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apparent beneficial effect of Bt-cotton crops

Blogging on Peer-Reviewed ResearchWay back in 2004, the first of the 'new' Scholarship exams asked students to:

Compare and contrast the ecological and evolutionary outcomes of releasing herbicide tolerant and insect resistant GM plants.
It's an interesting question. I suspect that a lot of the answers would have focused on the potential negative environmental effects of releasing these GM plants - the possibility of resistance/tolerance genes crossing to other species. But in last week's edition of Science there's a paper that suggests that GM plants may have a positive impact on their neighbours (Wu et al., 2008).
Transgenic crops are now planted quite widely, and just under 40% of the area planted in these crops  (42.1 million ha) is given over to plants containing some form of insecticide (Wu et al.2008). Perhaps the best-known of these insecticides is the "Bt toxin", originally produced by the bacterium Bacillus thuringiensis. Over the period 1992-2007, the researchers carried out an extensive field study of cotton plants engineered to express the Bt insecticide proteins.
 
Bt cotton is the only Bt crop registered in China; it was introduced in 1997 to help combat the voracious cotton bollworm, Helicoverpa armigera. In the early 1990s Chinese cotton farmers had to put up with outbreaks of H. armigera on a grand scale: crops were affected across entire provinces, and the bollworms produced 4 generations a year. However,  Wu et al. note that [by] 2001, Bt cotton had been extensively planted, especially in northern China, which resulted in increased yields and decreased use of insecticides.
 
Their research area covered 6 provinces in northern China & 3 million ha of cotton, plus 22 million ha of other crops that can also be attacked by the bollworm. They looked at the density of bollworm eggs on cotton & the density of larvae on these other host crops, & found that both were reduced following the introduction of Bt cotton. Bollworm egg densities didn't change over the study period (which would suggest that the Bt toxins don't deter bollworm females from laying their eggs). However, things were different for the larvae (the lifecycle stage that does all the damage to host plants): their densities were significantly higher on non-Bt cotton compared to Bt cotton over the period 1998-2006 (P<0.05). And because fewer moths were hatching, peak egg densities declined over the season. The team concluded, all data indicate that the commercial use of Bt cotton in northern China was associated with long-term area-wide suppression of H. armigera after 10 years.
 
They also noticed that there seemed to be a drop in bollworm populations on plantings of other host species, even though these species hadn't been engineered to express Bt toxins. One possible explanation is that every year the bollworms tend to lay their first lot of eggs on cotton, & then subsequent generations attack other surrounding plants. But because the larvae die in large numbers on Bt cotton, far fewer adults are produced and so fewer eggs are laid on the other plant species.
 
On the down side, Wu & his colleagues remind us that it's always possible for the bollworms to evolve Bt resistance, particularly if the host plants are grown as a monoculture & all express the same toxin. They suggest that one possible way to reduce the odds of this happening is to use high-Bt plants, but at the same time intersperse these plantings with non-resistant host crops, where Bt-susceptible insects will survive. In the US it's actually mandatory to plant these companion species when Bt-producing crops are cultivated. This would be a bit problematic in a country the size of China, because of the logistical problems involved in trying to educate and monitor the very large number of small-cropper farmers. However, Wu et al. conclude that [in] China, a multiple cropping system... is common. These crops also serve as hosts for H. armigera, and because they do not express Bt, they serve as refuges for non-resistant insects. Because cotton is not the only host crop, Bt cotton comprises about 10% of the major host crops in any province... [something which seems to have] warded off the evolution of resistance. However, because pesticide use has dropped, other pests have appeared in the fields. Using GM plants is obviously only part of the solution to the bollworm problem.
 
K.-M. Wu, Y-H. Lu, H.-Q. Feng, Y.-Y. Jiang & J.-Z. Zhao (2008) Suppression of cotton bollworm in multiple crops in China in areas with Bt toxin-containing cotton. Science 321: 1676-1678
 
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