Oil Beetle

2009 October 24

I was out for a walk with my daughters [1] last Saturday, and we went for quite a long way on the trails through the woods behind our house[2]. And, at the point furthest from the house, I spotted this big, fat insect lying in the grass, stunned by the cold[3]:

As it turns out, I knew what this one was right away, because I had seen something like this before, in 1995[4], and had it identified for me then by Doug Yanega. It is an Oil Beetle in the genus Meloe, most likely either Meloe angusticollis or Meloe impressus


This particular one is probably a female, because her antennae are pretty straight. Males have a pronounced kink in the middle of their antennae, and are a bit smaller than this.


It looks big, fat, and succulent, and it would seem like any passing bird would snap it right up, but appearances are deceiving. As it turns out, they have a good defense mechanism. That huge body is loaded with a blistering agent, so anything eating it is in for a very uncomfortable meal. When they feel threatened, they also ooze the blistering agent as an oily yellow liquid from the joints between their body segments (“reflex bleeding”). That’s why I was handling it with tweezers[5] in the previous pictures, it could have been quite uncomfortable to get that stuff on my hands.


The abdomen is so huge that it has trouble even walking, let alone flying, so the wings are kind of vestigial. There’s just enough of the wing covers left to identify it as a beetle, but not enough to actually do any good.


The head is pretty ant-like, but the antennae are distinctly segmented and are much more like what one expects from a beetle.


These are parasitic beetles, that prey on solitary ground-nesting bees. What they do is this: the eggs hatch out in the spring, into a small, fast-moving larva. This larva goes and hides out in a flower blossom, and waits for a solitary bee to come by. When one does, it leaps onto her while she’s gathering pollen, and then rides back to her nest. Once there, it slips in and waits for her to finish stocking it and laying her eggs. Once she finishes, the beetle larva molts into a grub, and settles down to consuming her egg and all those provisions before pupating into a beetle. The beetle then crawls out of the bee nest in the fall, finds another beetle to mate with, and then they lay eggs. Since the solitary bee nests tend to cluster together in spots with suitable soil, the beetles usually find other beetles nearby, so they don’t have to travel far to find mates. And, since the larvae end up getting carried around by the flying bees, they can get most of the benefits of flying without actually being able to fly themselves, so their wings have atrophied away.

Oh, this is interesting: while the species I have here probably doesn’t do it, there is at least one Meloe species that has a neat trick for getting onto their hosts. Newly-hatched Meloe franciscanus larvae will climb up onto something in a group and form a mass that smells, and to some extent looks, like a female bee. Then, when a male bee flies by, he thinks they are a female and tries to mate with the mass of beetle larvae. At which point the larvae swarm onto him and go for a ride. Then, when he really does find a female bee, the beetle larvae go onto her, and then get carried to her nest. Pretty sneaky.

[1] More of two walks and a carry; Rosie isn’t up to walking much on rough trails yet, so I had to carry her the whole way (luckily, she’s just old enough to ride up on my shoulders). She’s a real speed demon on smooth floors these days, though. Sam can walk in the woods just fine, although sometimes she complains that I walk too fast.

[2] Sam has been naming the trails. The main trail that goes through our back yard and into the woods is “Clover Weevil Road”, the first one branching off into the woods is “Leaf Road”, and the one running under the power lines is “Tall Grass Road”.

[3] It was about 38 F at the time, so I wasn’t really expecting to see many insects, especially considering that there had been a couple of inches of snow on the ground just a few days earlier. Later that day, it warmed up to the 50s and we actually saw a live grasshopper, which was a surprise.

[4] Back in 1995, before we were married, S_ and I were going for a walk on the Baraga Plains, about 40 miles south of here, and we found one of these hanging out at the top of a grass stem. Huge, electric-blue insects kind of draw the eye. Now, one thing we both knew was that a big, fat, uncamouflaged insect like that would normally be just screaming to the world at large, “Eat me! Eat me!”, and so the fact that nothing had eaten in yet was a pretty strong sign that there was a good reason not to eat it. Of course, I go and pick it up to get a better look. S_ asked, a bit apprehensively, “Are you sure it isn’t dangerous?”, to which I said something like, “This is Michigan, nothing is very dangerous”. All this time, I was looking it over, and noting, among other things, the yellow liquid oozing out of the joints between its body segments. Which, luckily, I did not actually get onto my skin. Then, once I got the identification from Doug Yanega[6], I felt pretty stupid once I realized what the yellow liquid was. I apologized to S_ for that later.

