In the Cayman Islands, genetically modified mosquitoes are on the prowl. The insects are all male, and they’ve been engineered so that all their offspring die before reaching adulthood. By having sex with local females, they could father a new generation that perishes prematurely, before it gets the chance to spread diseases like dengue fever.
These GM insects, engineered by Luke Alphey at the University of Oxford, are part of a growing number of initiatives designed to fight disease by pitting mosquitoes against mosquitoes. Alphey’s tactic of breeding mosquitoes that beget unfit larvae is just one approach. Some groups are trying to make the insects more resistant to the disease-causing parasites they carry. Others have loaded them with life-shortening bacteria that outcompete those parasites. While some scientists warn of the unintended consequences of releasing such insects into the wild, others acknowledge that we need innovative new strategies for tackling mosquito-borne diseases like malaria and dengue fever, which affect hundreds of millions of people every year. (Figures for mosquito-borne illness are notoriously unreliable, but the World Health Organization estimates that each year there are 247 million cases of malaria, causing 881,000 deaths, and 50 million instances of dengue.)
But all of these recent attempts to turn mosquitoes into malaria- and dengue-killing machines have something in common: The modified mosquitoes need to have lots of sex to spread their altered genes through the wild population. They must live long enough to become sexually active, and they have to compete successfully for mates with their wild peers. And that is a problem, because we still know surprisingly little about the behavior and ecology of mosquitoes, especially the males. How far do they travel? What separates the Casanovas from the sexual failures. What affects their odds of survival in the wild? How should you breed the growing mosquitoes to make them sexier? Big question marks hang over these seemingly straightforward questions.
Heather Ferguson from the University of Glasgow studies mosquito ecology. She views the knowledge gap in this field as a significant obstacle that stands in the way of the GM-mosquito initiatives. History tells us how dismally such initiatives can fare if they are not constructed on solid ecological foundations. In the 1970s and 1980s, several groups tried to control the mosquito population by releasing sterile males that would engage females in fruitless sex. The vast majority of the experiments failed.
Their poor performance is often blamed on the fact that the males were sterilized with damaging doses of radiation. But they had many other disadvantages. Lab-bred mosquitoes are frequently reared in large, dense groups, which produces smaller, less competitive individuals. The artificial lights of a lab could also entrain their body clocks to the wrong daily rhythms, driving them to search for mates at the wrong time of the day. And in several cases, the modified males ignored the wild mosquitoes and preferred to mate with their lab-reared kin instead. These problems went unnoticed in lab tests, where the modified mosquitoes were compared with unaltered ones that had been raised in the same conditions. They seemed to be perfectly competitive, but they proved to be feeble challengers to their wild peers.
Given that we ended up with puny, frail mosquitoes that looked for the wrong mates at the wrong times of the day, it is no wonder that most of these early experiments in modifying mosquitoes failed. Compare this example to the case of the citrus-eating medfly, a pest that scientists also tried to control by releasing sterile males in the early 2000s. Just like mosquitoes, mass-reared medflies initially lost out to their wild peers. But scientists realized they could improve the medflies’ reproductive chances by feeding them lots of protein, which gives them energy for mating hunts, or ginger root oil, which produces a female-attracting scent. These simple measures had a big impact on the programs, which have successfully controlled medfly outbreaks in North and South America.