Well, I guess I will start with the belief that carpet beetles, neither adults nor larvae, could possibly enter a home through the drains, meaning they could not come up from the sewer or septic system by crawling up the pipes and into the house. I have been properly corrected on American roaches before, which apparently DO have the ability to walk through the water barrier that should be there in the P-trap, but little beetles simply could not. Unless, that is, there is no water barrier due either to a lack of a P-trap or a defective P-trap that has no water in it. Even then I'm at a loss to suggest why carpet beetles would be in the pipes below.
So, assuming that you have correctly identified these bugs as carpet beetles there must be a source for them up in the house itself, and the reason they might be getting into the drain could be to feed on accumulated materials there. This still would be terribly out of character for carpet beetles, particularly if the drains are in use and are wet, but a dried drain with clogs of hair in it might be attractive to these scavengers. It's also possible that you may have some other bug at hand. But, the essence of carpet beetle control is to find THE SOURCE, and I hope you will avoid applying any more pesticide until you have discovered where the beetles are coming from. By treating everywhere inside and outside without having a specific problem in mind is not good policy, and as you have found it does not resolve a problem with these kinds of bugs. They are feeding on something that likely is hidden away, and until you find that food resource and eliminate it you cannot achieve control using insecticides.
Carpet beetles are pretty diverse when it comes to their foods. They are an important recycler in Nature, feeding on dead animal materials such as hair, feathers, skin, and the things we make from those materials, such as wool, felt, and other things. It's amazing how these little beetles eventually will find what we have in storage. However, they also feed commonly on grain-based foods, and these include many baking materials or other foods in the kitchen as well as pet foods that may be stored in other rooms, decorative items that use stalks of wheat or acorns or other seeds and grains, as well as rodent bait. I have seen plenty of carpet beetle infestations in old rodent bait tossed into attics and forgotten, or dumped into walls or in a crawl space. Fogging and spraying are not going to resolve these issues if you have not dealt directly with that food source, which in many cases should not be there.
So, it takes a LOT longer and you should charge this customer for your time, but rather than continuing to applying toxic materials to the house in the hope that some of it lands on the source, you should now use a flashlight and a careful and thorough inspection of the entire structure. Somewhere is a food source that the bugs are coming from. Perhaps it really is something in the drains, but take a closer look and perhaps physically clean out the drains by removing the drain stopper and pulling out accumulations of hair. Look in window sills for any buildup of dead insects. Ask if any rodent control has been done here, particularly by the homeowner himself who may have tossed bait in the attic or crawlspace and not removed it.
Take a long look at every package of food in the kitchen cupboard, and in particular packages that are old and have been forgotten. Check in the laundry room or garage and in bedroom closets for pet foods, such as hard foods and dog biscuits, bags of nuts or other things the kids may feed the local squirrels. Look at all decorative items to see if any real plant materials are used in them. Indian meal moth larvae love dried flower arrangements too. Do a careful inspection of the attic and crawlspace to see if there are dead rodents, rodent bait, dead birds, etc., all of which offer food resources for carpet beetles. Try placing some pheromone traps in different rooms to see if you can narrow the search by finding larger numbers in one area. Keep in mind the beetles and their larvae may fall through ceiling fixtures if they are in the attic, or come out of walls if they are feeding on something in a wall void.
But, please.........no more chemical applications until you find a source of this problem and correct that source.
Mr. Pest Control
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