August 29, 2026
Yesterday I was out looking for Monarch caterpillars (Danaus plexippus), not knowing if I would actually find any. But sure enough I found one in a patch of Common Milkweed (Asclepias syriaca). A Monarch caterpillar much smaller than I have ever seen in the past. The caterpillar in this picture is just 1/4 inch long, and this one is not newly hatched. They measure just 1/10 of an inch at that point and have no stripes.
There is a key connection between Monarchs and Milkweed plants. Female Monarchs seek out Milkweed plants on which to lay their eggs. Once hatched, the caterpillars only eat the leaves of the Milkweed which holds a milky, toxin filled sap, that is harmful to many animals, but is tolerated and stored not only in the caterpillars but also in the adult Monarch. This toxin creates a bad taste and can cause sickness in predators with backbones (vertebrates), protecting the Monarchs from being eaten.
Nature is amazing.
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Is that a first instar? I think they go through four or five stages before they’re finally mature enough to pupate.