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I'm suspicious that it might be crayfish, which I'd think was insane (I still kind of do), if I hadn't found a dead one around my house earlier this summer.

Is that definitely what it is, or do I have to worry it might be wasps?

I'm located in Michigan.

enter image description here

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It is indeed a crayfish hole (sometimes referred to as a "snakehole" in Michigan, even though it is not from a snake), albeit it has eroded from weather. If you have one in your yard, you likely have dozens, and they will range in their appearance due to soil and weather conditions.

Example 1 and Example 2.

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  • $\begingroup$ I'm inclined to agree. Is this something I need to be concerned about? I prefer not seeing miniature lobsters in my yard, and I don't live near above ground water, so it's odd, but will they cause any problems? $\endgroup$
    – dgo
    Commented Jul 2, 2015 at 15:04
  • $\begingroup$ Also, just to note: we have a sprinkler system and it has been a very rainy spring/summer which could account for the soil being pressed down. It has the right mud-like texture to match crayfish. $\endgroup$
    – dgo
    Commented Jul 2, 2015 at 15:07
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    $\begingroup$ @user1167442 No, they are harmless, and you'll rarely see one. If you don't like the appearance of the holes, you can knock over the burrows and kill them by putting a teaspoon of lye in the hole. However, you'd be fighting a losing battle. If you consider them to be a problem, then better drainage might reduce the number of them next year. $\endgroup$
    – user16391
    Commented Jul 2, 2015 at 23:01
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This is the nest of a cicada killer wasp, Sphecius speciosus.

enter image description here

Harmless, solitary and Cicada-killer wasp holes are ½ half to 1-inch around with thinly scattered loose soil, usually where there is no grass. Cicada-killer burrows have a U-shaped mound of soil at the entrance. Adults emerge June/July.

Source: All the Dirt on Gardening blog

According to the Wikipedia article linked above:

this species occurs in the eastern and midwest U.S. and southwards into Mexico and Central America.

so being in Michigan, you are certainly within the region in which the insect is found.

Parenthetically, also according to the blog from above:

Crayfish holes are 1-inch wide, 2-inches high and made of mud balls.

and the texture of that hole looks more even.

Related Bio.SE question: Cicada Killer Behavior

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    $\begingroup$ Great answer! Perhaps I should change my last name... $\endgroup$
    – AliceD ♦
    Commented Jul 1, 2015 at 1:06
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    $\begingroup$ The reason the texture of the hole looks even is due to the Michigan clay and erosion from rain. This is not a wasp nest. $\endgroup$
    – user16391
    Commented Jul 1, 2015 at 1:44
  • $\begingroup$ Interestingly, I have some experience with Cicada Killers ... See biology.stackexchange.com/questions/9830/bee-identification. I haven't seen any around and I suspect I would've noticed. $\endgroup$
    – dgo
    Commented Jul 2, 2015 at 15:02
  • $\begingroup$ @vallismortis. That is smart. I have a heavy clay based soil with sod on top. $\endgroup$
    – dgo
    Commented Jul 2, 2015 at 15:09

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