Pill Bug Mystery

Last November, we conducted a prescribed burn at a 3-year old restored prairie.  Two weeks later, I was surprised to be able to photograph green regrowth in that area.  Last week, I revisited the same burned area and got yet another surprise.  Thousands and thousands of dead pill bugs (aka sow bugs, roly polies, wood lice) lay scattered across the ground.

Pill bugs

Dead pill bugs in a prairie burned last November.

cluster

The distribution of dead critters wasn’t consistent across the site, but there were a lot of areas like this with big numbers.

After I found the first few, I noticed them everywhere.  White skeletons of pill bugs, lying on top of the ground – sometimes in large aggregations, other times just a few here and there.  If I’d randomly tossed a hoola hoop on the ground 100 times, I bet I’d have found at least a couple dead pill bugs inside the hoop after 97 out of 100 of those throws.

isopod

I think these pill bugs were probably Armadillidium vulgare, which is a pretty good descriptive name for them.

When I got home, I looked through my photos from last November to see if maybe the dead pill bugs had been on the ground back then too.  If they were still dark gray, instead of white, I might not have noticed them.  Out of about 30 photos, I did find one bug, and it was still dark.  It’s certainly possible there were many more bugs on the ground, hidden by a combination of their dark color and the remaining ash and debris that has since largely disappeared. I’d like to think I would have noticed that many, but I’m not very confident of that.

november

A single pill bug can be seen in this photo from mid-November last year.

I like mysteries, but I also like understanding ecological phenomena.  Pill bugs are detritivores; they feed on dead and decaying material on and below the soil surface.  One possibility is that the dead pill bugs had been feeding above ground, within the layer of thatchy dead vegetation, and were killed by the fire.  A second possibility is that they were belowground during the fire, but came up after the fire (maybe it got too cold and/or dry because the protective thatch was burned away?) and then died aboveground.  There are lots of other possibilities as well, not all of them related to fire.

I emailed photos and questions to several entomology friends, asking for help explaining what I’d seen.  None had ever seen something like this after a fire.  Based on their responses, though, my first proposed scenario (above) seems the most likely.  I just wish I’d looked more carefully after the fire last November, though there wasn’t any reason to do that at the time!  One of my friends also mentioned that I shouldn’t lose any sleep over what happened since the pill bugs are an introduced species (native to the Mediterranean region) and could be having negative impacts on the native detritivore community in prairies.

caught in grass

There were clusters of dead pill bugs in basal clumps of grasses like this one.  Maybe they got lodged here during a strong wind?

This is the kind of thing that keeps me interested in ecology.  Something killed a lot of pill bugs in that prairie.  It was probably related to the fire, but I can’t even say that for sure.  If it was the fire, were there other creatures similarly impacted, but in a less visible way? (We try not to burn entire prairies because of this kind of potential impact, especially on insects overwintering aboveground.) What impacts might the loss of that many pill bugs have on other detritivores, on the decomposition process in that prairie, or on other aspects of prairie ecology?  Lots to ponder; I love it!

Has anyone out there seen anything like this before?  Any other suggestions as to what might have happened?

PLANT GAME ANSWERS:

Thanks to everyone who played the plant game this week.  Over 360 people guessed on the first question, and over 60% guessed correctly that “Widespread Panic Grass” was made up.  Nice work, though I did purposefully try to make the first one easy.

The second question got many fewer guesses, only about 200, as of my writing this.  I got most of you on this one.  Almost half of the guesses were for Yerba Mansa, but that’s a real plant, folks.  It’s a rhizomatous semi-succulent plant in the lizard’s-tail family (Saururaceae) – I PROMISE I’M NOT MAKING THIS UP – and is most common in the southwestern United States, but has at least one record in western Nebraska.

The correct answer for the second question was “Jagged-edge milkwort,” which doesn’t exist.  It’s a little tricky because milkwort is a real thing, but there is no such thing as a “jagged-edge milkwort”.  Only 14% of guessers got it right.  Congratulations to you 30 people!

It looks like people enjoyed playing the game, so we’ll try it again in the near future.

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About Chris Helzer

Chris Helzer is the Director of Science for The Nature Conservancy in Nebraska. His main role is to evaluate and capture lessons from the Conservancy’s land management and restoration work and then share those lessons with other landowners – both private and public. In addition, Chris works to raise awareness about the importance of prairies and their conservation through his writing, photography, and presentations to various groups. Chris is also the author of "The Ecology and Management of Prairies in the Central United States", published by the University of Iowa Press. He lives in Aurora, Nebraska with his wife Kim and their children.
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10 Responses to Pill Bug Mystery

  1. Dave says:

    I think a good game would be a plant ID test. There could be 50 or 100 photos of native flowers or plants with 4 multiple choice answers (A through D) with the scientific and a common name.

    For example: photo of a Lilium philadelphicum (Wood Lily)

    answers:

    A. Lilium michiganense (Michigan Lily)
    B. Lilium lancifolium (Tiger Lily)
    C. Hemerocallis fulva (Day Lily)
    D. Lilium philadelphicum (Wood Lily)

  2. Dave says:

    Chris, if you want help with a plant ID test, I’d be happy to work on it!

  3. Steve says:

    I’ve seen similar appearing (white) pill bugs in plant pots which dried out and were left for several weeks.

  4. Susan says:

    What is the pill bug life cycle? Do they die in the winter anyway? and go unnoticed when there hasn’t been a burn? If not, maybe you are seeing an interaction of lack of cover and lack of food, leading to pill bug death. Check unburned and this burned area for pill bugs next season–are they different?

    • Chris Helzer says:

      Great questions. I don’t believe they regularly die in the winter, but don’t know that for certain. I will be looking at other unburned sites in the next few weeks to see what I find. It’ll be hard to see the little buggers if they’re there though!

  5. Not sure of who in this region would consume the pill bug carcasses, but if that area hasn’t been mostly frozen/snowy since November there and there are consumers I would imagine they would have gotten to them fairly quickly?

    There’s my conjecture.

    P.S. – This blog was fascinating. You are a great writer for broad audiences!

  6. Fascinating to learn that pill bugs aren’t native – I hadn’t realized that. I wonder when they were introduced? I’m guessing that the introduction was accidental rather than intentional, but that is purely a guess. So earthworms and pillbugs are both introduced… Each seems like such an intrinsic part of our ecosystems that it’s difficult to imagine what nutrient recycling was like before they arrived!

  7. shoreacres says:

    I’ve been visiting a prescribed burn site at the Brazoria Wildlife Refuge here in Texas for the past month or so. I got there six days after the burn, and have been going back every six days or so to document the changes.

    On my first visit, I found hundreds of thousands of small snail shells like this. They were everywhere, and just as chalky white as your pill bugs. Most were about 1/4″ to 3/8″ in diameter. By my second visit, on day twelve post-burn, the prairie was covered with fresh crawfish chimneys, and there were dead snails around some of them, as though they had been in the sandy mud that was excavated.

    On my last trip, this past weekend, part of the prairie had flooded because of rain and high tides, and large patches of Hymenocallis liriosme already were blooming. On some of the plants, there were live snails crawling about. I don’t know much about snails, but I do know they didn’t travel very far to get there. My assumption was that the dead snails had been above ground, and were victims of the fire. I don’t have a clue where the live ones came from, unless they were far enough down in the soil to survive. It surely is interesting.

  8. lili says:

    pillbugs have gills, so they need water to survive. Without water they can’t survive and if there was a fire it was no longer moist and they died.

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