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

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

doi:10.1186/s13104-018-3206-z

Abstract

ObjectiveMarijuana (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.ResultsLiver 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.

Highlights

The number and extent of marijuana (Cannabis spp.) growing operations (MGO) in California have increased substantially since the mid-1990s, with a mix of illegal clandestine operations and those growing for medical or, recently, recreational use [1–3]
Secondary poisoning of non-target wildlife from anticoagulant rodenticides (AR) has become a re-emerging threat in California, especially around outdoor MGOs on or near public lands, which are considered a primary source of AR in wild environments [6, 7, 9, 14]
Raptors found dead with signs of AR poisoning were not found at MGOs, they prey on rodents affected by AR at MGOs and possibly die elsewhere

Summary

Results

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). It was unclear what role brodifacoum had in the death of this bird. 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

Introduction

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Results and discussion

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