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Excerpts from Jim Conrad's
Naturalist Newsletter
from the January 25, 2015 Newsletter issued from Río Lagartos, on the Yucatan Peninsula's northern coast (~N21.60°, ~W88.16°), Yucatán state, MÉXICO
HIEROGLYPHIC MOTH CATERPILLAR
Last Sunday morning I set off bicycling to Playa Cancuncita to photograph seaside vegetation but about eight kilometers out of town I got a flat. Walking back to town I got to review the current state of our roadside weeds. Something to say about them is that maybe the most conspicuous weed right now is the yellow flowering Waltheria, which we've reviewed at www.backyardnature.net/yucatan/waltheri.htm.
Hundreds of thousands of Waltherias were passed by but I saw only one plant hosting a certain kind of caterpillar, about a dozen of them all of the same species but at various stages of development. The largest caterpillar in the group is shown above.
Volunteer identifier in frigid Ontario was happy to have a caterpillar to work with, and soon she'd figured out that this was the larva of the Hieroglyphic Moth, DIPHTHERA FESTIVA. Hieroglyphic Moths are a tropical and subtropical species found from South Carolina across the US to southernmost California south throughout Mexico to Bolivia and Brazil.
Walking along the road I had plenty of time to wonder about these caterpillars. Usually when such caterpillars are so boldly patterned it's a warning to predators that they taste bad, or else the caterpillars are mimicking other caterpillar species who taste bad. What's the case here? And why were all the caterpillars on just one Waltheria plant among hundreds of thousands, and how come they were of different sizes?
Having the name Bea provided, it was easy to look for answers on the Internet. Yes, probably the caterpillars do taste bad. There's at least one report of the moth being spit out by a kingbird after trying to swallow it. Studies show that Waltheria leaf extract not only may be "hepatotoxic," or toxic to the liver at high doses, but also it's used by the pharmaceutical industry as an "exfoliator" to lighten skin pigmentation. A caterpillar full of Waltheria's potent juice well might taste bad.
So, our caterpillar's bold patterning well may be a warning signal to predators to stay away, and because a reckless bird might kill a caterpillar before learning that it tastes bad, it makes sense for bad tasting caterpillars to hang together -- one sacrificed individual suggesting to the predator that all the others probably taste bad, too. How and why the caterpillars on this one plant could be of different sizes I'm still unclear about.
Though Hieroglyphic Moth caterpillars feed on many species other than Waltheria, Waltheria seems to be its favorite. In Florida, crop damage attributed to Hieroglyphic Moth caterpillars has occurred in pecan orchards and soybean, and sweet potato fields.