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Caterpillar’s Diet Determines its Appearance, Researcher Finds

February 3, 1989

WASHINGTON (AP) _ To a small caterpillar living in oak trees in the Southwest, the old saying ″you are what you eat″ is not just wise advice - it’s the code of survival.

Erick Greene, a researcher from the University of California at Davis, reports today in the journal Science that the food eaten by the caterpillar nemoria arizonaria causes bodily changes allowing the insect to mimic the seasonal appearance of its oak tree home.

In effect, the caterpillars hatched in spring eat their way into bright yellow and green outfits resembling a flower, while fall caterpillars munch on leaves to assume the subdued gray-green appearance of a plain twig.

The survival strategy, said Greene, probably developed as camouflage concealment from hungry birds.

″Both fall and spring caterpillars have identical genetic material,″ said Greene. ″The spring brood and the fall brood look exactly the same when they hatch. Their later change depends entirely on what the caterpillar eats. Diet cues turn on or off the right set of genes so that they turn into the right shape at the right time of year.″

Caterpillars, or larvae, hatched in the spring come out and immediately begin feeding on the oak catkin, or flower. Soon, these caterpillars turn bright yellow with green dots and develop the fuzzy, segmented shape that mimics the oak blossom.

After a few weeks, the caterpillar turns into a pupa and later into an adult moth. The adult quickly mates, lays eggs and dies, Greene said.

The new eggs soon hatch and caterpillars identical to the spring brood appear. But once this group start feeding, there are quick changes.

Greene said that by the time the fall brood appears, the oak tree has lost its blossoms and has leaves. The new caterpillars nibble on these leaves and soon develop a smooth, gray-green body that resembles the oak twig. Powerful jaws, capable of chewing the leathery oak leaves, also appear.

Oak leaves have a high content of a chemical compound called tannin. Greene said this compound apparently triggers the genetic response to create the fall caterpillar’s twiglike shape and color.

Greene made his discovery by collecting eggs of the insect, hatching them in the laboratory under controlled conditions and then feeding the caterpillars either oak flowers or oak leaves. It proved, he said, that diet made the difference.

″You could give me a spring or fall caterpillar and I could turn it into either form, depending on what I feed it,″ he said. ″If I fed the fall caterpillars spring pollen, they would all turn into the catkin (flower) form. It depends entirely on what the young caterpillars are eating.″

″I just caused a species name to go extinct,″ Greene said.

″Nemoria aemularia, was the summer form,″ he said. ″Now both forms are called nemoria arizonaria.″

The California scientist normally studies birds and said in a telephone interview that he ″stumbled″ into his caterpillar discovery while gathering data on the diet of birds living near Portal, Ariz.

″I was finding lots of caterpillars and I had to raise them to produce an adult form that could be identified,″ said Greene. ″While doing this, I realized that something much more interesting was going on.″

The caterpillars are just bite-sized for birds, said Greene. About half of each season’s caterpillar hatch is gobbled up, he said, and this may explain why the insect developed such a clever camouflage strategy.

″Birds are a major source of mortality for the caterpillars, so being very well hidden can pay off,″ Greene said.

The nemoria arizonaria caterpillar lives in Arizona, Mexico, New Mexico and parts of California. The adult form is a brillant emerald moth with a one-inch wingspan. Greene said it is commonly seen at night on windows and screens in the Southwest.

Science, which published Greene’s study, is the journal of the American Association for the Advancement of Science.

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