Man-Made Defense

Insects are among nature's most successful experts at chemical warfare. To protect themselves against enemies, they secrete many irritating substances. Certain grasshoppers and butterflies, for example, fight their foes with toxins that they accumulate by munching on milkweed plants. Moths pick up noxious alkaloids from other plants. Now it appears that some insects have gone one step further. They have managed to incorporate into their arsenal a chemical made by man.

The latest advance in insect weaponry came to light while a team of Cornell University scientists was studying a flightless Southern grasshopper called Romalea microptera. During egg-laying periods, when the female Romalea has its large abdomen stuck in the soil, and at other times when the grasshopper is vulnerable to attack by ants, it noisily emits from openings in its thorax a foul-smelling, brownish froth that halts predator ants in their tracks. To find out why the liquid is so effective, the scientists, led by Biologist Thomas Eisner, extracted it from several hundred grasshoppers and analyzed its contents. In addition to quinones, phenols, terpenes and other chemicals that are often used in insect warfare, they found an unexpected ingredient: 2,5-dichlorophenol, a derivative of a man-made herbicide.

Further experimentation, they report in Science, showed that although the grasshoppers eat herbicide-sprayed plants with no ill effects, ants will not touch food that has been doused with 2,5-dichlorophenol. Thus, the scientists concluded, the herbicide had become an effective component of the grasshopper's defense--the first known instance of an insect's using as a weapon a chemical "unleashed upon the ecosystem by man."

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