IN 1987, the US declared war on the Russian wheat aphid—a tiny insect that was annihilating harvests, forcing farmers to drench their fields with added chemicals and costing the US millions of dollars each year in pesticides and lost exports.
Entomologists scoured the aphid’s natural habitats from France to Uzbekistan for predators and parasites. When they found them, they bred and released them in numbers to rival a biblical plague. Aphid flies, flower flies, seven species of parasitic wasps and 12 of ladybirds (called ladybugs in the US)—24 different organisms in all were enlisted in a 10-year campaign that was so successful in some regions that farmers were able to put aside their anti-wheat-aphid chemicals.
Increasingly, “biocontrol” has been hailed as a cheap, effective way to deal with insect or plant pests, and a safe replacement for noxious chemicals. Chemicals destroy all insects—good and bad—and contaminate the environment, threatening the well-being of other creatures, humans included. In contrast, biocontrol insects can seem like smart bombs, able to home in on a pest with Terminator-like efficiency. Insects have even been deployed to expel non-native plants from wilderness areas, where chemicals are not an option.
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Biocontrol has yet to catch on in a big way in Europe, although this may change as more exotic pests arrive on the boots of the global economy. But in the US and other New World countries, a whirlwind of foreign arthropods is being unleashed —along with the warm fuzzy feeling that we’re finally working with Mother Nature, not against her.
But are we merely substituting one kind of plague for another? In the past few years, there have been a number of reports of good bugs turning bad. Even one of the hallowed warriors in the war against the Russian wheat aphid, the seven-spot ladybird, Coccinella septempunctata, has gone AWOL, establishing itself well beyond the areas in which it was released, muscling out native ladybirds in the process.
“A hideous ecological lottery” is how Donald Strong from the Bodega Marine Laboratory in Bodega Bay, California, a backer of biocontrol in principle, describes what he sees as the reckless release of insects. Ragan Callaway of the University of Montana in Missoula is also worried. “I’ve always been very pro-biocontrol,” he says. “But now I’m getting more and more negative. The more I read, the more I begin to realise we’re bringing in hundreds of exotic organisms.” Over 1780 different species have been released against insect pests worldwide since 1880, according to Brad Hawkins of the University of California in Irvine, who has compiled the figure with the help of the International Institute of Biological Control database.
No one wants to see an end to the dream of a kinder, gentler pest control, but Callaway, Strong and others argue that biocontrol must be more tightly controlled to avert biological disasters on the scale of the cane toad in Australia, or the mongoose in Hawaii. The toad was introduced to combat sugar cane beetles, and the mongoose to rid sugar plantations of rats. Both have become major pests in their own right.
Cavalier attitude
But whereas no right-minded government would now sanction bringing in foreign vertebrate species to fight pests, the attitude towards smaller critters whose ecological impacts are not so immediately obvious is positively cavalier, say the critics. President Bill Clinton has made it a national goal to use biologically based forms of pest control on 75 per cent of American agricultural land. To this end, the US Department of Agriculture (USDA) is calling on a network of entomologists to identify and distribute biocontrol insects.
There are some rules. In the US, to receive a permit for the release of an exotic biocontrol agent aimed at weeds, you must first run a battery of tests to work out whether your agent will be able to complete its life cycle in its new habitat, and whether it feeds on local plants—particularly endangered species, crops and plants closely related to the target weed. The process can take up to eight years, according to Jack Coulson, a USDA entomologist who is on the committee charged with reviewing the release proposals.
But for critics, the review procedure is flawed. Except where endangered native species are clearly at risk, the USDA can grant permits if any potential risk to a non-target plant is outweighed by the cost posed by the pest plant—a situation that is seen as biased towards agricultural profits. Worse, insects released to fight other insects need not be tested at all—a discrepancy rooted in the fact that if these critters go astray, they’re unlikely to damage anything of commercial importance.
There’s more. In an effort to ensure that biocontrol is effective in the field, its practitioners often test and release many different insect species against the same pest, upping the chances that one will do the job, but also increasing the likelihood that something will go wrong. Nor is there any rule that says you must track the long-term ecological impact of a biocontrol programme. “The temptation is to get on to the next project,” says Peter McEvoy, an entomologist at Oregon State University in Corvallis, who studies the long-term impact of biocontrol projects.
One of the first to suggest that biocontrol has a dark side was Francis Howarth, an entomologist at the Bishop Museum in Honolulu, Hawaii. For the past 20 years, Howarth has doggedly documented suspicious links between the deployment of biocontrol agents and the subsequent demise of native insects or plants. In Fiji in 1925, a parasitic fly from Malaysia, Bessa remota, was set on moths that damage coconut trees. The pests were eradicated from the island within a decade, but another Fijian moth, Heteropan dolens, disappeared over the same period. Other parasitic flies imported into New Zealand to control caterpillars that damage fruit trees have been found feeding on harmless native moths. And in Hawaii during the 1950s, three predatory land snails were pitted against the giant African snail, a foreign import that has been a notorious agricultural pest throughout the South Pacific. One of the introduced snails, Euglandina rosea, feasts on native snails, several of which are now extinct. The list goes on.
For the most part, however, the supporters of biocontrol have roundly dismissed Howarth’s concerns, pointing out that his evidence is anecdotal and claiming that he has exaggerated the extent of the damage to native species. Peter Kareiva, a population biologist at the National Oceanic and Atmospheric Administration in Seattle, understands the defensiveness. The supporters have probably always viewed themselves as environmentalists, he says. “They are providing the alternative to pesticides and they are taken aback. It’s like, `We’re the good guys, what are you picking on us for?'” What forced the good guys to sit up and take notice was a paper that appeared in Science in 1997 (vol 277, p 1088).
