I still maintain that the insecticide active ingredients we currently use, and which you have been using, are effective at killing German roaches. The important key is that the roach and the active ingredient must meet and stay in contact with each other long enough to get that lethal dose into the roach, and the best way to accomplish this is to place the active ingredient directly into the places where the roaches hide during the day - cracks, crevices, holes, and voids. In a restaurant these kinds of harborage could be in a great many locations, and perhaps your focus on that thermostat where you (or your customer) have seen them emerging is causing you to overlook other important places.
Since you feel that you had effective control for several months it may be time to go in with a fresh Inspection of the entire facility, using knee pads, a flashlight, and perhaps a C&C pyrethrum aerosol to find out just where the roaches may be now. It may be that they are also hiding in equipment, floor molding, within boxes in storage, or in other wall voids where you simply are not aware of them yet. If the interior truly is "overrun" then it sounds like you have a fairly large infestation that would most likely be using many areas for their harborage. These all need to be discovered and treated in some manner, and "treated" could mean simply filling in a hole or crevice permanently so that harborage is no longer available to the roaches.
You also need to evaluate what might have changed in the recent past. Has the sanitation effort by the customer declined so that more food resources are now available. Are their storage areas cluttered and filled with cardboard boxes. Are more things being stored on the floor. We really need to avoid complete reliance on insecticides to handle roach problems, and in particular if sanitation is poor then your bait products will not be as interesting to the roaches and your contact insecticides will be less effective. I would suggest altering bait products on a regular basis regardless, as this offers alternatives that may keep the roaches feeding on them. Dusting within wall voids is excellent, but you should try to access as many interior voids as possible and perhaps use an inorganic dust like boric acid or silica gel or diatomaceous Earth. These will last for many years, whereas synthetic actives like deltamethrin are going to degrade in a much shorter time. This is particularly so where heat and water may be contacting the treated surfaces.
Again, nothing wrong with cypermethrin as a contact treatment but it still pays to rotate your product choices for spraying to avoid any possibility of resistance or avoidance. Try one of the newer non-pyrethroids for a few months instead, and concentrate on placing all sprays directly into cracks and crevices using a C&C tip. This not only puts the most active ingredient into the most important places, but it also removes the a.i. from human exposure and from being washed off by the next wash down. Place lots of insect glue traps around the interior, such as under and behind all equipment, and take a look at them within a week to see what is captured and where. This may reveal that the roaches are coming from places you did not suspect, and you can then treat those locations. Advise the customer that you are placing these traps and that they should not be moved or washed down or covered in any way.
But, most important now, I believe, is to make a new and thorough inspection to find out just where all the roaches are hiding, and then attack them once more at their source.
Mr. Pest Control
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Please note, Mr. Pest Control is answering questions supplied by PMP customers across North America. His answers are generated from industry and manufacturer-provided information. The answer may not be specific to the laws and regulations for your State, Province, Territory or Country. In addition, products mentioned may not be registered and or available in all areas. Always check with your local Univar office for specific information to your area. Always read and follow label directions.