Ecological Risk Assessment for Use of Agricultural Rodenticides in California

Abstract

The California Department of Food and Agriculture holds registrations for four grain-based anticoagulant rodenticides used in agricultural areas in California to control the California ground squirrel. These rodenticides contain either chlorophacinone or diphacinone as the active ingredient at 0.005% or 0.01% by weight, and are applied by either broadcast or spot baiting techniques. Using residue data from recent field studies, an ecological risk assessment was performed for non-target species potentially receiving secondary exposure through consumption of squirrel carcasses. The species of concern included five birds (American kestrel, burrowing owl, common raven, golden eagle, red-tailed hawk) and one highly sensitive mammal, the coyote. Risks to nontarget species were estimated using a risk quotient (RQ) approach, with RQ values calculated using species-specific daily exposure estimates and levels of concern derived from subchronic toxicity studies. Exposure estimates for the birds of concern ranged from 0.05-0.21 mg/kg bw/day for chlorophacinone and from 0.04-0.16 mg/kg bw/day for diphacinone. For chlorophacinone uses, RQs for birds ranged from 0.172-0.724. RQ values for diphacinone were 16-18 times lower. Based on the methodology used and using EPA risk criteria, the RQ data indicate de minimus risks for all avian receptors of concern. Exposure estimates for adult and subadult coyotes spanned a range from 0.009-0.028 mg a.i./kg bw/day depending on the use pattern. RQs for mortality and blood coagulation were generally near, or slightly above, a value of 1.0 for all of the use patterns evaluated; however, because of the conservative exposure assumptions and other factors, it is highly unlikely that ecologically significant effects on coyotes does or could occur due to use of CDFA’s anticoagulant baits. Wildlife incidence data and population modeling studies corroborate this. Based on the weight of evidence, it is concluded that use of CDFA’s rodenticides for spot and broadcast baiting will not cause “unreasonable adverse effects” on coyote populations or those of other predators/scavengers that feed on squirrel carcasses.