Arapaho Prairie is one of the last remaining patches of wild habitat of its kind left in the US central plains and is a haven for such rare species as sand bluestem, a tall grass adapted to sandy soils, and the American burrowing beetle. For over 15 years, Svata Louda, a population ecologist at the University of Nebraska in Lincoln, has kept detailed records of the number and species of thistles that grow there, and the insects that prey on them. In 1994, she identified the egg casings of a weevil she hadn’t seen before on the thistles. At first, Louda thought the weevil was an accidental visitor. But over the next two years, its population exploded. Curious, Louda began reading about weevils and sent a package of the bugs to a weevil expert, who confirmed that the newcomer was Rhinocyllus conicus, a Eurasian immigrant that USDA entomologists had introduced into the US several times since 1969 to remove non-native thistles from ranchland.
In her Science paper, Louda and three colleagues presented unequivocal evidence that R. conicus is preying heavily on at least four native plants growing in Arapaho and other grassland in the region. Among the hardest hit is the Platte thistle Cirsium canescens, a plant found only in sandy and gravelly soils in parts of Nebraska, Colorado and Wyoming. Louda discovered that the introduced weevils were eating between 80 and 90 per cent of the Platte thistle seeds that survive predation by native insects, threatening the plant’s long-term survival.
Things could get worse. Louda and her colleague Amy Arnett recently found during lab tests that the same weevils have a taste for another native plant, Pitcher’s thistle, a threatened species that grows in the sand dunes of the Great Lakes. The weevils have yet to reach this territory, but they are in surrounding states and Louda fears they could easily be introduced if someone tries to eradicate problem thistles nearby. Meanwhile the weevil’s lust for thistles may be having a ripple effect on the ecosystems it has already invaded. Native picture-wing flies, which feed on thistles, appear to lose out in competition with the foreign weevils—an indirect effect that Louda is currently investigating.
And Louda’s weevils are not the only case of biocontrol gone bad. Caterpillars of the moth Cactoblastis cactorum, introduced to the Lesser Antilles in the Caribbean in 1957 to remove native cacti from ranchland, have since island-hopped their way into the US. On an island near Florida in 1989, a local botanist discovered the renegades feasting on a patch of cacti containing the semaphore cactus, Opuntia spinosissima, a species so rare that the few remaining individuals are kept in wire cages to protect them. żěè¶ĚĘÓƵs have followed C. cactorum ever since, and in October Strong reported having seen them on an island off the coast of Georgia.
Cases such as C. cactorum, R. conicus and the seven-spot ladybird may be the tip of the iceberg. The spread of C. cactorum came to light only because south Florida, with its semitropical climate and diverse plant life, is under intense scrutiny from botanists. The runaway weevils were spotted because an ecologist was in the right place at the right time. “That really makes me wonder,” says Dan Simberloff of the University of Tennessee in Knoxville. “How many cases like this have we missed?”
But biocontrol practitioners such as Ernest Delfosse, from the USDA’s head office near Washington DC, say the fears are overblown. Everyone knew that R. conicus posed a threat to native thistles, he says. It was introduced anyway, because the non-native thistles were deemed a greater threat to ranchland—a value judgement he could not rule out happening today. The testing of the potential ecological impact of insects before release, he adds, is far more rigorous today than it was for vertebrates such as the cane toad, guaranteeing that there won’t be any more disasters in store. As for the semaphore cactus in Florida, why should the caterpillar take the blame? “It’s a red herring to say that biological control is placing this species at risk,” he says. “It was placed at risk by land management strategies and lots of other things going back 200 years.”
Good for farmers
Critics are far from reassured. At the very least, they say, the number of insects released should be limited, biocontrol of insect pests must be regulated at least as well as that aimed at plants, and pre-release tests should consider the indirect as well as the direct ecological impacts. Those demands, counter the proponents of biocontrol, would lead to huge costs, bring much-needed biocontrol programmes to a halt, and result in a greater dependence on chemical pesticides.
More rigorous testing could even benefit farmers, says Callaway. Take spotted knapweed, a non-native plant despised by ranchers. Of more than a dozen different biocontrol agents let loose on this weed by the USDA since 1970, one of the most promising has been the knapweed root moth, Agapeta zoegana. But surprisingly, Callaway has found that the grasses cattle eat may actually fare worse when knapweed is under attack. In field plots, the number of seed florets produced by the native grass Festuca idahoensis decreased more than 70 per cent when root moths were allowed to eat spotted knapweed.
Callaway, who reported his findings in August at the Ecological Society of America’s annual meeting in Spokane, Washington, and Beth Newingham, a student in his lab, are currently investigating the cause of this collateral damage. One test shows that knapweed undergoes a compensatory growth spurt after it has been eaten by insects, and that this rejuvenation comes at the expense of neighbouring plants. However, the researchers used leaf-eating cabbage loopers, rather than root moths—a major experimental flaw, say the proponents of biological control.
Despite the apparent stalemate in the debate, there are signs of change. A meeting in October of the International Organization for Biological Control in Montpellier, France, was devoted to considering the possible indirect ecological effects of biocontrol. And last year, New Zealand began operating a 1997 law that prohibits the importation of any species that might cause harm to native species. Meanwhile, the USDA is now promising that follow-up studies will be required for all biocontrol programmes aimed at weeds in the US.
But will those changes be enough to avoid the ultimate irony of seeing biocontrol tarred with the same brush as chemical pesticides? Strong, for one, hopes so. “Biological control is an indispensable tool for use against our most serious threats,” he says. “But if we don’t clean up our act, we’re going to lose the public’s trust.”