“I found these (see attached video) underneath a paving slab, which was in the middle of a small 2m x 2m bed”, writes Sarah in her submission regarding the bright green worms pictured below. “I don’t think they are caterpillars. There was a big mass of them. Are they harmful? If so, could you suggest some options for dealing with them? I live in Cardiff, UK. Many thanks.” To start with, we have to point out that the video Sarah sent us (from which the screenshot below is taken) is really blurry, which makes it near impossible to make out any identifying information about the organisms other than their general shape and coloration. For that reason, we have to warn her that any suggestions we make as to their identity is not going to be 100% certain or accurate.

Now, Sarah states she does not think that these are not caterpillars, and we neither agree nor disagree. We think it is possible that they are. There are no worm species we know of that are this color and congregate in a mass like this. The only worm that comes to mind that is characteristically found in tangled piles like this is the Gordian worm, but they are red or pink in color, not this bright greenish yellow color. It is the coloration that has us thinking these are more likely caterpillars than anything else, though it should be noted that there are certain fly species whose larvae do resemble caterpillars, like the sawfly larva.

With that in mind, we were also thinking about fungus gnat larvae: a species of fly whose larvae are often found in soil as they feed on organic growths that form in it like algae, moss, and fungi. They also tend to pile on top of each other when they are fully-grown and will travel in such a group to find a place to pupate. This is called a fungus gnat larvae train, and it is exactly what it sounds like: a long, writhing, mass of fungus gnat larvae moving as one. It is quite the sight to behold. However, fungus gnat larvae are a translucent grayish color, not this green that we see in Sarah’s video, so we are doubtful of this potential identification. But this got us thinking about other insect larvae that display similar behavior, such as the tent caterpillar.

There are many species of tent caterpillars, but there are definitely ones that are similar in color to those that Sarah found. The only thing that doesn’t add up here is why they would have been under a paving slab in the soil. Tent caterpillar eggs are typically laid on the branches of a tree, which is also where they stay upon hatching. Maybe some caterpillars’ eggs blew off a tree and ended up in the soil. Or perhaps these are cutworms of some kind: a type of caterpillar that does tunnel through soil to chew through the stems of plants.

Without a clear picture of the creatures, the options are too many. With that said, we do not think that what Sarah found is harmful, and when it comes to dealing with them, we recommend moving the caterpillars elsewhere, outside. If she has a garden (which we assume from mentioning the “bed”), the best thing she can do is get protective mesh barriers for her plants, spray them with soapy water to stop caterpillars from being able to climb them, and sprinkle some diatomaceous earth around the bed. We do not recommend the use of insecticides as they are toxic for humans and pets, not only for worms and caterpillars.

In conclusion, we hope we were able to help somewhat, despite not being able to provide a confident identification. We would not rule out the possibility of these being caterpillars, but they could also be some type of fly species. In any case, we do not think they would be dangerous. Of course, allergic reactions can occur, so we do recommend avoiding direct physical contact. We wish Sarah the very best!

Summary

Article Name

Bright Green Worms Under Paving Slab Could be Fly Larvae or Caterpillars

Description

"I found these (see attached video) underneath a paving slab, which was in the middle of a small 2m x 2m bed", writes Sarah in her submission regarding the bright green worms pictured below. "I don't think they are caterpillars. There was a big mass of them. Are they harmful? If so, could you suggest some options for dealing with them? I live in Cardiff, UK. Many thanks." To start with, we have to point out that the video Sarah sent us (from which the screenshot below is taken) is really blurry, which makes it near impossible to make out any identifying information about the organisms other than their general shape and coloration. For that reason, we have to warn her that any suggestions we make as to their identity is not going to be 100% certain or accurate.

Author

Worm Researcher Anton