Bt: The lesson not learned

The more things change, the more they stay the same, as a Dec. 29 Associated Press report on genetically engineered corn notes.  Like déjà vu, this news story on emerging resistance to Bt toxin — a fabulously effective and popular insecticide to protect corn — brings to mind articles I encountered over the weekend while flipping through historic issues of Science News.

More than a half-century ago, our magazine chronicled, real time, the emergence of resistance to DDT, the golden child of pest controllers worldwide. Now much the same thing is happening again with Bt, its contemporary agricultural counterpart. Will we never learn?

The new AP story cites rather vague references to the fact that corn genetically engineered to produce the insect-targeting Bt toxin no longer knocks out a major scourge — the Western corn rootworm — as it recently had. These beetle larvae are developing resistance to the toxin (named for its initial source, the bacterium Bacillus thuringiensis). And the worst part: Early evidence of resistance occurs in secret as the voracious larvae again chomp away at roots buried beneath a masking layer of soil.

Although the AP report doesn’t cite research establishing rootworm resistance, it does exist. As I noted back in early August, Iowa State scientists published a report in PLoS ONE about rootworms able to feast on supposedly protected crops. “This is the first report of field-evolved resistance to a Bt toxin by the western corn rootworm and by any species of Coleoptera [i.e. beetles and weevils],” Aaron Gassmann and his colleagues noted. “Insufficient planting of refuges and [genetic] inheritance of resistance may have contributed,” they said.

A few weeks later, Mike Gray of the University of Illinois reported in the Aug. 26 issue of The Bulletin that he recently had been called in to “verify severe corn rootworm pruning on some Bt hybrids.” The concerned farmer had relied exclusively on genetically engineered Bt to protect his corn. When Gray arrived, “[rootworm] adults were numerous and easy to collect. It was also easy to find plants with two to three nodes of roots completely destroyed. A shovel was not required for removing the plants from the soil.”

This brutal pest lops off anchoring roots, after which corn stalks fall over like just so many trunks of felled timber.

Gray advocates tackling rootworms using “a long-term, integrated approach that includes multiple tactics, such as adult suppression programs, use of soil insecticides at planting, rotation of Bt hybrids that express different [toxins], and rotation to nonhost crops.”

In fact, he and other extension agents warn farmers that they must do this if Bt corn is to prove reliable into the future. And there are a range of complements to Bt that can be employed. (I reported more than a decade ago on a particularly innovative one the feds were developing, based on bitter melons.) But growers often go for expediency over long-term investments in multi-pronged and labor intensive crop protection. As Gray observes, “Many producers have relied on a single tactic for too many years, and unfortunate consequences are beginning to emerge.”

The irony: Bt toxin has been part of the agricultural arsenal for nearly a century. Farmers first began employing it — by seeding crops with spores of the parent bacterium — around 1920, according to a website run by Raffi Aroian’s lab at the University of California San Diego. But as spore-based products could wash away or be degraded by sunlight, biochemists sought a more effective way to ensure the toxin stayed with plants. And they found it: incorporation of the gene responsible for making the toxin directly into high value crops.

“The first genetically engineered plant, corn, was registered with the EPA in 1995,” the Aroian lab notes. Already, however, concerns about the invincibility of Bt were emerging in lab studies (see SN: 9/12/92, p. 166). And precisely because Bt toxin had proven such an effective insecticide for so long, crop-protection specialists warned that to safeguard Bt’s potency, growers would have to resist the temptation to overuse it.

That gets back to how we now appear doomed to repeat that history we failed to learn.

While perusing old issues of Science News, I encountered hosts of stories describing heavy and apparently indiscriminate use of DDT.

One January 1946 piece observed that dog shampoos laced with DDT can eliminate fleas for months. An August 1947 article described wallpaper manufacturers adding the chemical to their product so that it would kill flies on contact. And federal scientists had begun evaluating DDT’s safety in the paper used by stores to wrap groceries. A 1949 story described the insecticide’s utility as a treatment for rivers: Just two quarts were needed to deal with fly- and mosquito-infested regions up to 25 miles downstream. Our magazine also prophesied that thanks to DDT (and good sanitation), families could plan on soon kissing their flyswatters goodbye: “We are within sight of a flyless age.”

Five years later, pest control operators were singing a very different tune. Early claims of DDT resistance, initially shrugged off, eventually were shown to be prescient hints that a useful chemical had been overused to the point of abuse. Once a means to kill bedbugs and the lice that carried typhus — a major killer — DDT was quickly losing its potency. Malaria mosquitoes were all but laughing at the insecticide and Agriculture Department entomologists had bred a line of houseflies that could live in a jar coated with DDT (SN: 4/28/56, p. 266).

In 1957, Ralph Heal, executive secretary of the National Pest Control Association, all but conceded defeat. Along with wild houseflies, the German cockroach, bedbug, dog flea and brown tick were all exhibiting extensive resistance to DDT. Where this chemical failed to knock out pests, the newer malathion was proving effective. But Heal added that scientists already feared insects would soon develop resistance to these alternatives as well.

We’d like to think we learn from our mistakes, but collectively society can prove pretty stupid. Or selfish. Or oblivious. In the end, the bottom line is little changed: We still make way too many of the same mistakes.