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.
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.
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
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
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."
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.
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
"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
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
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
You can contact
* Debra Eberts, Bureau of Reclamation's
Denver Federal Center, Bldg. 56, Room 2010, Denver CO 80225
(303/445-2217); e-mail: email@example.com;
Greg Aplet at The Wilderness Society in Denver, Colorado, at
303/650-5818 or e-mail: