Issue link: https://read.dmtmag.com/i/73811
here is no question about the power of biocontrol. But this is a difficult and dangerous business, requiring much caution and humility. When an invasive species dramatically alters the rules of the game, biocontrol may provide a vital counterbalance. If only all conservation crises were this simple. It was 1920. The invasion of prickly pear cacti (Opuntia spp.) in Australia had become an emergency of national proportions. Millions of acres of grassland and forest were covered with these invaders, leaving domestic grazers with nothing to eat and native species with severely degraded habitat. So it was with high hopes that the Australian government began looking for natural enemies of prickly pears. Their goal was to harness the tightly evolved relationships between these natural enemies and their host plants to knock the prickly pears back to incidental levels. As luck would have it, they found an ideal natural enemy in a moth called Cactoblastis cactorum, and within a few years, the prickly pear invasion was nothing but a bad memory. And the best part was, because there are no native cacti in Australia, the introduction of a cactus-specific herbivore posed absolutely no threat to Australia's native species. But then there is the complex and unfortunate case of the endangered semaphore cactus (Opuntia spinosissima). A prickly pear native to the Florida Keys, the semaphore is in danger of extinction in part because it is under attack by the same biocontrol agent that was responsible for the successful control of prickly pears in Australia. Following on the heels of decades of habitat loss to development, the arrival of the moth in Florida may prove a deadly blow to what was already a quite rare species. The story is that various Caribbean nations imported Cactoblastis cactorum between 1957 and 1970. They released of knowledge in context that determines on-the-ground outcomes. Then and now, the trick to navigating the uncertainties inherent in biological control is taking the time to ask the right questions. And that is the angle we've taken in this article. When you're up against an invasive species crisis, biocontrol can look like the obvious answer. But it's critical to step back and evaluate the appropriateness of biocontrol to a given management problem—as well as the hazards of doing nothing in a world of invasions that are increasingly unmanageable by other means. it into the wild in a misguided effort to control native and introduced prickly pear cacti that had become invasive in overgrazed pastures or that were simply undesirable to ranchers. As in Australia, the reason for introducing the moth was largely economic—but the ecological context was entirely different. The Caribbean is home to many endemic prickly pear species, making it perhaps one of the worst places on earth to introduce a powerful natural enemy of prickly pears. The moth soon migrated to Florida via the horticultural trade and has had devastating impacts on the already threatened semaphore cactus. The key thing to notice here is the questions that were not asked. None of those responsible for importing C. cactorum to the Caribbean ever stopped to ask what the real problem was— and how best to solve it. Biocontrol never should have been used as a "band-aid" for overgrazing, and it never should have been used in an ecosystem with so many native species related to the target pest. Though we know far more about the ecology of introduced species now than we did 80 years (or even 30 years) ago, it wasn't lack of ecological knowledge that created the semaphore cactus disaster; it was the blinders that come with a narrow- minded focus on a limited, nonecological goal. No matter how much we know in theory, it is the thoughtful application Avoiding Collateral Damage The history of biocontrol is blemished with what today are judged as catastrophic mistakes. A few—most of them from the early years of the discipline—are the product of nothing but ecological ignorance. The introduction of mongooses to Hawai'i to control rats, for instance, ignored the basic fact that rats are nocturnal and mongooses are generalists and hunt by day. Many of the more recent examples, though, are less a consequence of ecological ignorance than of changing social norms. Until a few decades ago, many nontarget impacts were generally considered acceptable if the benefits to agriculture were large enough. The first rule of safe biocontrol, then, is to ensure that possible nontarget impacts are not only evaluated with care but also are taken seriously. The case of the non-native thistle-feeding weevil Rhinocyllus conicus is a case in point. R. conicus was introduced to North America in 1968 to control musk thistle (Carduus nutans) and other non- native thistles that are considered significant pasture weeds. It has since proven fond not only of its original non-native target but also of several native (and in some cases, rare) North American thistle species. Biocontrol researchers attempt to prevent problems like this one in part Continued On Page 28