulmahamad, BCE-Emeritus, PCA, is an urban and structural entomologist and consultant based in Ontario, Calif. He may be reached at entodoc@verizon.net.


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