Strange insect encounter: Carrion Beetle with Mites

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I’ve heard of “carrion beetles” but this is more like a “carry-on beetle”:

Amanda and I were outside the cabin in Cass County, Minnesota last week, cutting pieces of plywood for sub flooring, and we saw this creature among the debris. At first I thought it was some kind of wasp covered with tiny spiders, but on further investigation it turned out to be a beetle covered with mites. When we first saw it, there were many more mites than in this photo, and they were virtually roiling on the insect’s surface. It looked almost as though the insect was foaming.

We captured it for later identification, and Amanda ran it down. This is a carrion beetle. There are several species of carrion beetles, all in the family Silphidae. There are over 20,000 species of beetle in North America. Beetles are incredibly diverse. Therefore, it did not surprise me to learn that there are several dozen species of carrion beetle in Minnesota. This is probably in the genus Nicrophorus.

So, what are the mites doing there?

Carrion beetles face a special problem that we see here and there in the world of animals, and when it occurs there is often an interesting adaptive result. (This is how we know adaptation is a real thing and the diversity we see in nature is often the result of adaptatoin.) Carrion beetles lay their eggs in or on the body of dead mammals or birds. But dead mammals and birds are somewhat rare, especially the relatively undisturbed ones, the ones that are not about to be digested by something. Therefore, a carrion beetle that has a trait that maximizes the use of this rare resource will have a selective advantage over carrion beetles that don’t.

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Lots of things lay their eggs in dead critters, or simply find the dead critter’s body and eat it, but many of those species are less discriminating. Think of a simple world in which there is the occasional dead mouse, carrion beetles, house flies, and a large amount of poop. Never mind where the poop comes from, just assume it is there. In this imaginary simplified ecology, house flies will lay their eggs on poop and dead mice, but the carrion beetle, with larva that can only grow on a meat diet, will lay their eggs only on the dead mice. Selection that might lead to the development of a costly trait to monopolize the resource will not be strong for the house fly. If the house fly fails now and then because the carrion beetle out competes it on the mouse carcass, no big deal, there is always poop. But selection that might lead to the development of a costly trait in the beetle to make use of the mouse carcass may be strongly selected for. From the carrion beetle’s perspective, there is a strong possibility that there are already fly larva (maggots) on the carcass eating it when it arrives to lay its eggs, or soon after. This situation is probably made worse by the fact that house flies are quick and seem to be able to travel long distances, while beetles are slower and travel less far than flies in a given period of time. The only way carrion beetles will ever be able to raise their young in this scenario is to do a better job of accessing or using the mouse carcass than the fly does, so any variation that arises in carrion beetles that facilitates this will be strongly selected for.

Check out: Beetles of Eastern North America (Excellent resource)

This is known as the life/lunch dichotomy. In the competition for the use of the meaty carcass of a dead mouse, if the fly loses out it gives up the equivalent of lunch … there are still other opportunities, in this case, poop, for it’s young to eat. The carrion beetle, however, may be giving up its life (or the life of its offspring, really) because mouse carcasses are very rare, so if the one carcass it manages to locate is eaten up by fly maggots, it’s offspring will not survive.

This is where the mites come in.

The carrion beetle pays a huge cost carrying the mites around wherever it goes, because they are heavy and affect its ability to move and fly. But otherwise, the mites do nothing …. they just hang on for the ride, waiting for the beetle to locate a dead mouse. Then, when the beetle does located a dead mouse, the mites do not eat it. Rather, they eat the maggots, the fly eggs, and larva of anything that is not a carrion beetle. They clean the carcass of the potential competitors of the carrion beetle’s larva.

Now, if I can only find a dead mouse somewhere to feed to my new pet and her friends….

More info:

Carrion Beetle Mites
American carrion beetle (Wikipedia)
The Beetles at Cedar Creek Research Station, Minnesota

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38 thoughts on “Strange insect encounter: Carrion Beetle with Mites

  1. Adaptation. Life always finds a way. I am glad that you took the time to look this information up and share it with us. Thank you.

  2. Eww. I hope those mites are specific to the beetle.

    Now why is god torturing that animal? Oh, that’s right – the mites are abusing their free will. yeah, I pwn sophisticated theology.

  3. Your carrion beetle must not exceed a certain size, or must be placed beneath the seat in front of you.

  4. Dan: The ultimate answer to that is evolution … this is a mutualism of a sort. Mites that ate the eggs of their host species had offspring that had fewer host species, etc. At the proximate level, presumably some sort of detection/avoidance mechanism.

  5. Just give that beetle a hunk of meat. Put beetle & meat in a glass jar with a ventilated lid. Hours of fun for the whole family (until it starts to stink).

  6. I remember reading that one of the reasons for the demise of carrion beatles in NA was the extinction of the passenger pigeon.

    Passenger pigeons had the mobility to follow the mast events of oaks and other nut bearing trees so that tree biomass ended up as animal biomass instead of bacterial biomass.

    There is some thought that this might be a mast year for oaks in the Northeast.

