The Gila topminnow, a diminutive native fish species that tops out at a few inches in length, is not exactly the stuff of nightmares.
That is, unless, you’re a mosquito.
The larvae of the seasonal pests are a delicacy to the topminnow, which was first listed endangered in 1967 and nearly disappeared from the Santa Cruz River due to depredation by nonnative species, like the mosquitofish, and groundwater pumping, among other factors. In recent years, however, they have made a dramatic comeback, and now the Pima County Health Department is harnessing for the first time this coming season the topminnow’s taste for mosquito spawn.
At the department’s headquarters, two large glass tanks are teeming with roughly 500 topminnows, which came from the Phoenix Zoo, according to consumer health and food safety manager David Ludwig. Arizona State University workers helped get the department up to speed on keeping the fish alive, and both the state and federal fish and wildlife agencies supported the effort.
The fish mostly ignored dead mosquito larvae that were dropped into the tank as a demonstration, but when live larvae are introduced, they swarm “like piranhas,” a staff member quipped.
In coming months, the topminnows will be deployed in unmaintained pools, fountains and other sources of stagnant water that serve as breeding grounds for mosquitoes, cutting down on the need for chemical additives.
“It’s a win for the endangered species, I think it’s a win for the populace of Pima County,” Ludwig said. “Maybe a loss for the mosquito larvae, we hope.”
The county’s Multi-Species Conservation Plan, which was a key factor in the U.S. Fish and Wildlife Service’s decision last year to issue the county a so-called Section 10 permit, allows for the use of the topminnows for mosquito control. It also prohibits the county from using the mosquitofish for pest control in watershed tributaries.
Doug Duncan, a local U.S. Fish and Wildlife Service fish biologist, said he lobbied for the topminnows’ use in vector control to be included in the plan, and that there had been conversations about the prospect since the 1990s. He added that Pima, which is the first county in the state to start such a program, “has been the vanguard for a while.” Pinal County is developing a comparable program, according to Ludwig and Duncan.
A 2006 study published in the journal Western North American Naturalist found that “there is no evidence to suggest that nonnative (mosquitofish are) better suited for mosquito abatement” than the topminnow. It also advocated for avoiding the use of mosquitofish for vector control in the Gila River Basin, which includes much of Southern Arizona, including Pima and Santa Cruz counties.
Currently, the health department is permitted to use the topminnow only in contained sources of water unconnected to local waterways, though their application could eventually expand, according to Ludwig.
As it stands, the department responds to mosquito complaints by investigating possible breeding sites. When owners of pools and other standing water sources don’t have the means to properly maintain them, the department requests that they purchase chemical mosquito control products. If the owner is not responsive, the department can get a court order to do the work themselves.
Now, the topminnow can serve as a nonchemical alternative in either instance.
The principal shortcoming of chemical treatments, Ludwig said, is that they are often only temporarily effective, meaning his staff may need to repeat the process of gaining access to properties. Topminnows, on the other hand, can establish self-sustaining populations in such bodies of water and lay waste to mosquito larvae all season.
While there hasn’t been a locally acquired case, one of the mosquito species present in the county — Aedes aegypti — can carry the Zika virus, as well as Chikungunya and dengue fever.
Signs that will be placed outside of treated standing water read, “Fish at work! This neglected pool has been treated with fish to control mosquito breeding. Do not put chemicals in the water!”
The fish have a life span of less than a year, according to Arizona Game and Fish Department. Duncan acknowledged that some may find it strange to use an endangered species for mosquito control in contained bodies of standing water, where they will eventually die, but pointed to two principal benefits of the program.
For one, using topminnows instead of their mortal enemies, the mosquitofish, means the nonnative species will have fewer opportunities to spread and undermine the native species’ return.
Secondly, their widespread use by the county will have an educational effect, teaching residents about “the plight of the species — just more about what native fishes we have, what their traits are, and the things we’re trying to do to conserve them,” Duncan said.
“We hope it catches on throughout the range of the topminnow.”
