November 01, 2007

The Law of Expected Unintended Consequences

Self-evolving systemYesterday I attended a talk by Laith Yakob who described some of his current research on the ecology of controlling dengue fever. The best way of doing that is to control the populations of the Aedes aegypti mosquito. A very elegant way is the sterile insect technique: a large number of sterile insects are released, making most eggs nonviable and the next generation smaller. By releasing sterile insects each breeding season eventually the fertile insects dwindle to zero. It has been quite successful, and using sterile males this ought to be able to remove Aedes. No toxins, no risk of building up resistance, specific to one species.

Not so fast, Yakob warned. Aedes larvae compete with each other for food, making only a small number survive in each breeding clutch. Fewer viable eggs = fewer larvae = less competition = more survival. Running this through a population model he found that adding sterile males to the population could boost it by up to 70%! If enough sterile insects are released it will of course still collapse, but in a realistic situation this will only happen near the release sites. Nearby the boosting effect occurs. Oops.

The solution is likely genetically modified mosquitos. If they carry a dominant mutation that is lethal during the pupal stage the boosting of population does not happen. Similarly, a mutation that kills off any female will also cause a declining population. Of course, for breeding purposes it must be turned off, but that can be done using an artificial substance not found in nature. This is being explored by the company Oxitec, who not unsurprisingly appeared in the talk.

A far more elegant method than breeding insecticide-resistant females and covering them with insecticide.

I think this lecture demonstrated the utility of having detailed population models of different species. For once we can avoid a costly mistake and instead come up with a better solution. Similarly genetic engineering allows a targeted solution rather than just breeding and hoping for the best. Improved ability to predict and intervene, that is the way of making the law of unintended consequences predictable.

Posted by Anders3 at November 1, 2007 12:18 AM