Grass is not always greener: rodenticide exposure of a threatened species near marijuana growing operations

Alan B. Franklin, Peter C. Carlson, Angela Rex, Jeremy T. Rockweit, David Garza, Emily Culhane, Steven F. Volker, Robert J. Dusek, Valerie I. Shearn-Bochsler, Mourad W. Gabriel, Katherine E. Horak

Marijuana (Cannabis spp.) growing operations (MGO) in California have increased substantially since the mid-1990s. One environmental side-effect of MGOs is the extensive use of anticoagulant rodenticides (AR) to prevent damage to marijuana plants caused by wild rodents. In association with a long-term demographic study, we report on an observation of brodifacoum AR exposure in a threatened species, the northern spotted owl (Strix occidentalis caurina), found freshly dead within 669-1347 m of at least seven active MGOs. Liver and blood samples from the dead northern spotted owl were tested for 12 rodenticides. Brodifacoum was the only rodenticide detected in the liver (33.3-36.3 ng/g) and blood (0.48-0.54 ng/ml). Based on necropsy results, it was unclear what role brodifacoum had in the death of this bird. However, fatal AR poisoning has been previously reported in owls with relatively low levels of brodifacoum residues in the liver. One likely mechanism of AR transmission from MGOs to northern spotted owls in California is through ingestion of AR contaminated prey that frequent MGOs. The proliferation of MGOs with their use of ARs in forested landscapes used by northern spotted owls may pose an additional stressor for this threatened species.