[5] Those are special flexible insect-handling tweezers from BioQuip. They are springy enough that you can pick up fragile insects without crushing them, which makes them much more useful than conventional tweezers.

[6] (activating Geezer Mode) This was back when the big way to get your questions answered online was to post it on Usenet. The “sci.bio.entomology” newsgroup had only existed for a short time in 1995, so that was where I went to ask about the beetle. Yanega was one of the professional entomologists who used to read the group, and was very helpful to a lot of people.

27 Responses
  1. October 24, 2009

    I love the head-on shot with the beetle held in forceps. It’s such an odd looking insect.

  2. October 25, 2009

    What in the world is in that mammoth abdomen? Fat cells for energy storage?

  3. October 26, 2009

    Well, the females are markedly larger than the males, so probably a lot of it is eggs. The parasitic lifestyle they lead is pretty much a “famine or jackpot” proposition, with only a few of the eggs actually getting a crack at an appropriate solitary bee, so she is best off producing vast quantities of eggs.

    They are evidently also pretty full of the cantharidin-rich oil that they use for defense.

  4. October 26, 2009

    That particular style of forceps is very nice. I was never able to hold an insect with regular tweezers without crushing it, but these give a gentle yet firm grip. Much better than trying to hold it in my fingers, like I’ve been doing up to now.

  5. December 2, 2009

    Hey this is very interesting cause my garden was full of these this year. Up until this year I have never even seen an oil beetle here in Michigan. Here it is December and I am still seeing them on my porch. Don’t these thing hibernate or die after fall? What is their life span? Would they have come from tomato or pepper plants I purchased from a garden center? We have seen them from the back of our yard to the front, about an acre apart so do they usually stay in one spot, or move around a lot? Also, do they play dead? If you put them in a jar, they curl up for awhile, but pretty soon they are moving again. Just wondering because they are so BIG and scary looking!

  6. December 3, 2009

    They obviously emerge and mate in the fall, but I’m not seeing any specific statements of whether they then lay eggs and die, or whether the females then burrow back underground to hibernate and then lay eggs in the spring. Looking at the “Data” tabs on BugGuide for both M. angusticollis and M. impressus, it looks like M. angusticollis is often found in the spring (meaning they probably hibernate), while M. impressus is found in the fall (meaning that they probably don’t).

    I doubt that they came in with any plants that you purchased, they would have emerged from the burrows of the ground-nesting bees that they parasitize. I think that loose, tilled soil like you have in a garden would be very attractive nesting sites for the bees, so that’s where I’d expect to find the beetles.

    I don’t think it is so much that they play dead, as that they just aren’t very lively in the first place. And the blistering agent that they ooze out of their body segments is probably a good enough defense that they don’t need to do anything to defend themselves.

  7. Debbi permalink
    October 11, 2010

    Just found a large “herd” of these things in an unused garden area of my yard. There were about 30 of them hanging out in the clover. 2 imparticular were doing more than hanging out. In the words of my 8 year old daughter, ” their butts are stuck together” . UGH! I’ve never seen them ever before. I’m in Michigan and Fall is roaring in…

    The girl collected a couple in a jar. I think I’m going to tell her not to. We don’t need any skin irritations or UTIs!

  8. October 12, 2010

    They should be OK as long as she keeps them in a jar, but yes, they aren’t something that should be handled a lot. At least, not without gloves.

  9. Della3 permalink
    October 12, 2010

    I’m guessing a female excreted a mating pheromone into the air that attracted a lot of other beetles to her. At least, this is what the beautiful Blister Beetles do. Watching a horde of them mating is quite a site! But it usually occurs in the Spring.

  10. October 12, 2010

    We have at least two species of oil beetles around here: the species on this page comes out in the fall, while a different species comes out in the spring

  11. cathyrae permalink
    October 20, 2010

    I was digging up a stump in my backyard that was in the middle of what I was creating a perennial garden, this took about a week to complete. During the coarse of this project emerged these large bugs that appear to be the oil beetle. Other than handling them is there any other reason for concern. Will they harm my plants or cat? Will they be around forever….I hope not. Should I try to get rid of them and if so, how. There are hundreds of them and they just walk all around my backyard. But they were most prevalent where I was digging the stump. Years ago I had strawberries growing in that area so maybe I had ground bees, but that has since been grown over by lawn. The bee’s were way to abundant to even try to pick the strawberries.

  12. October 21, 2010

    I don’t think there is any reason to get rid of them. They don’t eat your plants, they won’t come in your house, it takes fairly aggressive handling to make them bleed on you, and if there are no ground-nesting bees in the area they won’t continue to breed there.

