Government officials warding off deadly diseases often praise the lowly mosquitofish as a public health messiah. But some environmentalists have dubbed it the “plague minnow” and the “fish destroyer.” Today, nearly a century after the finger-sized fish was first introduced to California in a Sacramento lily pond, it’s arguably the most ubiquitous freshwater fish in the world. But the mosquitofish also ranks among the world’s worst invasive species. Balancing the pest-control prowess and ecological destruction of the fish, nearly every mosquito and vector control district in California now deploys the creature with varying strategies.

Despite 74 deaths from the mosquito-borne West Nile virus in California over the last five years, the voracious fish helps greatly in the fight against infectious diseases. In the wild, however, it also threatens local biodiversity — outcompeting native species and infiltrating seemingly every corner of the Golden State.

“Every crazy-ass place in the state of California seems to have something to do with mosquitofish,” said Eric Palkovacs, a UC Santa Cruz professor of ecology and evolutionary biology who recently published a historical overview of the creature’s complicated relationship with humans.

When first identified in the 1850s, the mosquitofish was native to the southeast U.S. But the fish made its way to California in April 1922, and after being adopted as an alternative to the toxic insecticide DDT by the World Health Organization and various militaries, it spread around the globe.

“People put non-native fish everywhere,” Palkovacs said. “They didn’t give two thoughts to their impact on the food web and native environment.” The problem, experts say, is that the fish don’t discriminate between snacking on the rice-sized offspring of disease-ridden mosquitoes and critically endangered marine life. Chad Mitcham, a senior biologist in the Ventura office of the U.S. Fish and Wildlife Service, has seen the impact first-hand.

A few years ago, he sampled a pond in north Monterey County with thousands of larval Santa Cruz long-toed salamanders, a federally endangered species on the brink of extinction. Months later, after mosquitofish had been introduced to the pond, only 10 larvae remained.

“It was very disheartening to see,” Mitcham said.

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