Science: Pest Against Pest

Few plants are as useless, ugly and loathsome as the creeping puncture weed that straggles haphazardly across most of the western U.S. The puncture weed’s burrlike seeds can flatten bicycle tires, foul up cotton-picking machinery, rip through horsehide and gouge cattle. Humans get stabbed by the burrs when they garden, walk barefoot or when they pitch in to a harvest. Even the puncture weed’s scientific name, Tribulus terrestris—”earthly bed of spikes,” takes account of the tribulations it causes.*

Last week biologists were well into the decisive battle of a long campaign to bring the prickly puncture weed under control. From Texas to California, they were turning loose a species of weevil that destroys puncture-weed seeds without harming other plants.

Delicate Balance. Merely finding the selective weevil took some astute biological detective work. Entomologists James K. Holloway of the U.S. Department of Agriculture and Carl B. Huffaker of the University of California pored over old records and learned that the plant had not been found in the U.S. before 1903. Then they traced it to its native habitat in Italy and Spain, were surprised to find that on home grounds the weed did not thrive as it did in the U.S. Searching for an explanation, the biologists discovered that the puncture weed is peculiarly susceptible to a particular European pest called the puncture-vine weevil—a quarter-inch brownish beetle with a snoutlike head. The weevil’s life cycle is inextricably linked with the growth of the puncture weed.

During the summer adult weevils lay eggs in the seed pods of the puncture weed. In the spring the growing larvae feast on the seeds, killing them. Later the weevils even develop wings for a short time and follow seeds that the plants may have thrown to the wind. If there is a large crop of seeds, the weevils flourish along with their food supply. If there are more weevils than plant seeds, the little bugs simply die off. Thus nature maintains a delicate balance that allows neither the puncture weed nor its weevil to stir up a population explosion.

Occasional Bite. For five years Holloway and Huffaker tempted the weevil with other plants. They found that the worst the weevils would do was to take an occasional bite out of alfalfa or flax if there was no puncture weed around, but they also discovered that the weevil’s larvae could only grow in the pods of puncture vines. Convinced that the weevils were safe enough for large-scale experiments, the biologists imported 15 from Italy early this year and turned them loose to feast on a private garden of puncture weed. Now they have a crop of more than 100,000 weevils which they are distributing in the Southwest. Says Holloway: “If one weevil had been imported into this country at the same time as the weed, there would never have been any trouble to start with.”

* Other names for the puncture weed: goat-head, Mexican sandburr, ground burr nut.