Some agricultural experts, foresters and conservationists favor biocontrol over chemical solutions such as insecticides and pesticides to eradicate or maintain weeds, bugs, and sometimes disease. Yet, if not tested properly beforehand, in a community-wide context, introduced species can do more harm than the plant or animal they target, the new study suggests.
Jane Memmott and M. Laurie Henneman of the University of Bristol in the United Kingdom bring new scientific insights to the ongoing debate concerning widespread impacts of biocontrol.
The use of biocontrol to fight pests—and even invasive species, the second largest problem in biodiversity and conservation, next to habitat destruction—has clear environmental benefits, Memmott emphasized. But, she added, a biocontrol agent gone wild may have the potential to impose adversely cascading events in the food chain, either directly or indirectly.
Since 1945, Hawaii has had a history of non-native, or exotic species being purposefully introduced into the ecosystem. Memmott and Henneman wanted to examine, in a calculable way, the degree to which certain biocontrol species are interacting with native species. So, they constructed food webs of Hawaiian plants, butterflies and moths, and parasitoids.
The Lepidoptera (butterflies and moths) feed on the plants. Parasitoids, such as alien wasps, lay eggs on the bodies of Lepidoptera caterpillars, causing an early death before the parasite larvae metamorphose into winged insects. Researchers wanted to map out exactly “who eats what,” to keep track of the real impact of non-native species on the Hawaiian island of Kauai.
The food webs, which drew lines between the three members of the food web—plants, Lepidoptera and parasitoids—provide scientists with real numbers to draw conclusions about the proportions of organism types that depended on another for reproduction or food. These food webs allow ecologists to establish how much an alien parasitoid species has penetrated the food chain, and to predict the direct and indirect effects of biocontrol agents on species. These webs open up doors for more meaningful research to take place, according to the biologists.
Memmott and Henneman collected over 2,000 caterpillars, which they checked for parasitoids. Some 20 percent of the caterpillars died from parasitoid attacks, the majority of which were biological control agents people had introduced to fight pests. Accidentally introduced, or immigrant wasps, constituted the next most lethal group of parasites. Because native parasites only constituted three percent of all parasitoids, they were responsible for low levels of mortality, overall.
The food web approach to this ecological question still provides only a “snapshot” of the whole story, Memmott said, because the most vulnerable Lepidoptera species may have already disappeared from the radar screen. The low number of native parasitoids also presents a puzzle, because it is not known if the alien wasps out-competed the natives, or if the natives were already small in number.
“Some of the biocontrol agents released in early biocontrol programmes have left the agricultural habitats in which they were released, and turned to attacking native species,” Memmott noted. “However, no agents released post-1945 were found in the web, suggesting that biocontrol may be much safer today than in the past.”
Memmott and Henneman expressed the “critical importance” of completely understanding the ecological costs of biocontrol species over the species that humans intend to target, in Hawaii and elsewhere.
Support for this research was provided by the Leverhulme Trust Foundation, UK; as well as the University of Hawaii Department of Entomology; the State of Hawaii Department of Land and Natural Resources; and Kokee State Park.
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