In recent years, a number of crops have been engineered to carry a gene from Bacillus thuringiensis, a bacterium that produces a protein that is fatal when ingested by insects. These so-called Bt crops can get by with far fewer pesticides, given that insects ingest a toxin whenever they treat themselves to a meal on the crop. The widespread use of Bt crops has provided an unusual ecological experiment, and some studies of its impact have been published in the journal Science. A paper published earlier this year suggested that the reduced use of pesticides is having some unintended consequences, while one that was released this week shows that Bt corn protects neighboring fields where the corn is Bt-free. Meanwhile, a third paper in PNAS suggests that genetically modified crops may have impacts well beyond the areas we dedicate to agriculture.

We'll do the good news first. In the US, corn (maize) is the preferred meal of the European corn borer Ostrinia nubilalis, a species accidentally introduced in 1917. Bt is highly effective against this pest, which is great, but it presents a dilemma a bit like that posed by antibiotics: if it's overused, the selective pressure for resistance to the toxin would probably make it ineffective in short order. Fortunately, the US EPA did a smart thing: it mandated that between 20 and 50 percent of a farm's crop must be Bt-free, providing a refuge for susceptible insects.

Corn borer problems have generally been cyclical, with population booms and crashes dictated by a naturally occurring fungus. Since the introduction of Bt corn, however, the booms have largely stopped. Population surveys show that the borer levels have dropped and have continued to remain below historic levels for over a decade now.

The authors then performed an economic analysis, calculating the value of the increased yield of Bt fields while subtracting the higher price of the seeds. They estimate that, in five midwestern states (Illinois, Iowa, Minnesota, Nebraska, and Wisconsin), Bt corn has netted farmers who used it about $2.5 billion. But those who weren't using Bt corn also benefitted, since the population of corn borers has remained low. And there, the authors estimate that the benefit has been even larger—over $4.3 billion, for a total of $6.9 billion over the past 14 years.

So, at least when it comes to its primary pest, Bt has been good news.

Cotton is more complicated

Corn isn't the only crop that's seen the Bt gene inserted into its genome. Currently, 95 percent of the cotton growing on 3 million hectares in Northern China is Bt-cotton, and a study cited by this one indicates that it's highly effective against its primary target, the cotton bollworm. Again, indications are that this protects non-Bt corn, peanuts, soy, and vegetables by acting as a "dead-end trap crop" for cotton bollworm—when the moths lay their eggs in the Bt cotton, the hatching larvae are immediately killed, so they never have the chance to reach adulthood and attack other crops.

However, the effect of Bt cotton on non-target pests turns out to be not quite so positive. Mirid bugs have historically been considered minor or occasional pests in Northern China. However, mirid bug populations and infestations in cotton and other crops have gradually increased as the proportion of Bt cotton has risen. The decrease in insecticide use permitted by Bt cotton has had the somewhat ironic effect of reversing the ecological role of cotton in Northern China from a dead-end trap crop for mirids to a source of them.

Mirid bugs move from their early season host to crop fields in mid- to late-June and, once in the crop fields, they build up their populations. As cotton is one of the few flowering host crops during that period, mirid bugs love it. Spraying against H. armigera used to destroy early mirid populations, limiting its impact on later crops. But now that Bt cotton doesn’t need to be sprayed, the mirid population has thrived and spilled over into other flowering crops, so spraying against mirid bugs has had to increase.

It's not just in the fields

After harvest, maize detritus—leaves, cobs, and stalks, all expressing the Bt protein—is left on agricultural fields. Throughout the upper Midwest, these fields are riddled with tile drains to prevent flooding by transporting water, along with any associated detritus, through stream networks and ultimately to the Mississippi River and the Great Lakes. The concentration of the product of one Bt gene, Cry1Ab, has been quantified in detritus on land, where it can be detected for at least seven months, but not yet in water. A recent report assessed whether Cry1Ab makes it into the stream networks crisscrossing the Midwest.

They looked at 217 stream sites in an intensely farmed northwestern Indiana county six months following the maize harvest. While 67 percent of the stream sites contained maize leaves, only 13 percent of the sites contained Cry1Ab expressing maize, and 23 percent of the sites had Cry1Ab dissolved in the water. All of the sites with maize detritus were within 500 meters of planted maize, but otherwise the sites were randomly distributed across the stream network.

Geographical Information Systems (GIS) analysis indicates that 91 percent of the stream length in Indiana lies within 500 meters of a maize field, and the same holds true for the other Corn Belt states of Iowa and Illinois. Bt maize detritus that contains Cry1Ab can be transported by these streams, and can remain submerged for at least six months. The authors of this study suggest that Cry1Ab’s ecological consequences on nontarget organisms, such as the caddisflies, earthworms, and isopods that inhabit the streams, should be examined.

Bt crops are grown in more than 20 countries and, as the first study makes clear, they have the potential to provide enormous benefits. While the crops' landscape-level impact on target pests, nontarget natural enemies, and our favorite species (the monarch butterfly) have been analyzed, not as much work has been done to track their impact on nontarget insects. Clearly, such work is essential to ensure the sustainability of the transgenic crops many hope will safely feed us in the future.