Mosquitofish Gobble Up Growing North State Public Health Threat All Summer

Sep 3, 2015

Mosquitofish netted, weighed and on their way to be released into a rice field near Willows, Calif. The mosquitofish are used as a biocontrol method for mosquitos. The fish eat mosquito larvae, which are found in (among other water sources) the standing water of the rice fields while they’re flooded in summer.
Credit Sarah Bohannon / NSPR

With West Nile Virus on the rise, county vector control departments are working hard to kill mosquitos. They’re fumigating roads and fumigating by plane. But they’re also using a less noticeable biocontrol method too – a grey finger-long fish.

In its natural habitat the appropriately named mosquitofish eats buffet-style. Its meals are mostly made up of zooplankton and invertebrates. Its tiny fish plate is piled high with beetles, mayflies and mites.

But for the hundreds of thousands of mosquitofish living in five human-made ponds in Glenn County the menu is more vegetarian. These fish are on a diet of cheese, which Jack Cavier, manager for the Glenn County Mosquito and Vector Control District, said is donated by North State based Rumiano Cheese Company.

“They like Cheddar better than they do the others. But they’ll eat Monterey Jack and just about everything.”   

Driving around with Cavier near his office in Willows makes it pretty easy to see why his department is raising loads of mosquitofish. There’s a lot of rice being grown in Glenn County. That means for the past few months there have been a lot of mosquitos breeding in the standing water of flooded rice fields.

“That’s one of our biggest problems here is all these rice fields so close to the proximity of town that they’re able to come in so prevalent,” Cavier said. “That’s why we’re getting so many calls.”   

Luke Niblack, assistant manager for Glenn County Mosquito and Vector Control District, releases mosquitofish into a rice field. The mosquitofish are used as a biocontrol method for mosquitos. The fish eat mosquito larvae, which are found in (among other water sources) the standing water of the rice fields while they’re flooded in summer.
Credit Sarah Bohannon / NSPR

One of the ways Cavier has been cutting down on that mosquito population is with the fish. They’re released into the rice fields where they’ll happily spend the rest of their days gobbling up mosquito offspring. Goodbye Cheddar, hello mosquito larvae.  

To get the fish from the county’s ponds to the fields, Cavier and his team transport them in an oxygenated tank carried in the back of a pickup truck. Once at the destination, the fish are scooped from the tank with nets. Then they’re weighed based on the field’s size.

It’s two tenths of a pound of mosquitofish for every acre. A number Cavier said came from a study by UC Davis looking at achieving mosquito control with the fewest fish.  

Once weighed, the mosquitofish are then walked to a place on the water’s edge where they’ll have a clear path to the rice and won’t get trapped in mud or weeds. Then the fish are freed.

It’s a pretty simple process that helps prevent, what can be, a pretty complicated disease.

Although West Nile Virus goes undetected in 80 percent of the people it infects, the disease can cause severe neurological disorders – meningitis and encephalitis. Sometimes it can even cause death. Two virus related fatalities have occurred this year. The first was in Nevada County. The second was in San Bernardino.

Last year, California had the most human cases of West Nile Virus it’s had in nearly a decade. This year rates remain high. Cavier said some studies are linking the rise in West Nile Virus to the drought. The idea being that lack of water is bringing mosquitos and birds closer together.

Glenn County Mosquito and Vector Control District’s Jack Cavier, manager, and Derek Nunes, field technician, look for a place free of mud and weeds to release mosquitofish into rice fields outside of Willows, Calif.
Credit Sarah Bohannon / NSPR

“Then they’re in more proximity to each other,” Cavier said. “So it really actually kind of ramps up the virus activity, because the birds are the host for the virus and the mosquitos are picking it up from the birds, and they spread it from there.” 

Glenn County had the highest rate of human cases of West Nile Virus in 2014. Four out of every 10,000 Glenn County residents ended up with the disease.

This year it’s a neighboring county that’s the frontrunner. As of Friday, the California West Nile Virus Website was reporting 36 confirmed human cases of West Nile Virus in Butte County. That’s 40 percent of the 83 confirmed cases in the state this year. Glenn County currently has the second most with 12 cases.

With this year’s warm summer, Cavier expects that West Nile Virus cases will continue to rise – maybe until November. After that, fall should set in and the cool weather should make the mosquitos inactive.

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The Glenn County Mosquito and Vector Control District offers free mosquitofish to county residents that can be used for biocontrol in standing water around homes.

Some other North State counties also provide free mosquitofish to residents. Check with your local vector control district for more information.