Another story the other day, this time from the San Jose Mercury News, showing the perils of importing predators to control pests.

This time it’s the mosquitofish, a guppy-sized fellow, brought into California from the East Coast in the 1920s to control, you guessed it, mosquitoes. The fish are voracious—can eat 500 mosquito larvae in a day. This is very good, especially since West Nile virus hit the state in 1999. Mosquitofish have helped keep the virus, which is frequently carried by birds and then transmitted by mosquitoes that bite the birds, in relative check.

The problem—there’s always a problem, isn’t there?—is that the mosquitofish are virtually omnivorous, and one of their favorite noshes is tadpoles, including those of the threatened red-legged frog and the endangered Santa Cruz long-toed salamander.

The problem is compounded by the fact that, as frog and salamander habitat has shrunk, the creatures have taken up residence in stock watering holes and backyard ponds—just the places that are being outfitted with mosquitofish to combat mosquitoes and West Nile virus.

To combat this problem, the state has banned the release of mosquitofish into natural waters—creeks, lakes, and streams—and has drained a handful of ponds to remove the fish and salamanders (drain the pond, gather up the tadpoles and put them somewhere else that’s fish-free, then allow the fish to expire, then refill the pond and bring back the amphibians). This is obviously a complicated and time-consuming process that’s impractical over wide areas.

No easy answers. There’s no treatment for West Nile virus and it can be a killer of humans. Too bad for the frogs.