Organisms introduced to control agricultural pests may end up permanently altering indigenous ecosystems, new research reveals. The results, announced today in the journal Science, suggest that the long-term effects of natural alternatives to chemical pesticides need to be carefully considered.

Jane Memmott and M. Laurie Henneman of the University of Bristol studied the effects of parasitic wasps and flies (parasitoids) brought in to protect against agricultural pests on native leaf-feeding caterpillars, which were not the intended targets. To assess these nontarget effects, the researchers mapped the web of interactions among numerous species of plants, butterflies and moths, and parasitoids in the Alakai Swamp, an isolated wilderness preserve on the island of Kauai, Hawaii. They found that parasitic waspswhich kill their prey by laying eggs inside the victims bodyhad invaded roughly 10 percent of the 2,112 caterpillars collected during a two-year period. More than 80 percent of these wasps were biocontrol agents originally introduced in lowland agricultural fields; only 3 percent of the parasitic insects were native to the region.

The new research hints at the ability of an introduced species to take on a commanding role in an isolated ecosystem. Since 1901, parasitic wasps and flies have been released at least 122 times in Hawaii. "Some of the biocontrol agents released in early biocontrol programs have left the agricultural habitats in which they were released, and turned to attacking native species," Memmott says. "However, no agents released post-1945 were found in the web, suggesting that biocontrol may be much safer today than in the past."

The food web approach can only provide a snapshot in time, the scientists say, and it is impossible to understand the complexities of a system with only two years of study. And, although it is encouraging that later biocontrol agents were not implicated in the study, "there is little doubt," the authors conclude, "that the community structure has been altered considerably from its original state.