Despite Pest Progress, Black Flies Still Hang Around

Greg Garatea used to wear mosquito netting when he irrigated. But it wasn’t mosquitoes the Murtaugh area rancher was trying to protect himself against — it was black flies.
Clouds of black flies, also known as Buffalo gnats, swarm around people and torment grazing livestock. The problem used to begin as soon as farmers started irrigating in the spring and lasted until a hard frost. But since the Twin Falls County Pest Abatement District began controlling black fly larvae over the winters, the population doesn’t become problematic until later in the season.
Garatea, a member of the pest abatement district, was able to graze his light calves along the river until the middle of September this year. That’s about a month longer than he usually can. Measures to control stable flies or house flies have no effect on the black flies.
“You have to control them at the larval stage,” Garatea said.
That’s what Manager Kirk Tubbs and the staff of the pest abatement district do. Last year, Tubbs and his crews identified areas in canals and laterals where the black fly larvae were most prevalent and treated those areas in November, after the water dried up. They went back in January and February to treat the egg stage.
That meant a lower adult population was present early when warm temperatures finally arrived this spring. But the wet spring also meant river levels were higher than usual, and they remained high all summer.
Larvae anchor themselves to rocks or vegetation in moving water and let the water bring them food. Depending on water temperature and food, they can develop into an adult in as little as a week, Tubbs said. He has identified particularly productive stretches of the Snake River in which a million black flies per mile are developing every day.
He has treated some of those sections of river later in the summer when water levels are lower, but the higher flows this year made treating the river cost-prohibitive. The district has spent $180,000 controlling black flies this year.
Higher water levels coupled with the abnormally warm temperatures in September and early October have allowed the population to continue to build. That’s generated calls about why black flies are worse this year than ever.
Actually, data Tubbs collected from traps show the black fly population is lower this year compared to previous years. The difference now is the season has lasted longer.
Because each female can lay up to 200 eggs, the population increases exponentially. Twin Falls is the only county in south-central Idaho that treats black fly so as populations build in neighboring counties, the flies — which can travel 20 to 30 miles — migrate into the county.
Tubbs knows there are areas within the county where black fly populations have become troublesome. He would like to treat the larvae in those areas again this fall, but isn’t sure he will be able to. He is waiting for a new federal permit to be approved so that he can apply a natural bacterial product called Bti (Bacillus thuringiensis v. israelnsis) to the water.
“People don’t notice when they don’t have black flies around, but they do notice when they have them,” Tubbs said. “But this is as good as control can get unless we go to a Magic Valley-wide control program.”


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