California alfalfa growers have a new weapon against meadow voles and are rediscovering an old one against pocket gophers.

Zinc phosphide was recently registered for in-season broadcast applications over alfalfa. Previously, the poison had been allowed only when alfalfa was dormant or on the edges of fields.

Terry Salmon, University of California extension wildlife specialist, expects zinc phosphide to quickly become the rodenticide of choice against meadow voles in alfalfa.

“It's a very effective, acute toxicant, which means it kills immediately,” he says. “It can be put out by mechanical spreader or by aircraft. You are allowed up to two applications per year, although you can probably get by with just one. If a second application is needed, there is a 30-day interval required between treatments.”

It's important that the toxicant be put out at the right time and in the right amount, says Salmon.

“If the vole doesn't eat a full dose, he will get sick but won't die. And he won't approach the zinc phosphide again because he knows he got sick the first time.”

Salmon says zinc phosphide is a restricted-use pesticide. Personal protection, including a mask and respirator, is required during mixing/loading and if applied by belly grinder. It cannot be applied where grain-eating birds are present, and waterfowl must be hazed for 24 hours after treatment. It's available only through county ag commissioners' offices.

Salmon says the key to getting the registration was establishing a safe level of exposure so that no zinc phosphide would enter the food chain. That meant collecting sufficient data to make a scientific determination. It was paid for out of a 50¢/lb surcharge for rodenticides purchased through ag commissioners.

He says the pesticide may soon be registered in Idaho, too. He knows of no other states that have started the registration process, “although this may spur them to do so.”

Aluminum phosphide, on the other hand, has been around since the 1980s and is labeled for pocket gopher control throughout the country. Yet its use in agriculture is only now beginning to pick up, says Salmon.

“Aluminum phosphide is a fumigant, and since no other fumigants had been shown to be effective for pocket gopher control, people gave up on them. But the landscape and ornamental industries have shown aluminum phosphide to be effective, and I expect it to become very popular with alfalfa growers.”

Other fumigants emit a strong odor that scares away the gophers. Aluminum phosphide has an odor, too, but apparently not one that wards off gophers.

“We haven't been able to document it, but we think the gophers even take the aluminum phosphide pellets back to their nests,” says Salmon.

To apply it, growers will need to use a soil probe to locate the gophers' underground tunnels. Then a piece of PVC pipe or other tube can be used to enlarge the hole and put two to four pellets into a tunnel. The hole must then be closed or the gopher will see the light and repair it himself, covering the pellets with soil in the process.

Salmon says the pesticide is inexpensive — about 10¢ a pellet. And it's “not as much work as trapping and not much more work than baiting. I think this will be picked up quickly by growers. They're always looking for new ways to control gophers.”

For more information on gophers, visit the University of California's IPM Web site: www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7433.html. For more on voles, visit www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7439.html.

Information about a UC-Davis publication called Integrated Pest Management for Alfalfa Hay can be found at: www.ipm.ucdavis.edu/IPMPROJECT/ADS/manual_alfalfa hay.html.

The book presents an ecological approach to managing the crop, pest insects, weeds, vertebrates, pathogens and abiotic disorders. The 98-page book, with 200 color photographs, costs $22 and uses the expertise of more than 30 University of California researchers, extension specialists, farm advisors and pest-control professionals.