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An insecticide that kills 'grandmother' mosquitoes but gives a stay of execution to their young offspring could be the key to the long-term eradication of malaria, say scientists.

For decades people have sprayed their houses with insecticides to control the malaria-carrying Anopheles mosquito. But the insects swiftly develop resistance to insecticide because a tiny number of their offspring have natural resistance — and thus survive the spraying to produce new generations of mosquitoes immune to the chemicals.

A paper published this week (7 April) in PLoS Biology points out that while resistance develops among younger mosquitoes, it is the older ones that pass on malaria. This is because it takes time for the malaria parasite to develop within the mosquito.

An insecticide that selectively killed older mosquitoes could exploit this distinction, argue the authors.

"Selection for resistance is very weak in old individuals," says Andrew Read of the US-based Pennsylvania State University, lead author of the study. "If the mosquitoes have already done most of their reproduction by the time they see the insecticide, there is very little chance for them to become resistant," he told SciDev.Net.

"As the mosquito becomes more dangerous, most of her reproductive life is over." So you have to kill the "grandmothers" and leave the young ones alone.

Using a mathematical model to follow the evolution of resistance in Anopheles gambiae, Read and his colleagues predict that killing only mosquitoes that have completed at least four cycles of egg production reduces the number of infectious bites by 95 per cent.

They are developing fungal biopesticides that take10–12 days to grow inside the insects, killing them towards the end of their lives. 

"We've been working with this fungus for five years now and we knew that it had the right properties to reduce the transmission of malaria. But we hadn't realised that it could stop the evolution of resistance. That's the new insight," says Read. "The ultimate goal will be to provide a single malaria control that will last forever."

But the main obstacle, according to Read and other malaria experts, is that people will still get bitten — by younger, non-infectious mosquitoes — so they may perceive that the insecticides are not working.

Link to full paper in PLoS Biology

References

PLoS Biology doi:10.1371/journal.pbio.1000058 (2009)

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