Home & Garden Columns

Garden Variety: Costly ‘Free’ Mosquitofish Belong in a Barrel

By Ron Sullivan
Friday July 28, 2006

It’s high hot summer and the mosquitoes are peaking, along with the rest of the annoying arthropods. 

People are getting nervous about West Nile virus, though the next wave of ordinary flu will doubtless carry off more of us; hey, we’ve heard of flu, but what’s this new thing? 

I get my flu shot every year, myself, but I’m more worried about the crows and jays and the magpies over the hills, all of whom are more susceptible to the virus than we are, even the old and frail among us. (Me, for example.) 

The plague has been playing hob with the magpies in particular—corvids and raptors are even more badly hit than most bird species—and you do know, don’t you? that our yellow-billed magpie species exists nowhere else on Earth. Scary.  

It’s a good idea to kill lots of mosquitoes. The local bats, swifts, and swallows can’t get them all. Even PETA hasn’t yet stood up for them, as far as I know. 

The only reason a mosquito bites is that she—always she—wants to be a mommy and needs a blood meal to make eggs, but I haven’t seen the lacto-ladies or the think-of-the-children groups picketing the vector control office. The quibble I have is the means people use to kill them.  

We’re well past the days of innocents happily disporting themselves in the cooling fog from the DDT spray trucks on the neighborhood streets. I hope. But one bit of official panic can send helicopters over the marshes, killing everything that happens to hatch and have six legs. And the “greener” weapons can be even worse in the long run.  

You’d think we’d remember mongoose invasions in Hawai’i, cane toads in Australia, and such disastrous good ideas before setting another “biological control” loose on a landscape. 

But Gambusia affinis, the cute little mosquitofish that public agencies give away free and dump into public waters, gets an approving pat on its scaly head despite its threat to hard-pressed natives like pupfish, minnows, frogs, newts, and salamanders.  

The species has been introduced worldwide for mosquito control. It’s voracious enough to gobble the young of native fish, amphibians, insects, and other critters that already eat mosquitoes themselves, and they’re not so particular about eating mosquito larvae. 

Peter Moyle, who wrote the authoritative Inland Fishes of California, says there’s no evidence that gambusia control mosquitoes in natural bodies of water where native fish or mosquito-eating invertebrates are already present, and that some native fish, like the endangered pupfish species, can be more effective than mosquitofish in natural situations, and that goldfish and small koi are better control agents in ornamental ponds. 

The only place for those free mosquitofish is in a barrel or artificial pond that has no connection at all with any natural waterbody—even when it overflows. 

And you’d be better off buying a netful of feeder goldfish at the pet shop—they eat just as many mosquitoes and they’re handsomer than gambusia. I’ve never heard of a goldfish invasion in a stream or lake, and if anyone has, I’d appreciate a pointer to the place. Meanwhile, beware of the freebies! 

Thanks to Joe Eaton for material originally published in the late Faultline webzine.