MISSOULA, Mont. — First came the knapweed. Then came the gall fly. And now the mice population is exploding - the mice that carry hantavirus. In a classic case of unintended ecological consequences, an attempt to control an unwanted plant has exacerbated a human health problem. Spotted knapweed, a European plant, is a tough, spindly scourge that has spread across hills and mountainsides across the West. In Montana alone, one of the worst-hit states, it covers more than four million acres. In the 1970’s, biologists imported a native enemy of knapweed, the gall fly. The insect lays eggs inside the seed head, and the plant then forms a gall, or tumor, around the eggs. When the larva hatches, it eats the seeds. Dean Pearson, who works at the Rocky Mountain Research Station of the United States Forest Service, said the fly had not halted the spread of knapweed. In a report in Ecology Letters, however, Dr. Pearson reports that the introduced fly has changed the ecosystem’s dynamics. The fly larvae provide an abundant food source for deer mice in the winter, above the snow. Instead of dying out, as is often the case in cold and […]