    Of course, that assumes that what you have is actually oil beetles, and not something that resembles them but has very different habits. Maybe some kind of bark beetle or wood-boring beetle. Which is possible – there are a lot of kinds of beetles and I don’t know all of them by a long shot. I’d have to see a picture to be sure whether or not they are oil beetles.

  13. Toni permalink
    March 21, 2011

    I think these are the beetles I have in my yard in South New Jersey. Have lived here all my life and always in the garden, just saw 4 of these in the last 2 weeks for the first time. Anyone else see these in NJ?

  14. Elisabeth permalink
    January 23, 2012

    I’m in Indiana, and we saw these for the first time this year. My husband took a liking to them in midsummer when we saw the first few. I lost whatever fancy I had for them in late summer when they completely demolished my chive patch, ate it to the ground in two days. I’m not sure if it will grow back next year.

    Has anybody else experienced this? I’ve never seen a predator in my chives (other than myself). If they go after my onions and garlic next year, it’s war.

  15. Kameron permalink
    February 1, 2012

    im in indy also, came home from dinner tonight to find him/her on my front step. scared to death, i took pictures because i have never seen a bug like this. are they poisonous just to touch? or only if the yellow stuff touches you?? rather neat, yet very ugly…..GO GIANTS!!!!!

  16. February 2, 2012

    Kameron: As long as you don’t get the yellow stuff on your hands, it is no problem. And since it is a blistering agent, if you wash it off right away it still shouldn’t be a problem. It’s only an issue if you handle one a lot, get a bunch of the liquid on your hands, and then don’t wash it off.

  17. Dave V permalink
    November 12, 2012

    My neighbor and I have a bunch of these around our yards right now (metro detroit)… I found your site trying to identify them. I feel bad now because his 5-yr old was playing with them all weekend, I’m going to have to see if his hands/fingers are bothering him. We have both never seen them before, and there does seem to be a lot by the flower pots. Oh, and also a big shout out to Atlantic Mine, I’m a Tech alum and I sure do miss it up there!

  18. November 13, 2012

    Dave V: Your neighbor’s 5-year-old is probably OK, these beetles don’t seem to ooze their juice onto people unless they are really roughed up. It’s eating them that would be really dangerous.

    And Houghton is probably just about the way you remember it – we just got several inches of snow last night.

  19. Rodney permalink
    February 9, 2013

    I found one of these in central Alabama and was wondering if this is normal to see one here as this is the first I’ve seen?

  20. February 10, 2013

    Rodney: The BugGuide range map doesn’t show anybody reporting them from Alabama, but their range maps are generally kind of spotty, and these beetles have been found in Georgia and Tennessee. So they probably are pretty rare where you are, as you are likely right on the very edge of their range. This is the time of year when I’d expect you to find one there, they seem to like cooler conditions.

  21. April 9, 2013

    Thanks for posting the details! I found one of these just out my back door this morning (central rural Ohio, 50 degrees F, 7 am) and when my dog stepped on it the Oil Beetle oozed the golden liquid from all of its visible joints. When we came back it was burrowing under my back step! Luckily I had a good look to reference when Googling “black segmented insect”. I was thrilled to find your post. THANKS again!

  22. April 9, 2013

    Sarah: You’re welcome. It sounds like Ohio is a lot warmer than us – we just had a snowstorm yesterday, and temperatures are still hovering around freezing – so we probably won’t see more of these beetles for some weeks yet.

  23. Breanna permalink
    September 26, 2013

    Thank you that was very well explainatory. You did a very Great job on this!

  24. liz permalink
    November 4, 2013

    I saw two of them in my yard this past weekend in Rhode Island . It was unusually warm. I thought they were carpenter ants at first. Will they harm anything?

  25. November 6, 2013

    Liz: As long as you don’t touch them, they are harmless. The adults don’t eat anything as far as I know. If you handle one, just wash your hands afterwards, and don’t let a pet eat them.

  26. February 23, 2017

    Is it possible to get permission from you to use one of your images (like the oil beetle ventral view being held with tweezers) on a website promoting a project for kids at the San Diego Public Library? We are having kids sign out a bug collecting kit from the library that will be returned for DNA barcoding. I’d like to use a small image on an accompanying website.
    Thank you,
    Mary Ann Hawke PhD
    San Diego Barcode of Life

  27. February 24, 2017

    Dr. Hawke: You are quite welcome to use my images with appropriate acknowledgement on the website for the library project. Thank you for asking.

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