  7. I realise that my last post was somewhat incoherent, but then it’s Friday night and Messrs Brahms and Lizst are paying a visit. What I mean is, wouldn’t you rather live in a world where carrion beetles foster mites that eat maggots than one where “he” made them all, low and mighty, the rich man in his castle, all things “bright and beautiful” and ordered their rotten estates?

    What poverty of imagination the other lot must suffer from!

  8. As I learnt just two days ago (from a carrion beetle that was also crawling with mites), they don’t just eat dead mice and such but are also very fond of cat food.

  9. If you really want to try a mouse, most pet stores, including chains, sell frozen mice. Get an adult and let it thaw at room temperature or in warm water. If you want to see a wider range of behavior, put enough soil in with the beetle that she can bury the carcass (if it’s a male I believe he’ll simply wait for a female rather than burying it himself).

    @ #14 daedalus2u

    Specifically it is the largest native species, American burying beetle (Nicrophorus americanus), that is now extirpated throughout most of eastern North America. They prefer chipmunk-sized carcasses, and the elimination of passenger pigeons as an abundant food source is one of several hypotheses proposed to explain their decline.

  10. The mites on Nicrophorus are not always beneficial: Blackman, S. W. 1997. Experimental evidence that the mite Poecilochirus davydovae (Mesostigmata: Parasitidae) eats the eggs of its beetle host. Journal of Zoology 242(1): 63-67.

    The classic paper that showed mutualism is: Springett, B. P. 1968. Aspects of the relationship between burying beetles, Necrophorus spp., and the mite, Poecilochirus necrophori Vitz. Journal of Animal Ecology 37: 417-424.

    but far more extensive work done in the field by D. S. Wilson showed the relationship was much more complex, sometimes mutualistic, sometimes only mutualistic for 2nd generation offspring, sometimes commensalistic (no benefit, no harm) and sometimes parasitic – see:
    Wilson, D. S. 1983. The effect of population structure on the evolution of mutualism: a field test involving burying beetles and their phoretic mites. The American Naturalist 121(6): 851-870.
    Wilson, D. S. & W. G. Knollenberg. 1987. Adaptive indirect effects: The fitness of burying beetles with and without their phoretic mites. Evolutionary Ecology 1(2): 139-159.
    Nice post!

  11. If you requested a dead mouse from a local bakery they’d likely have one in a trap somewhere… though whether they’d own to it is another matter. Also, they’d have all kinds of larder beetles. Stored grains = so many nifty pests.

  12. My favorite spotting of one of these beetles was floating in a swimming pool. The beetle was not moving anymore and the mites were huddled away from the slowly encroaching water. It was “Titanic” in miniature.

    Derek, any suggestion that the beetles/larvae eat the mites?

    I just want to add to the more general audience that these types of interactions, traditionally termed mutualism, almost always vary all over the place in terms of proximate effects. They can range from mutualistic to competitive to parasitic.

  13. While the relationship between beetle and mites is not as black and white as described it is interesting to see evolutionary principles applied to this detail of Nicrophorus behavior. Of course, the beetle’s efforts to secure the rare resource for its offspring go much further – when the pair prepares the carcass for burial they reshape it and may well get rid of or destroy other eggs/larvae. They also stay underground and feed their young larvae – overall a level of involvement that’s rare in beetles and understandable under the premises that you describe in your third paragraph.

  14. Wow, this is absolutely fascinating! I used to feel sorry for beetles with mites. Interesting to know that those mites are actually friends rather than foes.

  15. Why do spider mites do this to other insects that don’t feed or breed on carcasses. I’ve seen them do it on insects that don’t devour meat. There’s quite a lot on YouTube videos saying that the mites feed on creatine when swarming like above. I’ve even seen insects die while they swarm over it.

  16. The mites do eat the insect … The free ride version is a subset of all the possible arrangements.

  17. You are right, I doubted you but you are right. They are phonetic mites that beneficialy ride along for a free lunch on dead prey. Lol

  18. I just found this post while looking for information on a strange experience I just had. A small fly/gnat was flying around me inside the house, so I smushed it on my shirt. Looking more closely at my shirt, I noticed two little mites that had survived the fly’s demise! They looked pretty similar to the picture at the top of this page. Could they be the same or similar mites? Unfortunately the best description I have of the fly is a black smudge. I suppose it could have been a drain fly.

  19. OMG! I found one of these at work about a week ago and was wondering what the heck was up with the beetles little passengers!

  20. This bug, it flew into my apartment…. I thought it was a hornet or a wasp…. Yeah, i killed it…. But then all the mites started running so i was grossed out…. And now i know it buries into dead animals…. I don’t want to be in my room anymore o_o

  21. …. and here was I thinking I had spotted something really unusual when I was out moth watching last night. Thanks everyone for all the information on these mites, truly fascinating.

  22. I just had a rather large nicrophorus sayi nearly divebomb me andnoticed a few mites on its back. I thought it mightbe itsoffspring at first but this blog cleared that up. Thanks!

  23. I would watch these guys bury mice from the shed in my parents backyard in Colorado. I also constructed a trap for fly larvae with different levels and widths of screens. The flys seemed to become frustrated and would just drop their eggs above the mice, or if I was using a simple flank steak, that. I got off track, I only saw the mites a few times..

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