Tucson’s long-awaited rainy season traditionally gets underway sometime around the July 4 weekend, bringing not only some relief from triple-digit temperatures, but the scary possibility of flash flooding, the return of croaking Colorado-River toads and those damned mosquitoes.
Fight the Bite, Day and Night helps the Pima County Health Department in their effort to combat the disease-carriers. This summer, they’ve got a new ally in the battle against mosquitoes in the form of native fish, the federally-endangered Gila topminnow, which love to feast on the larvae of the seasonal pests. Call them a non-chemical alternative to mosquito reduction.
Last year’s rainy season brought on a rash of complaints from citizens. They were about mosquitoes breeding in the green pools of stagnant waters, despite the placement of nearly 800 traps throughout the county. Most of the collected insects were of the “flood water” type that doesn’t cause illnesses, while a minority belonged to the Aedes Aegypti category that can potentially carry the Zika virus.
Regardless of what brand of biter they represent, they’re a pain in the posterior, arm, face or leg when they drill into you. The health department’s David Ludwig thinks some 500 Gila topminnow fish, from a breeding population at the Phoenix Zoo, can begin to help cut down on the unwanted pests. As its name implies, the native Gila topminnow is a two-inch-long, short-lived fish that spends the majority of its time close to the water’s surface feeding on small invertebrates—and they love to feast on mosquito larvae.
The tiny fish that can survive in waters up to 100 degrees once populated the Santa Cruz River, until they were listed as endangered in the late 1960s. They’ve made a dramatic comeback however and are now starting to be stocked in unmaintained bodies of water that have turned into stagnant mosquito breeding grounds.
“Once we verify that there are mosquitoes in green water, we will treat the site to reduce the number of larvae and then introduce 10 to 12 of these tiny fish to do a clean-up job,” Ludwig said. It’s hoped that the minnows, with a lifespan of about a year, will reproduce naturally in their new homes and continue their foraging behavior to keep things clean.
Ludwig and U.S. Fish and Wildlife Service biologist Doug Duncan have been strong supporters of using topminnows in vector control, making Pima County a vanguard in that movement.
“In the late 1990s, all Arizona counties were invited to a meeting of federal and state fish biologists to then consider using native fish for mosquito control,” Duncan said. “Pima County was the only county in the state to send a representative.”
“The Sonoran Desert Conservation Plan and the associated Habitat Conservation Plan both call for Pima County to use Gila topminnows for mosquito control (instead of problematic non-native mosquito fish) and that was a logical step because of the county’s interest in native species conservation,” he said.
Ludwig called the tiny torpedoes voracious. While they generally ignore dead mosquito larvae, they attack live larvae like miniature—and very hungry—sharks.
“If these fish see something wiggling in the water, they’ll go after it and devour it,” Ludwig said. “Within a minute, everything will be gone.”
It doesn’t take long for standing water to turn into a mosquito-breeding pool.
“Conditions can go from egg to larvae to winged insect in somewhere between 48 to 72 hours,” Ludwig added. “Water doesn’t have to stand around very long or be very deep, no more than enough moisture to fill the little plastic cap of a soda bottle, to allow them to lay eggs.”
“Adding to the problem is the fact that mosquito eggs that didn’t get a chance to hatch last season can sit dormant for months, even years, and as soon as monsoon rains arrive, you’ll get rehydrated larvae,” he said.
“Mosquito control falls under the aegis of integrated pest management, and both the Fish and Wildlife Service and the Arizona Game and Fish Department support using native fish to help accomplish that,” Duncan said. “Except in very simple aquatic systems, fish will not completely eliminate mosquito larvae so multiple tools may be required. Fish are just one tool in integrated pest management.”
“This program isn’t a panacea, but it’s an option, another tool in the toolbox to get rid of mosquitoes,” Ludwig said. “The best option of them all is a community that works together to reduce the number of potential breeding grounds because the insects don’t recognize fences or property lines—your mosquitoes are my mosquitoes. We all need to work together to empty potential water sources and not allow any standing water for two days or more. Check your property after a storm passes through, empty any containers and wipe things dry.”
And while the Gila topminnow may not solve the entire problem, Ludwig added that “we were able to put this program together through grants and donations, so there was no real cost for permits or the fish. It’s an inexpensive use of taxpayer dollars to eliminate potential breeding in green pools.”
Initially, the health department will be allowed to use the topminnow only in contained sources of water unconnected to local waterways, although Ludwig is hopeful that application could expand in the future.