Citrus greening is threatening to collapse the American citrus industry: a cold glass of orange juice or a refreshing mojito could become a luxury commodity. The citrus industry is pouring millions of dollars each year into trying to find a cure for the disease, which causes citrus trees to produce small, bitter fruits with damaged seeds.
Since 2005, citrus greening has laid siege to citrus is Florda. A little bug just a tenth of an inch long—the Asian citrus psyllid—carries the disease, and these critters have been spreading across the country, most recently popping up in California. Now, on the West coast, as Hillary Rosner reports for National Geographic, researchers are hoping to turn one invasive species against another to help stem the spread of citrus greening.
The psyllids in California don't seem to be carrying the disease-causing bacterium, yet. The federal government has a strict quarantine in place on the movement of citrus crops from infected areas to help keep California disease-free. But the psyllids are in California, and the worry is that the disease bacterium could arrive at any time. To help prevent California from sharing Florida's fate, parasitic wasps from Pakistan are being bred in the state, Rosner says.
As part of their life cycle, the wasps, which are even smaller than the psyllids, lay their eggs on the psyllids' bellies. Parasites are, in general, highly specific, and the wasp in question—Tamarixia radiata—only goes after Asian citrus psyllids, not other native psyllids, so far as we know. The researchers were careful to look out for possible ecological side effects before they started releasing the wasps a few years ago. These sorts of safeguards are incredibly important. After all, there are a number of prominent examples of this sort of project gone wrong.
Likely the most well-known example of a biocontrol disaster took place in Australia in the 1930s. In the days before agricultural pesticides, Australia's sugar cane industry was being battered by beetles. To stop the bugs from killing their crops, the Australians brought a hardy predator in from South and Central America, the cane toad. The large, poisonous cane toad has no specialized predators in the land down under, and no diseases to keep it in check. The toads eat all sorts of insects and snails, and their spread over subsequent decades wrecked havoc on Australia's ecosystem. Other examples abound of humans deliberately tweaking the balance of the ecosystem to deleterious effect.
The field of biological control—using one species to keep another in check—is a growing one. Cornell University's Anthony Shelton's biocontrol website lists dozens of wasps, flies, bacteria, fungi, beetles, and other bugs that have been approved for use to control the populations of other species.
Assuming an imported predator or pathogen takes and there's no negative consequences on the rest of the ecosystem, biological control is extremely efficient—far cheaper than constantly relying on pesticides. When it works, biocontrol is great, say Russel Messing and Mark Wright in a review article on the issue in the journal Frontiers in Ecology in 2006:
In successful biological control, the results can be dramatic. Invasives that threaten entire regional economies or vast areas of natural land can be reduced to a fraction of their previous abundance and sustained at low levels indefinitely, without additional cost of management inputs.
The problem is that most introduced predators aren't so picky with their diets, they write.
A substantial number of introduced biocontrol agents do indeed feed on non-target species. In Hawaii, 22% of 243 agents were documented to attack organisms other than their intended targets, while across North America, 16% of 313 parasitoid species introduced against holometabolous pests (insects that undergo complete metamorphosis) also attacked native species.
We don't have a particularly strong track record when it comes to biocontrol, but scientists have been getting much, much more careful in recent decades.
In an ideal world, we wouldn't be trying to tamper with the balance of the ecosystem by bringing in predators from elsewhere. But we don't live in an ideal world—we brought the citrus greening psyllids to America, and now we have to deal with them.