Small farmers benefit from transgenic insect resistant crops... but what about cheaters?
Economic impacts and impact dynamics of Bt (Bacillus thuringiensis) cotton in India
The authors write that:
"Bt has caused a 24% increase in cotton yield per acre through reduced pest damage and a 50% gain in cotton profit among smallholders. These benefits are stable; there are even indications that they have increased over time."So far, so good, but what will happen when the pests evolve resistance? The high-dose / refuge strategy seems to be working to slow the evolution of resistance, prolonging the useful lifetime of Bt crops. But...
Success of the high-dose/refuge resistance management strategy after 15 years of Bt crop use in North America can be undermined by "failure to use high-dose Bt cultivars and lack of sufficient refuge."
Getting the dose high enough is the responsibility of the seed companies, but how do we keep individual farmers from "cheating" -- growing Bt crops on their whole farm, rather than allocating some land to the Bt-free refuge this strategy requires? Should we rely on pressure from individual neighbors, or something more organized?
An economic experiment reveals that humans prefer pool punishment to maintain the commons "...where the punishment act is carried out by a paid third party, (e.g. a police system or a sheriff)"
Bacillus thuringiensis, the original source of the Bt gene, also has problems with cheaters. Making the toxin is expensive, so "free riders" (which don't pay the cost of making the toxin) can potentially multiply faster than "cooperators" that make the toxin. But they can only achieve that potential inside host caterpillars that are simultaneously infected by a toxin-making strain. Co-infection is more common at high bacterial population densities, but as cheaters become more common overall reproduction decreases bringing down the population density. So cheaters and cooperators co-exist in the field, as reported in "The Dynamics of Cooperative Bacterial Virulence in the Field."
We have suggested a somewhat similar mechanism for the co-existence of cooperative (N2-fixing) and cheating rhizobia: host sanctions against nodules that fix less nitrogen reduce the fitness of cheaters when mixed nodules are rare, but high rhizobial population densities lead to more mixed nodules, where cheaters may escape sanctions by sharing nodules with strains that fix more nitrogen.