The presence of carpet beetles wandering around inside a structure suggests that you have a source someplace inside where the larvae are contentedly munching on their chosen foods, and these foods often are materials that we meant for our own use or consumption and did not plan to share with the beetles. Carpet beetles are one of Nature's main recyclers, and they feed not only on animal fibers and things made from them (hair, feathers, skin, wool, felt) but also on most grain-based dry food products. I myself played the part of Carpet Beetle Rancher for awhile as a large population of them fed for months on a large dog "biscuit" that had fallen where I did not see it. I have seen infestations of them coming from rodent baits left too long in attics. They manage to find wool sweaters, socks, and other clothing stored in boxes or other hidden places, including within cedar-lined chests that people believe will keep them out. They feed on decorations with feathers, on felt pads under lamps, etc. These are very, very efficient insects.
The adults feed on pollen and other materials on flowers, so they are not the damaging stage. But, they will fly into homes and eventually find suitable materials for their larvae to feed on, and this is your challenge. Rather than focusing on killing the wandering beetles or larvae that become visible in the home, you need to seek out the source and deal with it at that level. Any old insecticide is capable of killing these insects, but the larvae are feeding on something that most likely is hidden away and will not be contacted by anything you spray in the air or along baseboards. You and your customer need to work together to start going through all the kitchen cupboards to see if any dried food products are infested, through the closets and other clothing storage areas to see if the beetles are on woolen products there, and even check out decorative items in vases and on walls. You might be surprised to find susceptible materials being used for decoration. Turn over everything sitting on tables and shelves to see if there are felt pads underneath.
You should get up into the attic and into the crawlspace if there is one to see what might be there. This could be rodent baits left in place, dead birds or other animals, or even personal materials stored there and forgotten. Despite their name "carpet" beetles rarely feed on carpets any longer as wool carpets are pretty rare. But, don't overlook that possibility and do question the customer about this. Consider the use of pheromone traps specific for carpet beetles to see if you can narrow the search by finding larger numbers of the adult beetles in certain locations.
But, try to avoid any applications of insecticides until you are able to focus those applications where they will do some good. Even when you find the source it should be cleaned up, disposed of, laundered, or whatever is appropriate to eliminate the larvae feeding on it, and then the area vacuumed thoroughly and perhaps treated lightly to eliminate hidden larvae.
Mr. Pest Control
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