Polka dots! We knew pretty quickly what these caterpillars were likely to be. Last year, we posted about the Spotted Oleander Moth (Empyreuma affinis), a Caribbean species that has been moving its range steadily north for the last 30 years or so. It feeds, as should be extremely obvious, on oleander (Nerium oleander).
The Spotted Oleander Moth is similar in appearance to the Common Oleander Moth, also called the Polka-Dotted Wasp Moth (Syntomeida epilais), both as adult moth and and as a caterpillar. The common oleander caterpillar is familiar to anyone who has oleander in their yards, as a bright orange caterpillar with black spiky hairs (click here to see a picture). The Spotted Oleander Moth caterpillar is also orange, but with white spots and mostly white hairs, with longer black hairs at the front and back.
All parts of the oleander plant are very toxic, so it seems likely that any caterpillars that feed on it would be toxic as well. The bright coloring of the spotted oleander caterpillar is a good indicator of that danger - it's a form of defense called "aposematism". These bright colors basically indicate to possible predators that the organism in question is likely to make them sick if eaten. The hairs of this caterpillar, or setae, are harmless to humans, but likely very irritating to anything trying to ingest it.
The common oleander caterpillar is a known pest on oleander plants, but the spotted oleander caterpillar shown here is not considered to be as destructive as they are not as gregarious. We found only three altogether on our oleander plants, so that seems true.
Not that it matters to us. Since our yard is designed to attract wildlife, we rarely consider any new creatures to be pests in My Florida Backyard. This recent find was a good reminder that summer heat doesn't keep wildlife from visiting the garden, and it shouldn't keep us from visiting either!