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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.


Seems to me you can have human Bt cheaters come in two stripes: one would be the farmer you mention who would choose to plant the whole farm to the Bt variety (forgoing refuge), but the other is the farmer who forgoes the Bt technology altogether and reaps the benefit of living among a sufficiently high density of Bt users as to have less insect damage. This latter 'cheat' saves the expense of the Bt trait in seed cost. One can observe that this latter cheat is actually serving as a refuge against cheats of the first stripe (and I'll agree this is real).

But there comes a subtle difference in cheat control. It is possible to control the first cheat in hybrid crop production (e.g., corn) where new seed is essential each year and the cost of seed production is beyond the means of most farmers. Simply add refuge seed into the retail bag of seed (refuge in a bag) and -poof- cheating is eliminated.

The latter style of "cheating" is more akin to the rhizobial system you're working with... forgoing some real cost and still gaining some advantage due to the action of neighbors. Now recall that above we noticed the human cheat of style two might also be considered a refuge to the "all in" style one cheat.

So my question for pondering is whether there might be some yet to be discerned value of the rhizobial "cheats" in a population of comensal bacteria??


Thanks for the insightful comments.

I wouldn't call people growing nonBT crops "cheaters." They might benefit from lower pest levels if their neighbors grow BT crops, but people buying BT crops may have signed a contract agreeing to refuge requirements. And having BT crops nearby may cause additional problems. Some organic farmers, for example, use BT sprays. These might stop working if pests evolve BT resistance on their neighbors' farms.

I'm told that mixing nonBT seed doesn't work as a refuge. It's more like a low dose, which speeds evolution of resistance.

Someone has claimed that cheating rhizobia are "beneficial" because they impose selection to maintain host sanctions against cheaters. Seems like a stretch to me.

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