Happy Ending

Man often gets his knuckles rapped when he monkeys with the balance of nature. Starlings were introduced into the U. S. to crowd out English sparrows. Now the starlings are a great nuisance in some eastern cities, while the replacement of horses by automobiles hit the sparrows harder than the starlings did. When rabbits were taken into Australia they proliferated enormously for lack of natural enemies. Wholesale slaughter has not suppressed them. Australia also had a distressing experience with the prickly pear -- but in this case there was a happy ending. The prickly pear story, abstracted from a publication of the Imperial Bureau of Pastures & Forage Crops, was told last week in Science by Botanist Francis Ramaley of the University of Colorado.

North and South American prickly pears, members of the cactus family, were taken to Australia in the 19th Century, planted for hedges and as a source of fodder. By 1925 they threatened to crowd out native vegetation on 30,000,000 acres of land, and on 30,000,000 more acres the pears had completely won, standing in a dense, solid growth two to five feet thick. The cost of fighting them with chemicals, by digging or plowing, stacking and burning, would have been more than the land was worth. So, year after year, more land was abandoned, more homesteads deserted.

Meanwhile the Australian Government sent agents to the Americas to see what the prickly pear's natural enemies were. The agents investigated about 150 insects that feed on cactus and nothing else, set a few of the most promising to work in Australia. By far the most potent destroyer proved to be a little moth borer, Cactoblastis cactorum. The larvae of this insect eat the inside of the pear plant, even the roots, and their depredations promote rotting due to bacteria and fungi. Armed with strings of moth borer eggs glued to strips of paper, fieldworkers swarmed through prickly pear land, pinned their deadly eggs on the plants.

Cactoblastis waged its war so well that prickly pear infestation has now been reduced 75% or more. Writes Professor Ramaley: ". . . The scattered remaining plants are not a menace--indeed they are of value for breeding Cactoblastis. Areas of former dense prickly pear are now being used for crops, for dairying and for grazing. . . . [This] land will never revert to its previous useless state."

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