Fungus gnats and dark-winged fungus gnats, Sciaridae sp. and Mycetophilidae sp., are a commonly encountered species of small flies in a variety of homes and businesses. Most, if not all, of the literature leads pest management professionals to the source by finding the “over watered” house plants in someone’s home or business. That’s a simple enough and easy IPM solution (if, of course, you can find that elusive source).
These small flies get their name from the fact that one of their major food sources is fungi that their larval stages feed upon. While there are other food sources and points of origin, most PMPs do in fact find a house plant that has been so over watered it practically needs scuba gear to survive.
Correct identification is of course paramount and if not done properly it can lead you in wrong directions. You need proper magnification to see the wing venation on both species.
Once you have the proper identification, finding the typical source of the population is relatively simple. However, what if there is not an overwatered house plant, nor any leaks around plumbing lines or sinks? What if there are no leaks that you can find yet the gnats are still present? And what should you do if you knock down the adult populations with a pyrethroid aerosol but the population rebounds within just a few days?
This case study took place during the cooler months of March and April in Cincinnati, Ohio. That’s just warm enough to spike populations of just about everything in this region but it’s also fairly dry and cooler than the later summer and fall months. The call came in from customers living in a single-family house in a nice neighborhood. It was a well maintained, two-story brick house (built in 1979) with a basement and a poured foundation.
The inspector brought back samples that were positively identified as Sciaridae sp. — the dark-winged fungus gnat. A contract was written and information was given to the service department for scheduling and treatment.
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The typical treatment strategy of inspection and a crack and crevice treatment began. Addtionally, the “education” of the customer began with that first treatment. The customer seemed happy enough for about a week. Then the phone call came in from the customer that the flies were back — and so were we. This time a much more thorough inspection was performed with no apparent “source” found.
Further treatment and inspections yielded the same results — there was a small window of time where the adults were not seen but in every case, after a week or so, the adults emerged from some out-of-sight breeding source.
Moisture meters were brought in and a room-by-room inspection using the meter on every wall, floor and ceiling was completed with no evidence of any water source or moisture. Once again a space treatment was performed. The customer was instructed to pinpoint the exact location of the adult fly’s first sighting and to call the office immediately. Six days later the call came and we were on site within hours. Two adults were found in the basement on an outside wall. Absolutely no moisture of any kind was found but still another inspection was initiated around this area. A small closet in the front corner of the house was opened and two more flies emerged. No moisture was seen or measured. A number of glueboards were placed on the floor, walls and shelving of the closet and we told the customer we would be back the next day.
Upon returning, the two glueboards at the corner of the closet were found to have about 20 adults on each. The carpeting in the closet was removed and a ¼-inch crack in the concrete floor was found. The moisture meter was inserted and a positive, but not unusually high, reading was noted. The exterior was inspected with no apparent moisture issues noted.
During my career in this industry I have often instructed inspectors, supervisors and technicians that if what they are doing is not making any difference then stop and think about it. I have always made it a point that when nothing makes sense, I walk across the street, yard or lot and look at the whole building or site to see if I have missed anything. In this case I noticed the downspouts off the gutter continued down into the ground around the foundation. As it happens, one of these downspouts was at the corner of the house where we were finding the gnats.
Upon consulting with the homeowner we found out the downspouts connected with PVC piping and traveled along the foundation underground to the backyard where there was a ravine into which the rain water was spilling out.
A plumbing company was called to send a video camera down the downspouts and into the PVC pipe. That firm found the PVC pipe had sagged and cracked, creating a gunky mixture right next to the foundation in close proximity to the crack in the foundation floor. When it was excavated and repaired we found this was indeed the source of the fungi and the population of fungus gnats. Problem solved.
The moral of the story is “don’t pound sand…all you get is sand.” Take some time and look at the whole picture at the first signs of issues and concerns.
The author is director of sales for Rose Pest Solutions in Chicago. Email him at email@example.com.
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