A new weapon has been found in the quest to control disease bearing mosquitoes: noise frequencies irresistible to sexed-up males. Catching males won't stop bites directly, but it can help us track when dangerous species are in an area, and assist with programs that rely on releasing sterilized males to prevent the next blood sucking generation.
Aedes aegypti mosquitoes carry dengue fever and yellow fever. Like other blood-sucking mosquito species, only the gravid females bite, needing the protein in vertebrate blood to develop their eggs. Males, and females not bearing eggs live on nectar and plant sugars. Catching so many males that the females are unable to breed is an almost impossible task, so most attention has focused on trapping the females, particularly those ready to bite.
However, according to James Cook University's Professor Scott Ritchie catching males is useful too, and easier. Ritchie is co-author of a paper in the Journal of Medical Entomology on building a better mosquito trap.
"Males of most mosquito species of medical importance use their superb auditory sense to detect and locate female mosquitoes by recognizing the female's unique flight tone," Ritchie and co-author Brian Johnson wrote.
A aegypti females have a wingbeat of around 484 hertz, so this frequency is very attractive to the males (and sadly familiar to people trying to sleep in the tropics). Ritchie and Johnson placed a mix of hay and water in four traps inside tents and attached them to speakers producing 484 hertz, 560 hertz, 715 hertz (the frequency of males in flight) or silence. The traps were rotated between different locations to ensure it was the sound, not the spot where it was being emitted, that proved effective. Ritchie told IFLScience that the traps could include either an insecticide to kill the mosquitoes, an adhesive to trap them, or a thin layer of oil that would cover their wings and prevent them escaping.
On each occasion 30 males or 30 gravid females were released near the traps. Within two hours the 484 hertz trap had caught an average of 95% of the males. The 560 hertz trap showed some value, catching around half of the males, while the other two traps were almost empty. A follow-up experiment tested the traps in a large cage with about 1,000 mosquitoes of both sexes, with similar results, while under field conditions 484 hertz traps captured twice as many males as alternatives.
The females, on the other hand, were no more attracted to any of the sounds than to the silent trap. Apparently the sound of a male mosquito has no attraction to the female, who presumably have figured he will come after her soon enough.
Ritchie explained to IFLScience that while existing traps are effective, they rely on the use of fans to suck mosquitoes in, and need either mains power or heavy batteries to last. Speakers, broadcasting at noise levels barely audible to the human ear, on the other hand, can be powered by watch batteries, opening up the possibility for placing long lasting traps in remote locations.