Parasitic wasps that were introduced into Hawaii more than 50 years ago to prey on sugarcane pests are now dominant players in the food web of a remote native forest. Experts say the finding, reported in the 17 August issue of Science, justifies more stringent prerelease evaluations of biocontrol agents, including tests to assess the number of species they attack.
The introduction of parasitoid wasps, which kill other insects by laying eggs in them, is a popular strategy for trying to control pests. There have been hints that such wasps--for instance, those introduced to combat the gypsy moth in the eastern United States--can harm native species. Ecologists M. Laurie Henneman and Jane Memmott of the University of Bristol, U.K., wanted a more extensive idea of how the biocontrol insects interact with native species in the Hawaiian food web.
Henneman and Memmott went to Kauai's Alakai Swamp--a boggy forest much higher, cooler, and wetter than the lowland fields where more than 122 parasitoid species have been released in the last 100 years. In the swamp, Henneman collected moth caterpillars from 58 species and reared 2112 of them in the lab to see if parasitoids would emerge. All told, the researchers estimate that about 20% of the caterpillars had been parasitized. A whopping 83% of the parasites were biocontrol agents.
All three species of biocontrol wasps found in the caterpillars had been set loose more than 50 years ago. This makes it difficult to assess whether the wasps have damaged the swamp ecosystem, because little is known about the original community. Memmott says that the native moths have most likely been attacked for decades and some can sustain the rate of parasitism, while others may have been driven to extinction. But she is encouraged by the absence of more recently released parasitoid wasps. "It means that biological control is much safer today," she says.
But others aren't sure of that. Frank Howarth, an entomologist at the Bishop Museum in Honolulu, points out that the more recently released insects simply may not have arrived yet at Alakai Swamp. The finding "is a call for safer practices" and for finding specialized parasitoids that will devour only pests, adds Robert Pemberton, an entomologist and botanist at the U.S. Department of Agriculture's (USDA's) Invasive Plant Research Laboratory in Fort Lauderdale, Florida.