Thanks to the wasp’s strategy, a field of badly farmed cassava will be filled with male wasps. Since males don’t lay eggs, they pose no threat to the mealybugs, which have a chance to quickly rebuild their population. “We’ve told the farmers, ‘Look, biocontrol can only work when everything else is in good shape. If you don’t weed your field, nothing will work.’”
Herren told me the story of the cassava mealybug one sparkling day in Nairobi. He had moved there in 1991 to become the director general of the International Center for Insect Physiology and Ecology, a massive complex on the outskirts of the capital with sculptures of dung beetles out front. The job is one of his many rewards for having saved the staple crop of 200 million people. The center is filled with entomologists trying to find ways to use insects to make human life better by producing honey and silk and by destroying pests. A stem borer has been chewing its way through the corn of East Africa, but Herren’s scientists have found a wasp from India that parasitizes it. When I visited, they had already set it loose in Kenya to see whether it would survive in the wild. It did, and now they had no idea how far it had spread. And that sort of ignorance was fine with them.
Lafferty and Kuris wanted to do for the green crab what Herren had done for the cassava mealybug. They knew that in Europe many green crabs were plagued by parasites such as Sacculina, but the crabs they dissected from San Francisco Bay were parasite-free. That might be one of the reasons why it could outcompete other crabs in its new home. So Lafferty and Kuris began to contemplate bringing Sacculina to California as well. Sacculina-infected green crabs could be dropped into the Pacific waters. They would act like miniature parasite crop dusters by spraying Sacculina larvae into the water. The larvae would seek out uninfected crabs, burrow into them, and spread their tendrils out. Bringing Sacculina to California wouldn’t have the same effect as the parasitic wasps had on cassava mealybugs, because the biology of the two parasites is very different. The wasp kills its hosts by devouring their innards and then chewing its way out of their bodies. Sacculina doesn’t kill its green crab hosts, but it does castrate them and then make them compete for food with uninfected crabs. Lafferty built mathematical models that suggested that if Sacculina came to the Pacific, it would make the crabs decline, but more slowly than the cassava mealybugs. It would be the missing crab eggs that would bring down their numbers, rather than dead crabs. So when Sacculina and the green crab finally reached an equilibrium with each other, the crabs would be reduced but not gone.
But to Lafferty and Kuris, it didn’t seem as if there were any other choices. “All other alternatives are way worse ecologically,” says Kuris. “Antibarnacle paint on boats is polluting our estuaries in a major way. Up in Oregon there’s someone in a crop duster spraying mud flats against ghost shrimp, to protect the goddamned introduced oyster production. It’s killing Dungeness crabs.”
For a few years, Lafferty and Kuris couldn’t drum up any funds to study Sacculina, but by 1998 the green crab had reached the shores of Washington State. It was poised to move into Puget Sound, with its huge Dungeness crab fishery. At last Kuris and Lafferty got the money they needed. They contacted the world’s expert on Sacculina and related parasitic barnacles, a scientist in Denmark named Jens Høeg. Høeg sent them coolers filled with infected green crabs packed in ice.
Mark Torchin, Kuris’s graduate student, set up the crabs in a quarantined room. He couldn’t simply seal off the room completely, though, because the crabs and the parasites needed circulating sea water to survive. Torchin built pipes that pumped sea water in from the ocean; the water poured into a group of tanks, and the overflow, which might carry the invisible parasite larvae, traveled through a series of filters and tubs of gravel before pouring into an outgoing pipe headed for a nearby lagoon.
For months, Torchin slowly got acquainted with Sacculina and its bizarre life cycle. He figured out how to recognize when a crab was getting ready to release a new batch of parasite larvae from the sac on its abdomen (the sac would turn from butterscotch-colored to a dull caramel). He would put the crabs in little plastic cups to collect the larvae, and then he’d siphon off some of the Sacculina-laden water. He would pour it into another cup with a healthy green crab and wait for the female Sacculina to get into its new host.
Each day he would grab a crab by the claw and pinch it with his fingers. To escape, the crab would sever its own limb from the inside and drop back into the water. Torchin would take the limb to his microscope and look for larvae grabbing onto the hairs of the crab’s claw and digging into the soft pits that anchored them. When a female Sacculina succeeded in infecting a crab, he’d let it develop into a knob on the crab’s abdomen, and then he’d try to get males into it.
After a few months, Torchin was able to shepherd Sacculina from larva to adult. Then, at the beginning of 1999, he applied what he had learned to native California crabs. He chose the common shore crab, Hemigrapsus oregonensis, and exposed it to Sacculina. This was probably the first time in the history of these two species that they had ever met—a crab from California and a parasitic barnacle from Europe. Torchin waited to see what would happen.
A female Sacculina, he discovered, had no trouble getting inside the shore crab. It could even send its tendrils out through its new host’s body. But then something went wrong. In a European green crab, the parasite can carefully wind its tendrils around the nerves, not only causing no damage to them but passing mind-altering signals to their host. In the shore crab, though, Sacculina’s tendrils just seemed to destroy its host’s nerves. Torchin would come in some mornings and find shore crabs on their backs, still breathing but completely paralyzed. Within a few days the infected shore crabs died, and Sacculina died with them.
The biologists had come up hard against the trouble with parasites: their flexibility. Parasites may become specialists on a single host thanks to their evolutionary arms race. But that doesn’t always mean that a parasite can’t use the same tricks to infect another species. If it should come across a new host with a similar physiology and a similar way of life, it may be able to eke out an existence in it. The parasite may simply never get a chance to try out that new host because of its ecology: if a species of tapeworm lives in a stingray in the Amazon, it probably won’t get a chance to try out stingrays in New Guinea. But sometimes parasites do get a chance—when, for example, continents slam together and animals on one of them colonize the other. That, in fact, seems to be how parasites survive through mass extinctions that claim so many of their hosts. They just jump from one host to a new one.
And so parasites carelessly introduced to new habitats can cause disasters, for all the reasons that make them so impressive when they work well. They have a sophisticated set of tactics they can use against their hosts, and they can fine-tune them through evolution to take on new hosts and new defenses. And once they get into a new habitat, there’s no way to get them back out. It is a one-way experiment.