What kind of beetle do I have

Asked January 13, 2018, 2:00 PM EST

I live on a farm southeast of Loveland. I recently have been experiencing an invasion of very tiny black beetles. At first I thought they were mouse droppings, till I saw one move. They are in my kitchen, well away from any food storage, and don't seem attracted to food. They are migrating around the corner on the floor into the next room. So far these are the only places I have seen them, except for a single one here or there that I think got carried by me or a dog. I think they have wings but I have never seen one fly. I first noticed hundreds of tiny yellowish larvae in some drawers on that end of the kitchen. I cleaned them up and kept after it, leaving the drawers empty till I didn't see any more. Then I started to see the beetles. I have a big crack between the house and a sidewalk on along the exterior wall at this end of the kitchen, and a cat recently alerted me to mice in this crack. I have set traps there and caught several. I had also moved a bird feeder out of the rain this fall and it is closer to the house, and I think I turned it into a mouse feeder. I also think mice have carried bird seed into this crack and think maybe this is the source of whatever kind of beetle it is. I think it is either a drugstore beetle or a carpet beetle. I wipe off the counters at that end of the kitchen and the floor there and around the corner two or three times a day and always get dozens of these tiny black beetles on my cloth. They like to climb into the bowl of the stand mixer I have on that counter, and when I left a green plastic bowl there it soon had a dozen or so bugs in it. They don't seem attracted to the windowsills so light might not attract them I need to know (1) what they are and (2) how to get rid of them. I am working on getting the mice out of that crack and sealing it up, hard to do in such cold weather as some sealants don't work well in the cold.

Larimer County Colorado

1 Response

It is a bit too fuzzy a photo to tell for sure, but almost assuredly this is some kind of dermestid beetle (aka carpet beetle). A sheet on dermestid beetles can be accessed at http://webdoc.agsci.colostate.edu/bspm/Hexapoda%20(Insects)/Dermestid%20Beetles.pdf

And I would further guess it is one of the Trogoderma species, maybe the one known as the warehouse beetle, Trogoderma variable.

And I think you seem to have figured out correctly what the situation is. I would guess that the origin of the infestation is primarily the seeds brought in by the rodents, along with other nest debris, that the insects have been feeding on. The yellowish insects you saw were full-grown larvae, migrating from the food in search of sites to pupate. And now you are seeing adults that have developed from the larvae.

And you probably will see adults continuing to dribble out over a considerable period of time, as more develop to the adult stage.

This is a common situation in homes but not one that involves an insect that normally does much, if any, harm to household items. They pretty much limit their feeding to scavenging seeds and dead insects, and are largely a nuisance.

The primary strategy for control starts by removing all the food sources, if you can. Often these insects are breeding on dead insects and rodent cached foods behind walls that are inaccessible. It can help some to seal the openings that allow the larvae and adult beetles to migrate into the living area. And, perhaps, there is some use in spot spraying the site with a household insecticide that can provide residual control for a few days or weeks. (Most such insecticides that are sold for this purpose are called pyrethroids and have names ending with "thrin" - bifenthrin, deltamethrni, cypermethrin, cyfluthrin, and cyhalothrin are common examples of presently available pyrethroid insecticides that have labeled indoor uses).