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It is always possible when a German cockroach infestation continues despite repeated control efforts that the population may have insecticide resistance. It is more likely, however, that one or more active harborages have been overlooked and left untreated. In some cases, it is as simple as cockroaches hiding in a box under the wait station in a restaurant or a major oversight or lack of effective control procedures (e.g., relying on space treatments or surface treatments as the primary control strategy). In the author’s 40 years of experience, the following are key overlooked areas that lead to chronic German cockroach infestations in commercial buildings, particularly in commercial kitchens.

Fig. 1. So many cockroaches were present in a duplex that cockroach nymphs were clustered together exposed on the ceiling throughout the home.


By far the most overlooked area in a commercial kitchen that allows continued infestations is wall voids. A major reason that wall voids go untreated is the time it takes to professionally drill holes and treat the voids. The wall voids of most importance are those behind sinks, the dishwasher area, and behind stoves and ovens. The more poorly maintained the walls, such as deterioration from moisture, the greater the likelihood they will harbor cockroaches.

In one case in a restaurant of a marina country club in Texas, repeated efforts to control an infestation failed to eliminate cockroach sightings. The pest professional servicing the kitchen had diligently treated every available, accessible crack in walls and equipment, but had not treated any wall voids. Drilling and treating the walls behind sinks, the dishwasher, and stove lines caused sightings of cockroaches to drop to zero within a week.

In another case involving a badly infested duplex in Memphis (see Figure 1), an attempt was made to control the infestation using a new bait product. Although large numbers of cockroaches were killed after two weeks, hundreds of live cockroaches were still being found so additional baits, along with other crack and crevice treatments, were applied. While the numbers of cockroaches decreased significantly, it was not until after the wall voids were drilled and dusted through the back of all kitchen cabinets and bathroom vanities that the infestation was finally eliminated.

Fig. 2. Conduit boxes are commonly overlooked as harborage sites in commercial kitchens.


The second most important overlooked areas in commercial kitchens are electric conduit boxes and conduits (see Figure 2). Again, it takes time to service conduit boxes, and so pest professionals may overlook them. A case in Florida involving a fast food restaurant revealed the source of cockroaches to be a single conduit box behind a small refrigerator. The numerous cockroaches harboring on the outside of the conduit box were removed by vacuuming. The conduit box was opened, the cockroaches within vacuumed, and the conduits treated with a light amount of residual dust. The other conduit boxes in the kitchen were checked for activity and treated with dust as a precaution.


The third most overlooked potential source for continued infestations is false ceilings. German cockroaches are not often found in false ceilings, but such cases do occur. False ceilings take time to inspect due to the need for a ladder and having to move it about the kitchen to check the entire ceiling for activity. Ceilings should be checked for cockroaches during the initial service and in cases with chronic infestations. In particular, check ceilings above stove lines and the dishwashing area.

Applications of dust insecticide — except into cracks and voids — should not be done above false ceilings. Dust applied loosely above a false ceiling will drift out of cracks and light fixtures into the room below, particularly whenever ceiling tiles are lifted.

The best way to treat cockroaches in ceilings is to remove as many as possible with a vacuum followed by spot treatments to the wall area above the ceiling in areas where cockroaches are active. When cockroaches are hiding in cracks where the false ceiling meets the wall, they can be treated with a residual dust or aerosol product applied to those sites with activity. A gel bait also may be applied into these cracks instead of a residual product.

Infestations in false ceilings must first be removed as best as possible with a vacuum, followed by spot treatments.


In some cases, the source of continued cockroach sightings involves items removed in preparation of cockroach control services. A case in Indiana in a high-rise apartment was traced to a stack of plates the tenant removed from her cabinets and covered with a sheet. The dozens of cockroaches hiding between plates were escaping detection and treatment as the pest professional had not thought to examine the removed items. Another Indiana case in a larger institutional kitchen involved cockroaches living in a slicer that was being moved and covered prior to pest control services.

A unique case in a Tennessee country club was encountered where cockroaches repeatedly were seen on food tables during banquets, wedding receptions and other events in the banquet room. After each report of cockroach sightings, the pest professional and his supervisor would find no signs of cockroaches around the banquet tables, behind baseboards or other sites in the banquet room. They had diligently treated every crack in the tables and had even drilled and treated the voids of the wall where the food tables were located.

When the author became involved to look at the problem, he asked the banquet manager how the tables were decorated during an event. The tables would be outfitted with cloth tablecloths and table skirts, and then mirrors placed on the tables and food dishes placed on the mirrors. The mirrors were stored under a table in the kitchen and, when examined, dozens of cockroaches were hiding in the crack along the rim on the underside of every mirror. Several times a week, these cockroaches were carried out in the mirrors and served up an easy buffet. The cockroaches on the mirrors were removed by vacuuming and the facility maintenance person filled the cracks on the back of the mirrors with a silicone sealant to deny cockroaches any future harborage.

The lesson here is when a source cannot be found, the PMP should ask what is stored in the area when the cockroaches are being seen that is not present when cockroach services are performed.