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Resistance to Rodenticide Found in Meadow Voles in Artichoke Fields

The Issue

In a recent UC Cooperative Extension study of meadow vole (Microtus californicus) in artichoke fields in Central California, the rodenticide chlorophacinone was evaluated and found to be much less effective than when introduced about 20 years ago. On close examination, the researchers found that the baiting strategies used were likely to increase the chances of developing genetic resistance in the target population. Typically, chlorophacinone has been used annually, sometimes with multiple treatments, with no alternative control materials or strategies available or used. This puts intense pressure on the vole population to select for animals that are less affected by the rodenticide and has resulted in an artichoke field vole population with a significant degree of resistance to chlorophacinone. These findings highlight the importance of developing resistance management strategies for field rodent control programs and demonstrate the need for alternative control materials.

What Has ANR Done?

To help artichoke growers deal with this serious pest situation, UC wildlife experts conducted trials to find alternative control materials and strategies. In the laboratory, they studied the efficacy of zinc phosphide-treated artichoke bait on meadow voles and found it to be extremely effective. They also conducted trials in artichoke fields to compare the laboratory findings with those under field conditions. Zinc phosphide-treated artichoke bait was very effective in reducing meadow vole populations in the artichoke field plots. Overall effectiveness was 95 to 98 percent, compared to about 50 percent with chlorophacinone. The low persistence of zinc phosphide in the environment and the fact that it does not accumulate residue in the vole carcass makes it a viable rodenticide for voles in artichoke fields.

The Payoff

Alternative Rodenticide Found to Control Resistant Voles

To make this a useful solution, zinc phosphide must be registered for use in artichoke fields. Data from the UC trials are being used to support the registration of this material. Since this bait works in a completely different way than chlorophacinone, it will be effective on those animals that have developed the resistance we identified, giving artichoke growers a tool to reduce the serious damage to their crop. In the future, alternating treatments with zinc phosphide and anticoagulants like chlorophacinone will be an important strategy in fighting rodenticide resistance.


Supporting Unit: Wildlife, Fish & Conservation Biology

Terrell P. Salmon, Ph.D.
County Director and Wildlife Specialist
UC Cooperative Extension - San Diego

(858) 694-2864