If you have ever seen what looks like clumps of dirt ascending your walls, you are actually seeing an insect. The “dirt” is actually a case that surrounds a household casebearer caterpillar (Figure 1), which is the larval stage of a Tineid moth (Figure 2). The caterpillar creates its case out of silk once it hatches from the egg. You might have actually been correct in believing the case was dirt because, in some cases, the household casebearer will attach soil particles to the outside of its case. Other materials that the caterpillar attaches to the silk include sand, hairs, and insect droppings. The case serves to protect the insect during its immature stages, and it only foregoes its case once it becomes an adult.
The caterpillar never leaves its case, only partially coming out, using its front legs for crawling. The household casebearer caterpillar expands its case as it grows by adding more silk. If you find an immobile case with both ends closed, this is most likely a household casebearer in its pupal stage. The caterpillar seals each side of the case and goes through its metamorphosis within and will emerge as an adult (Figure 2).
Figure 1. Household casebearer, Phereoeca uterella, caterpillar walking along a surface. Photograph by Lyle Buss, University of Florida.
Figure 2. Adult, female household casebearer, Phereoeca uterella. Photograph by Lyle Buss, University of Florida.
To learn more about the household casebearer, check out the Featured Creatures page!
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This guest post co-authored by Shari Linn, a member of the UF/IFAS Gillett-Kaufman Lab.