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Most pest control professionals are familiar with fruit flies. Red-eyed fruit flies, Drosophila melanogaster, are a common pest. They breed in sugary, slightly decomposed substances such as rotting fruit, or forgotten sugary beverages that find their way through the liner of a trash can. As the name suggests they have red eyes and are easily recognizable with a little training. These flies can be an unexpected and sudden nuisance as their life cycle only takes 8 to 10 days. One day a person’s living space or work space is pest-free, then suddenly there are hundreds or even thousands of little red-eyed flies causing frustration.

The seemingly obvious solution, and the one the annoyed customer almost always expects, is the elimination of adult flies. After all, the adults are the ones flying in their face and present in large numbers. It seems simple! There’s just one little (or possibly large) problem. At any given time, the number of visible adult flies only represents about 10 to 20 percent of the total population. Where are the rest of the flies you might ask? Well, they are cryptically hidden as immature stages in the sugary, decomposing breeding substrate. With this knowledge, it’s easy to see why killing the adult flies is a Band-Aid, at best, for the problem. A thorough treatment may kill every single adult fly present at the time, but those adults will quickly be replaced by new flies emerging from the breeding substrate. In fact, it only takes about 24 hours for the dead adults to be replaced.

The experienced Integrated Pest Management professional knows all of this, and thus he or she knows what needs to be done. An investigative search of the area, usually wherever food is stored or disposed of, often will reveal rotting fruit or another substance that is housing tiny white maggots (the majority of the fly population). Proper disposal of the breeding substrate leads to a rapid improvement of the situation. However, the adult flies can live for several weeks, so an application of an adulticide following the breeding substrate elimination may be warranted depending on the situation.

DARK-EYEDS ARE DIFFERENT. Most pest control companies are familiar with all this information, and have no problem eliminating red-eyed fruit flies and satisfying customers. However, there’s a relatively new fruit fly on the block that’s causing problems for customers and pest professionals alike. It’s known as the dark-eyed fruit fly, Drosophila repleta, and it’s a very dirty little fly. The frequency of dark-eyed fruit fly infestations in the U.S. and Canada has been gradually increasing since the early 2000s.

The dark-eyed fly is believed to be native to South America and has only recently been introduced into North America. But apparently the fly likes it up here and intends to stay. It has the same life history as its red-eyed cousin, with a few very important differences. One is that it has dark eyes and a darker general coloration than the red-eyed fruit fly, so it is easy to mistake it for a phorid fly. These pests are lazy fliers, and tend to aggregate and rest on surfaces to a much greater extent than red-eyed fruit flies.

The most important difference, however, is where dark-eyed fruit flies breed. They like their breeding substrate to be more decomposed than that of their red-eyed cousins, so you won’t likely find them in rotting fruit or anything similar. Instead, you can find dark-eyed fruit fly maggots developing in what I like to call “black sludge.” It’s what rotting fruit becomes after it has been rotting so long it loses its shape. The most common location for black sludge is in the drains of a busy restaurant, especially the drains for the drink machines. The constant, slow flow of liquids full of organic matter over a surface is the perfect way to create a utopia for a large family of dark-eyed fruit flies. This wet and heavily decomposed material isn’t restricted to drains, of course. Cracks in floors also can be a breeding site. Basically, anywhere that can hold wet decomposing debris is where you want to look. What makes this difficult is that it’s usually a drain or area that NO ONE has looked at in a very long time. This is essential for the accumulation of heavily decomposed materials that the flies crave.

So, if you are having issues with this fly, don’t just spray walls that are covered in resting adults. Instead, look for the smelliest, dirtiest areas you can find and make sure they get cleaned. Sometimes a foaming drain cleaner will clear things up after several treatments, but sometimes this just isn’t enough. Get a stiff-bristled brush, such as a toilet bowl cleaner, and physically remove the sludge from the drain.

CASE STUDY. I once helped one of my technicians with some dark-eyed fruit flies in a trash room. There were thousands of flies in the room, and several large, smelly green trash bins. My technician had sprayed the walls of the room with a broad-spectrum insecticide several times, but this failed to help for much more than a few hours post-treatment. The technician had recommended that the dumpsters be pressure-washed, but building management wasn’t interested in that. Inspection of the bottom of the dumpsters revealed wet-looking black sludge that was several inches thick in some places. I liberally applied borate dust to the bottom of the dumpster, and then mixed the black sludge and the borate dust together with a long (and disposable) stick. The flies were considerably less numerous thereafter.

More commonly, check the floor drains throughout the kitchens, especially the one associated with the soda machine drain. On several occasions I’ve found floor drains that are hidden inside voids in the base of cabinets that drain poorly. The managers of the food-service establishment are usually completely unaware that these drains even exist.

FINAL THOUGHTS. When confronted with a dark-eyed fruit fly infestation, prepare to get down and dirty. Find the black sludge and do whatever it takes to get it cleaned up. It may not be the most glamorous experience you’ve ever had as a pest management professional, but you’ll form a bond of trust and appreciation with your customer that no amount of rotten goo can break.

The author is an entomologist with American Pest, Fulton, Md.