The little guppy Poecilia reticulata has developed a big reputation. For decades, the fish has been championed as a mosquito fighter and dumped into ponds and ditches to eat up the insect’s larvae. But among scientists, it has a different reputation—as an invasive species with a remarkable ability to reproduce and spread.
Now, as health officials in regions facing mosquito-borne viruses like Zika consider expanding use of these predatory fish, ecologists are urging them to think twice. In a paper published online today in Biology Letters, a group of ecologists argues that the guppies—and other nonnative fish used for mosquito control—haven’t actually proven very effective mosquito fighters, but are known to pose ecological risks.
“It all sounds like it’s magical—you put the guppies in, they eat the mosquitoes, everything is fine,” says Rana El-Sabaawi, an ecologist at the University of Victoria in Canada and lead author on the new paper. “Our concern is that you have a potentially invasive species that is being introduced haphazardly.”
Larva-gobbling guppies may have been cutting-edge technology for U.K. colonialists aiming to rid the empire of mosquitoes at the turn of the century. But to El-Sabaawi, the strategy seems so old-fashioned that she was surprised to find out large-scale projects are underway. While “randomly Googling guppies,” she came across news reports from Pakistan that health officials had released thousands of the fish into the ponds and sewers of Karachi in 2013 to fight the transmission of dengue fever. And in a widely circulated news video documenting Zika control efforts in Brazil, El-Sabaawi was troubled by footage of a municipal government worker apparently “wandering around with a bunch of guppies and basically just introducing them in ditches.”
That’s unnerving for El-Sabaawi and her co-authors because they know guppies are efficient invaders. They’re hearty and fertile, surviving in relatively polluted water, reproducing often, and giving birth to fast-growing, live young. A combination of accidental aquarium releases and mosquito control projects have spread the species from its native range in the Caribbean and the northern coast of South America to at least 69 countries, according to a 2011 survey.
And several studies suggest that introduced guppies threaten biodiversity. Researchers in Hawaii found that guppies released in the 1920s drove down native fish populations, perhaps by competing with them for food and living space, and had likely changed the cycle of nutrients in water: Guppy-rich areas showed increased levels of dissolved nitrogen—from ammonium in fish urine and gill excretions—which, in turn, stimulated algae growth. (Another fish commonly used in mosquito control—Gambusia affinis—has also been associated with declines in native fish species.)
The authors also question whether guppies are reliable mosquito slayers. Studies that back their effectiveness tend to have flaws, they say. Lab tests often starved the fish before exposing them to a diet of exclusively mosquito larvae. And studies in the wild have been small and poorly designed.
That critique may be correct, but dismissing guppies as a control strategy is counterproductive, says John Hustedt, senior technical officer of the nonprofit Malaria Consortium in Phnom Penh, which has been releasing the fish into water storage jars in rural households to combat dengue fever and other mosquito-transmitted diseases. Hustedt hopes that a study his group has just completed will provide new evidence for the guppies’ value. Preliminary results showed that reductions in the number of adult mosquitoes were two times greater in households with guppies than in those without.
“If someone comes out and says, ‘Actually it doesn’t work and it’s going to cause you a problem,’ that can decrease the chance that the government would be more open to trying [guppy release] on a large scale,” he says.
As for ecological risks, guppies in isolated containers may be less likely to spread than those dumped into urban sewers and ditches. But Hustedt also questions the distinction between native and nonnative for a species that is already so ubiquitous. The guppies used in his project were found in a farm in a province outside Phnom Penh; their original source is unknown. “It seems to me that they’ve been here for quite a long time, and they’re already in the environment,” he says.
Although the benefits and risks of guppy releases may be highly context-dependent, some researchers are simply taking a hard line. “The use of fish to control mosquito disease vectors should be abandoned by authorities,” says Valter Azevedo-Santos, an ichthyologist at São Paulo State University in Botucatu, Brazil, who co-authored a letter objecting to the strategy published in Science earlier this year. He believes resources would be better spent on other control measures: insecticides, sanitary measures such as eliminating standing water in homes, and even the experimental release of genetically engineered mosquitoes to spread a lethal gene. As health workers cast around for ways to combat Zika, he hopes this paper will give them pause. “This mismanagement must be abandoned, or new fish invasions will occur in the near future,” he says. “This is a special moment.”