WHEN A PLAGUE of tree-climbing aphids afflicted pecan orchards in the southeastern United States in the 1970s, federal biologists released a tree-climbing ladybug from Asia to devour them. The multicolored Asian lady beetle (Harmonia axyridis) did a superb job. Unfortunately, it also revealed two unwelcome traits: the wanderlust of a hobo and the appetite of a gourmand. Swarms of the little beetles eventually marched up the Atlantic seaboard into New England and westward across the Mississippi. Along with another foreign import, the seven-spotted ladybug (Coccinella septempunctata) brought over earlier from Europe, the ravenous Asian insect has eaten so many aphids so fast that many native ladybugs may have been left with too little to eat. To make matters worse, the newcomers are apt to eat the hometown ladybugs, too. Possibly due to this onslaught, even New York’s official state insect, the nine-spotted lady beetle (Coccinella novemnotata), is now extinct in the state.

In the field of classical biological control—the use of exotic natural enemies to counter invasive pests—examples of biocontrol insects that have themselves gone out of control are relatively few. When it works, biological control is more benign, more efficient, and more precisely targeted than the usual method of controlling pest species, which is spraying their general whereabouts with toxic chemicals.

But just as an exotic plant can turn invasive when freed from the enemies that kept it in check back home, so too can an exotic biocontrol insect run amuck itself in the absence of the predators and competitors it evolved with. (The organisms used for biocontrol include not only insects but also other arthropods, viruses, fungi, bacteria, nematodes and other life-forms.) And when good bugs go bad, they can make big trouble. Whereas a chemical pesticide weakens over time, living creatures have a way of multiplying.

“With a chemical spill, you could theoretically recover all the molecules,” says entomologist May Berenbaum of the University of Illinois. “With a biological organism that reproduces, once it’s out there, it’s out there. You can’t round it up.”

The first successful U.S. deployment of a biocontrol insect occurred more than a century ago. An Australian ladybug, the vedalia beetle (Rodolia cardinalis), was released in California citrus orchards in 1888 to combat cottony cushion scale. In decimating the citrus pest and keeping it in check ever since, without collateral damage or becoming a pest itself, the beetle has performed superbly—possibly a little too superbly, suggests Erin Stephens, a Cornell University insect ecologist. “It led people to think they could use biocontrol agents as a cure-all,” she says, “when in fact they just got really, really lucky.”

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