Entomologically-minded readers of the Federal Register (you know who you are) might have noticed an interesting item shortly before Christmas: in the 19 December issue, the Department of Agriculture posted this note asking for the public’s thoughts about genetically modifying insect pests. Specifically, they’re working on inserting some choice genes into fruit flies and pink bollworms, then releasing the re-engineered critters into the environment. I’m sure the usual naysayers will soon be screaming about Frankenflies (which, by the way, would be a good name for a band), but this project could actually be a tremendous boon to the environment.
Pink bollworm life cycle, image courtesy USDA
The focus of the effort is actually a decades-old insect control strategy called the Sterile Insect Technique, or SIT. Unlike most pest control approaches, SIT has the potential to eradicate an insect species completely, without any harm to non-target species. Somewhat counterintuitively, the first thing you do in SIT is to build a hatchery and start rearing thousands of the insects you want to wipe out. Then you kill the females, and expose the males to some chemical or physical insult that sterilizes (but doesn’t kill) them. Finally, you release the sterile males into the wild, where they mate with females, who then lay clutches of eggs that will never hatch.
Because many insect species mate only once in a season, each sterile male that competes successfully for a mate wipes out one female’s entire reproductive output for the year. The next season’s crop of wild flies will be that much smaller, but you release the same number of sterile males, giving them an even better chance of bedding an otherwise fertile female. After several generations of this, the wild population will go extinct, and you can fold up the hatchery and go home.
It’s a stunningly elegant strategy, and even more impressively, it works. Edward Knipling, the entomologist who invented SIT, proved that 25 years ago, with Cochliomyia hominivorax, the screw worm fly. Screw worm (another good name for a band) gets its common name from a habit of laying its eggs on live animals, letting the maggots burrow into the flesh (like a screw) to feed on blood and develop. After pupating, the adult hatches out of the now-festering wound and flies away. It’s very gross, and very costly; historically, screw worm infestations were an enormous problem for livestock farmers in the Americas, especially cattle ranchers. Using SIT, Knipling and his colleagues eradicated the fly from this country by 1982. It’s since been extirpated from several other countries the same way.
Now the Department of Agriculture is trying make SIT more efficient. Using current technology, SIT programs are damnably expensive, mainly because of the challenges of separating the sexes of tiny insects (can you tell girl bollworms from boy bollworms? ten thousand times a day?), and making sure that the sterilization works just right. Too much radiation or chemical treatment, and you get dead males. Too little, and you get fertile males. Neither is useful for SIT.
Genetic engineering could address both problems. Inserting an inducible gene regulation system that selectively kills females in the larval stage, and another that sterilizes males, would make the breeding program a snap: just induce the genes, grow up the flies, and release whatever survives pupation. One could also get more sophisticated. How about building a paternally-inherited, female-lethal gene expression system into the flies rather than sterilizing them? Males coming out of the hatchery would mate with wild females, who would then lay eggs that would only produce more males – males that would carry the same trait to the next generation. This sort of genetic pesticide, if it worked, could wipe out all of the females in a few generations, and the whole species right after that.
Beyond a few interesting laboratory findings, all of this is theoretical right now, which is exactly why it’s good to see the Department of Agriculture asking for comments on it. Maybe there will be an informed public debate about the risks and potential of this approach. Or maybe there will be a lot of screeching and shouting from various corners, followed by a permanent stalemate that keeps this promising technique on ice.