[Tech Talk] Beneath Our Feet — Solving Phorid Fly Issues Below the Slab

When phorid flies are coming from decaying matter somewhere in a kitchen or above ground, they are handled in the same manner and just as easily as any other small fly problem. When they are coming from under the slab, however, they present a significant challenge to both the PMP and the client. The process for solving these problems can be daunting. How do you know exactly where under the slab the flies are living? Simply being able to answer this question is half the battle. Here is one way to get a clear picture of what is happening underground.
 

Below the ground.

After you have confirmed the identification of the problem as a phorid fly issue, a careful inspection should be done to rule out any above-ground sources. If any sources are found, they should be eliminated and monitoring should be done to determine if further sources exist. It is also helpful to tape plastic bags over each floor drain for 24 hours or at least overnight. If any of the bags have phorid flies in them, examine the drains carefully and consider cleaning and re-bagging them. Let subsequent monitoring indicate whether or not there are still sources of flies.

If it is apparent that phorid flies are present and likely to be emerging from under the floor, set out insect light traps (ILTs) in a logical pattern to help focus your search. Each trap should be numbered and enough traps should be used that many of them catch zero flies. It is important to know where the flies are, but it is just as important to know where they are not.

Check the traps daily and record the new flies caught each day. You may have useable information within one or two days, but it may take a week before a clear picture emerges. It may be evident that the trap placement pattern needs to be altered before analysis can be made. You should have four or more traps that catch at least some flies. Use your data to make a graph or draw the trap plan on a floor plan of the facility. With highlighters or colored pencils you can indicate which traps get light, medium, or heavy activity and which traps have zero.

If the resulting data show that the flies are found in all traps from perimeter locations or near exterior doors, you may consider placing a trap outside for a night or two. If you are going to do this, cover the trap with coarse screen or fine hardware cloth to reduce the number of moths that will inevitably show up. If the trap catch from an exterior trap holds more phorids (same species) than the interior traps, you should examine the possibility that the flies are coming in from outside.
 

Likely Sources.

The resulting picture should allow you to draw a line around the most likely area containing the source or sources. With this map to guide you, walk through the area and determine which areas of the floor may be over the source. It is helpful to study a plan of the plumbing that may be under the slab. Sometimes these plans are not available, but often they are. Have holes drilled through the slab at carefully chosen points and securely tape plastic bags over the holes. Each hole should be numbered. Drilling a hole in the slab may be well within the capability of the facility, or it may be a service that you provide. Of course there are many procedures and precautions that must be taken when drilling concrete slab floors; whoever is to do the work must be competent to drill floors.

Occasionally, the flies will emerge from the drill holes as soon as the drill bits are removed. Most often, the flies show up over the next few hours or days. Check the bags daily and record the number of flies found. Within 48 hours, you should have sufficient data to make some decisions about where to dig. It is the holes that yield zero flies that usually indicate a position outside the boundaries of the infestation. There may be other reasons a hole yields no flies even though it is within the infested area, but if the other holes have been well placed, the picture should be sufficiently clear to begin remediation.

The holes that first yield flies and that yield the most flies are normally those drilled into the center of the source. When the source is large and spread out, flies may emerge from widely separated holes. Some holes farther from the source may not show flies for a day or two if it takes them a while to get there.

Start where the activity is greatest. Once you know that you are right on target, it’s all over but the digging.

Rose Pest Solutions’ Mark Sheperdigian is a 1982 graduate of Michigan State University with a bachelor’s degree in entomology. He has held a variety of positions in service, sales, management and technical support. He is vice president of technical services for Rose Pest Solutions in Troy, Mich.

Copesan is an alliance of pest management companies with locations throughout North America. To learn more, visit www.copesan.com.