One of the most successful forms of genetically modified crops are the species that have been engineered to express bacterial proteins that are lethal to insects that ingest them. These crops have picked up the name "Bt," for Bacillus thuringiensis, the bacteria that originally made the toxins. There are Bt versions of food crops such as corn and soy beans, as well as the commercial crop cotton.

The danger with these crops is that they'll do what every other insecticide has done throughout history: select for the evolution of resistance. In the US, government regulations require that Bt crops be planted along with some fields sown with their non-Bt versions, called refuges. This ensures that any rare resistant individuals will likely mate with non-resistant animals that fed on the insecticide free crops, diluting out the resistance genes.

But China, which grows lots of Bt cotton, has no such regulations. Instead, it relies on the fact that insect pests aren't always picky about the crops they feed on. Neighboring fields of non-Bt corn, peanuts, etc. are expected to provide the equivalent of refuges. Until now, however, that idea has never been tested. But the results of tracking Bt resistance over several years are now in, and they are somewhat mixed. Refuges of other crops do work, but they've only delayed the spread of resistance.

Bt crops have been highly successful in China. Studies have indicated they've reduced insecticide use while reducing accidental poisoning of farmers; profits on farms that used them also increased. However, as noted above, China has been somewhat less cautious with their deployment. It's also somewhat behind the times technologically. In the US and Australia, crops now carry two different Bt toxins, reducing the chances that any pests will evolve full resistance.

In the new study, researchers track the roughly 2.7 million hectares of cotton that are grown in China, mixed in with 25 million hectares of other crops. Data on the frequency of resistant insects are available through the years 2010 to 2013. The authors compare the spread of resistance to estimates generated using a population genetics model.

Population genetics suggests that, in the absence of refuges, the starting percentage of resistant insects (just under one percent in 2010) would have produced a 98 percent resistant population by the end of the study. If the remaining crops were fully effective as refuges for non-resistant animals, however, the percentage would only be expected to reach 1.1 percent.

The results, however, were somewhere in between. By 2013, resistance had risen to 5.5 percent. Thus, the other crops are functioning as refuges, though with an efficiency the researchers calculate as roughly half that of non-GMO cotton.

That's the good news. Part of the bad news is that, over the study period, there was a shift in how the resistance was inherited. At the start, the majority (63 percent) of the animals had resistance that was recessive, meaning the animals needed to inherit two copies of the resistance gene in order to safely eat Bt crops. By the end of the study, 84 percent of the animals had resistance that could be inherited in a dominant fashion, meaning only a single copy of the resistance gene would be needed. Worse still, the dominant alleles do not seem to cause any other health issues for the insects that carry them.

This will allow the resistance to spread much more rapidly in the future. In fact, plugging the data back into population biology models suggests that over half the insect pests will be resistant by 2017.

The authors also note that it could be worse. China only really uses Bt cotton; Brazil uses that plus corn and soybeans and also doesn't pursue a refuge policy.

There is some hope on the way. As noted above, commercial versions of crops that have two different Bt proteins are already available, and the authors of the paper note that a version carrying a third insecticide is in development. Unfortunately, by the time these arrive on the scene in China, they may be facing insects that already shrug off one of the toxins.

Nature Biotechnology, 2014. DOI: 10.1038/nbt.3100 (About DOIs).