Ants, catepillars, and frass (Dave, Lakeland, FL, USA)


I'm not an ant observer, but I recently noticed ants on my sidewalk behaving in a manner I hadn't seen before.

The sidewalk is shaded by an oak tree that is dropping thousands of tiny, hard cone-like kernels about 1/8" long and slightly less in diameter. Simultaneously this tree also drops mature acorns, so I don't know whether the tiny cones are related to the acorn process or not. The ants are also very small - 1/8" or less long.

About a week ago, I noticed occasional tightly gathered 'circles' of cones - usually about 1-1/2" to 2" in diameter, and about one to three or four cones in height. Upon further examination we discovered the circles were being 'built' by the ants, and also discovered that these 'circles' were being built around dead or dying caterpillars. Some ants were continuing to build the pile - while a steady stream of others were going back and forth - presumably carrying bits of the caterpillar back to the nest. After the ants abandon the pile, there is nothing left of the caterpillar.

I suspect this is not an unusual ant activity, but since I know little about ant behavior, I'm curious about the reason for the temporary burial of the caterpillar. Are they hiding or protecting their food-source from other predators - restricting the movement of a still living caterpillar - or what?

Here are a couple of photos - the first showing the cones as they distribute when they fall, and the second shows a caterpillar in the process of being surrounded and covered.

Thanks for your help.

Dave Owen
Lakeland, FL

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Dear Dave,

As you live in Florida, we reached out to a Florida ant expert, Lloyd Davis, and here is what he had to say:

"About this ant behavior: First, the "cones" are caterpillar dung. I suspect from the color of the ants on the caterpillar that the ants are imported fire ants. I have seen fire ants bring dirt onto a glue trap to gain access to food trapped in the glue. This however looks too far fetched. Unless someone watched the process and saw the ants positioning the caterpillar dung, I suspect some other reason for their arrangement and that it is just a coincidence that the ants are feeding on the caterpillar in the middle of the pile."

In the insect world, we often call insect "dung" or waste by the name frass. This may explain why the frass is accumulating near the caterpillar as this is the source.

Red imported fire ants (Solenopsis invicta) are unfortunately common in Florida. You can see photos of them here and here.

To learn more about red imported fire ants you can read some of our previous posts including here and here.

Thank you for contacting AntBlog,
Corrie Moreau & the AntAsk Team

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UPDATE:

Hi Corrie -

Thank you for pursuing an answer to my 'ant' question and notifying me of Mr. Davis on-line response; however I'm still puzzled by a number of points that likely have been explained had I been more thorough in my question.

If the 'cones' are, as Mr. Davis says, caterpillar frass, I am astounded by the quantity, distribution, size and shape uniformity (they look very much like sub-miniature pine cones or a tiny shelled ear of corn). When I spoke of 'thousands' of the little 'cones', it was not an exaggeration. Although no longer happening, when it was the entire portion of the sidewalk beneath the oak tree (roughly 160 sf) would be littered with these 'cones' within 24 hours after the sidewalk was swept. Incidentally, the appearance of the 'cones' started about two weeks before I wrote, and stopped almost totally and suddenly a few days after my previous note. Except when there was a 'circle', the distribution of the cones was almost uniform over the entire sidewalk area beneath the tree - and oddly there never any on the sidewalk beyond the foliage of the oak tree.

I should have noted that I did personally see the ants constructing these 'circles', and there was always a dead caterpillar or other insect in the middle. It was amusing to see these tiny ants (perhaps 1/16" in length is more accurate than 'smaller than 1/8"') swinging the 'larger-than-themselves' cones to and fro as they carried them to the pile. As the 'cone-circles' grew, the cones around the pile almost totally disappeared, defining an increasingly larger 'empty' circle largely devoid of cones (the 'cleared' circles got as large as 6" in diameter from the pile center).

I'm very familiar with imported 'fire ants', and the 'circle-builders' may well be that type, although they seem smaller and darker in color than the many other colonies we constantly battle on our large lot. These ants are almost black in color - with just a hint of red, and both front and back seem to be rounded - rather than 'wasp-shaped'. The fire ants we have in our yard all have piles of excavated sand around them that continue to grow until we poison them. The small ants about which I wrote seem to live beneath the sidewalk, and I've not seen any excavated dirt or sand, although I know it must be there.

Neither my sister nor I have ever noticed this phenomenon before in the many years we have lived here. If it does happen again, I'll try to get more information and a close-up photo of an ant moving one of the cones.

Again, I want to thank you and Mr. Davis for responding to my inquiry.

Dave
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Dave,

Regarding your second inquiry, here is what another ant expert, James Trager, had to say:

"I agree that the "cones" are caterpillar frass, and from their size, would even go as far as to suggest that they were produced by some sort of oak-feeding saturniid. Leaves are not an especially nutritious food, especially tannin-filled oak foliage, so caterpillars have to eat prodigious amounts to fuel their rapid growth rates, from tiny egg to several-inch long caterpillar in just a couple of weeks or so. This explains the rapid appearance and disappearance of the frass, and why all the frass was under the drip-line of the tree. .

Fire ants almost invariably bury large items for butchering. It doesn't surprise me that, in the absence of convenient soil particle, sawdust, or what-have-you, the ants used caterpillar frass for their burying ritual. The smaller, darker description of the ants also fits for a "butchering party" of fire ants. These gatherings usually comprise mostly or only the smaller workers, which are more uniformly dark colored."

I hope this helps!

All the best,
Corrie Moreau, James Trager, & the AntAsk Team

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