Which animals kill the most humans? Lions and tigers and bears? Oh no, malaria-transmitting mosquitoes! The risks of using insecticides to kill mosquitoes may be outweighed by the benefits, but those benefits only last until mosquito populations evolve resistance. Careful use (insecticide-treated bed-nets, for example, rather than spraying wetlands) can slow the evolution of resistance, but we haven't yet achieved a goal I recently saw on a bumper sticker, namely, to "Stop Evolution Now!"
Can we do better? A paper published today suggests a new approach. "How to make evolution-proof insecticides for malaria control" was written by Andrew Read and colleagues. It's in the open-access journal, PLoS Biology, so you can read the whole article for details, but here's my summary:
When insecticides kill mosquitoes quickly, the rare insecticide-resistant mutants make a disproportionate contribution to the next generation, so resistance evolves quickly. But reproduction of the malaria parasite inside mosquitoes is slow enough (and mosquito life-spans are short enough) that most mosquito eggs are laid by mosquitoes that have not yet become infectious. So the authors suggest developing methods that only kill older mosquitoes. This could be a slow-acting chemical insecticide, a slow-killing virus, or something that preferentially kills older mosquitoes. They wrote:
" in principle at least, public health advances can be achieved with minimal selection for resistance by an insecticide that kills after the majority of mosquito reproduction has occurred but before malaria parasites are infectious."
One problem they note is that slow-acting insecticides would provide longer-lasting protection from malaria, but they wouldn't keep people from getting bitten. People often have a short-term perspective, so might be reluctant to adopt this approach, unless it were combined with other methods. For example, window screens keep people from getting bitten at home, while also imposing selection on the malaria parasite for lower virulence (because people too sick to leave the house don't get bitten and transmit the more virulent variants).
Although most mosquitoes live only a short time, I wonder whether there are places or times where long-lived mosquitoes constitute most of the population. For example, most Monarch butterflies have short life-spans, but those that migrate back to Mexico from the US are much longer-lived. If long-lived mosquitoes lay most of the eggs after a drought, for example, then insecticides that kill (susceptible) older mosquitoes could lead to more rapid evolution than the models in this paper predict. But I don't know enough about mosquitoes to know if that's a real concern.
Why Evolution is True discusses human-biting mosquitoes in the London Underground that may have evolved since it was built. Their sister species (or subspecies) on the surface mostly bites birds.