Note: This article is a sidebar to this issue’s feature story.
Is releasing exotic insects to control exotic plants, such as the tamarisk, a good idea? The answer depends on whom you talk to.
Scientists who specialize in biological control say exotic plants often explode in foreign soils because they have left behind their natural predators, which attacked their leaves, roots and stems. Bringing in a few of those predators, they say, is the most natural way to slow down the exotics and give the natives a chance to reclaim lost turf.
“Because we bring in part of an ecosystem from another part of the world, it’s right that we bring in some controls from that same ecosystem,” says Jack DeLoach, the U.S. Department of Agriculture entomologist who would like to introduce two species of Eurasian insects to control the Eurasian tamarisk.
In his many papers on the subject, DeLoach highlights some of the stunning successes of biological control. At the turn of the century, for example, scientists released 22 species of insects into Hawaii to effectively control the exotic lantana, an ornamental shrub from Mexico which had become a serious rangeland weed.
But for every successful case of biological control, there are dozens of others where the insects fail to get established or are only marginally successful, or, more rarely and insidiously, where they begin to attack non-target native species. In Hawaii, for instance, the rosy wolfsnail was introduced in 1955 to prey on the African tree snail, an exotic agricultural pest. According to Robert Devine, the author of Alien Invasion, it soon decimated some of Hawaii’s 800 species of non-marine snails, contributing to the extinction of 15-20 species.
Such horror stories make some scientists wary of biocontrol. “An animal in a cage will do something different than an animal in the wild,” says ecologist Bob Ohmart, who is fighting DeLoach’s proposal to release tamarisk-eating insects. “There’s no telling what those insects will do once they eat through all the tamarisk.”
Supporting Ohmart’s fear is a long history of exotic introductions that have gone bad. In the United States alone there are the well-known disasters of forest-smothering kudzu in the Southeast, the bird-killing mongoose in Hawaii, and the aggressive starling, now found virtually everywhere in North America.
Of course, these introductions were not done by qualified biocontrol workers, as DeLoach and others are quick to point out. But that doesn’t mean that even federally approved biocontrol wouldn’t potentially imperil non-target native species. Twenty years ago, scientists introduced an exotic weevil to control musk thistle, an invasive Eurasian plant found throughout much of the West. The weevils have been fairly effective, but they have started attacking native thistles; this worries Carol Spurrier, a Denver-based botanist with the Bureau of Land Management.
“These rare native thistles are very important ecologically,” says Spurrier. “Before all of the European thistles arrived, their seeds were an important food for native birds.”
But Spurrier says not all biological control agents are equally dangerous. Introducing insects to fight weeds that have no close relatives in the native flora is generally much safer, she says. The tamarisk falls in this category.
Debra Eberts, a botanist with the Bureau of Reclamation, says federal scientists who approve insects for introduction to the United States have become more careful over the years. “I’ve seen the approval process, and these people do a great amount of testing. These processes weren’t in place when the musk thistle weevil was approved.”
Greg Aplet, an ecologist for The Wilderness Society in Denver, Colo., agrees that testing has improved over the years, though he says he still sees a lack of follow-up work on released insects.
“There is so little post-release follow-up on what the insects have actually done to the ecosystem,” says Aplet, who in 1993 coauthored a law review article entitled Biological Control: A Little Knowledge is a Dangerous Thing. While many hundreds of insect species have been set loose upon the North American continent, he says, “We really don’t know what has happened to most of them. There could be cases of genetic mutation taking place that would allow an insect to switch its host.”
Genetic mutations do happen in the natural world, says Eberts, but the dangers of that are greatly overblown. “What would be the chance of, say, a ladybug mutating to start eating apple trees? she asks. “It just isn’t likely to ever be a problem.”
Though critics may not take solace in those assurances, the severity of the exotic weed invasion has convinced some of the need for biological control.
“It’s the only thing that really holds promise for the future,” says Aplet, “because hand-pulling weeds and the use of chemicals have either been ineffective or an outright disaster.”
You can contact …
* Debra Eberts, Bureau of Reclamation’s Denver Federal Center, Bldg. 56, Room 2010, Denver CO 80225 (303/445-2217); e-mail: deberts@do.usbr.gov;
* Greg Aplet at The Wilderness Society in Denver, Colorado, at 303/650-5818 or e-mail: greg_aplet@tws.org;
This article appeared in the print edition of the magazine with the headline Fighting exotics with exotics.
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