Mosquito control aided by tiny fish with big appetite

Since 1970, mosquito abatement programmes in the state of California have reduced their use of insecticides by more than 75 per cent. Part of the credit for this is due to a hardy minnow from the rivers and lakes of the southeastern United States, the gambusia, whose prodigious appetite for mosquito larvae has earned it the more common title of "mosquitofish". Although the full-grown female is only as long as a human finger and the male is even smaller, each can eat dozens of mosquito larvae and pupae a day. The fish occupy some amazing habitats: tunnels, abandoned swimming pools, bonds, rainbarrels, stagnant rice fields, and waterways that dry out only slowly in the spring. Mosquitofish have been introduced statewide into sewage-treatment ponds, roadside ditches, and watering troughs on ranches. The city of Fresno has them in drains under the streets. Disneyland has them inits aquatic exhibits and its many storm drains.

Yet the discovery of the species' effectiveness in controlling mosquitoes is not new. The tiny grey or grey-and-black fish were first used for mosquito control in the United States in the early 1900s. Stocks were sent to Hawaii in 1905, where they were credited with reducing the populations of the encephalitis mosquito. Mosquitofish also helped conquer the yellow Fever epidemic that threatened to halt all work on the Panama Canal. They were dumped into flooded excavation pits, into plants that trap water in their leaves, and even into gutters around houses.

In the 1940s, however, the spectacular early success of DDT and other synthetic compounds made the tiny minnow seem obsolete. Its use declined. This natural form of mosquito control is only now coming back.

The fish require no special egg-laying site: they bear live young. Under favourable summer conditions a single female can produce 200 young every 21 days. Small size allows them to penetrate most sites where mosquito larvae and pupae are found. They thrive when combined with other control techniques, such as bacterial pesticides, and even chemical insecticides used to spray mosquitoes do not hurt them. They tolerate wide ranges of temperature and salinity, as well as moderate sewage pollution. "Mosquitofish can take terribly poor water," notes Graham Gall, an aquaculture specialist at the University of California. "The oxygen level can be down to practically nothing. They're a surface fish, they work on the interface with air, and are okay."

Most of California's 53 mosquito abatement programmes use some mosquitofish. Some have full-time fish biologists on staff. Mosquito control personnel have fish trucks and other special equipment for handling fish. (Mosquitofish have been dropped out of helicopters in plastic bags; the beg explodes, releasing the fish into little pools that are hard to find except from the air.) The study of the metabolism, feeding, and aquaculture of mosquitofish are leading programmes in statewide mosquito control research.

For mosquito control in rice fields, the fish reduce the number of pesticide treatments by 90 per cent and reduce both inspection time and costs per acre by around 65 per cent. Only insufficient supplies of the fish are hampering the increased use of this integrated control. Many mosquitofish die during the cool California winter, and there are not enough available in the spring when mosquito populations are small and most vulnerable. No one yet knows how to culture the fish, although researchers are attempting to mass rear it. To recover the fish, rice growers now drain their fields before drying them out in the autumn. The fish wash down through canals, where mosquito control personnel recover them. They are overwintered in warm water ponds at power plants or sewage treatment facilities.

Elsewhere in the United States mosquitofish have proved singularly successful as well, and they could have worldwide possibilities. They are not, however, without their potential problems. Mosquitofish are so effective that they can consume most of the plankton in an area, and scientists worry that this might harm other fish species. There are no quantitative data to confirm any adverse effects in field practice - indeed, the mosquitofish are preyed upon by larger fish and in some waterways they seem to have improved the numbers of food- and sport-fish species - but it is still possible that problems may arise in the future, especially in areas where the fish do not die off over the winter. Most biologists are opposed to introducing the mosquitofish to new habitats.

Instead, countries worldwide should investigate their own indigenous fish and identify those that consume mosquito larvae. (This is done by observing the faeces under the microscope; the species that eat mosquito larvae are identified by the undigested outskeletons, which are easily seen.)

The World Health Organization has already begun this process for some areas. Mosquito-eating fish that have been identified include guppies, blackfish, the desert pupfish, goldfish, and even "annual fish" whose eggs resist desiccation when a pond dries out.

Therefore, the most important message of California's success is that fish in general can be valuable in mosquito control. In the future, this natural form of pest management is likely to be used the world over, employing dozens of local species and involving many fish biologists in suppressing mosquitoes and preventing diseases.

Noel Vietmeyer