White caterpillars with black spots were found in this woman’s house in Ypsilanti, MI, and she would greatly appreciate our assistance in identifying them. She has found three so far, crawling under and on top of her couch, and even under some cushions.
Fortunately, further context was provided in conjunction with the picture, which we greatly appreciate. Our reader mentioned that she has a “minor garden”, but no houseplants indoors, and adds that it is rare that she finds caterpillars on her vegetables. She also states that she has never seen caterpillars like these before, either in her house or neighborhood. Our reader speculates that perhaps this could have arisen from moths wandering into her home, but we do not think this is the case.
Though this worm does resemble a caterpillar, we believe it is, in fact, a figwort sawfly larva (or Tenthredo scrophulariae). When fully grown, figwort sawflies resemble wasps, though can be distinguished by their orange antennae. Additionally, unlike wasps, these creatures do not build nests. This is, in fact, a curious case, and our reader has a right to be confused at discovering this unfamiliar caterpillar. Figwort sawflies are predominantly found in Europe, including the British Isles, Belgium, and other countries, but not in the United States. However, that is not to say that it is impossible to find alien species in one’s country. Given how much produce is imported from other countries, it is possible that this species of sawfly is only now spreading through the US.
One may argue that the worm in the picture also resembles a species of sawfly that is indigenous to Michigan (see image below from the Michigan State University’s department of Integrated Pest Management), and this would make more sense. However, if one compares the pattern on the back of the larva in the photograph sent in by our reader to the pattern on the larva in the photograph below, the difference becomes apparent. In the photo from our reader, the dark patches are more circular, similar to the figwort sawfly larva, while the one in the image below has stripes formed by small oval-shaped splotches.
So, why would there be figwort sawfly larvae in Michigan? What makes us come to that conclusion? It comes down to the figwort sawfly’s diet. The larvae of the figwort sawfly feed on leaves, and specifically prefer figwort plant leaves. Interestingly, according to Britannica the figwort plant was actually brought and introduced to Eastern North America (which Michigan is a part of) to be cultivated. It is perhaps possible that figwort sawflies migrated to Eastern America via the importation of the figwort plant.
Furthermore, the larvae of the figwort sawfly can cause significant damage to crops, and should thus be moved to a different location where they can feed off leaves in peace (perhaps a forest). When disturbed, these larvae curl up in a S-shape, but pose no threat in terms of being venomous. Even the grown sawflies do not sting like wasps do. However, we still advise handling them with caution, just as with any larva, insect or creature. Allergic reactions can still occur that one is unaware of, so we recommend using garden gloves if picking them up. If our reader finds that these larvae become a pest in her garden, Pests.org provides detailed steps of how to deal with sawflies in the garden.
To conclude, the caterpillars our reader in Ypsilanti found were figwort sawfly larvae. They pose no threat to humans or pets, but definitely do toward crops and plants. Removing them from one’s home is easy enough, especially in the case of our reader who does not have any houseplants, but dealing with a sawfly-infested garden can be more tricky, and thus we encourage our reader to follow the link above if this becomes the case.