Marc R. Matchett Travis M. Livieri Richard A. Bowen Airn E. Hartwig Stephanie Porter Mary L. Wright Jason Fly Madisen Hartlaub Phillip Dobesh Paul Roghair Eddie Childers John P. Hughes Michelle L. Hladik Gregory P. Dooley Brian J. Smith Rachel A. LaCasse Kristy Bly Dean E. Biggins David A. Eads 2026 <p><span id="_mce_caret" data-mce-bogus="1" data-mce-type="format-caret"><span>We evaluated the effects of fipronil bait pellets on two cricetids that commonly occupy colonies of black-tailed prairie dogs (</span><i>Cynomys ludovicianus</i><span>; BTPDs): western deer mice (</span><i>Peromyscus sonoriensis</i><span>) and northern grasshopper mice (</span><i>Onychomys leucogaster</i><span>). In one experiment, bait pellets (0.96 mg fipronil/bait) were applied at 75 baits/ha to three 1.44-ha plots on a BTPD colony. Mouse abundance declined by 70% from before to 6-10 d after treatment. In a second experiment, bait pellets (0.46 or 1.52 mg fipronil/bait) were applied at 125 baits/ha to four plots (0.85-1.86 ha) on two BTPD colonies; two non-treated plots were baselines (1.09 and 2.06 ha). From before to 11-15 d after treatment, mouse abundance declined by 51%- 67% on the treated plots vs. a decline of 9% on the non-treated plots. Mouse survival from before to 11-15 d after treatment was 51% lower on the treated plots. In a third experiment, bait pellets (0.84 mg fipronil/bait) were applied at 125 baits/acre on two 1.44-ha plots on a BTPD colony; two 1.44-ha non-treated plots were baselines. Mouse survival from before to 30-44 d after treatment was 45% lower on the treated plots; the abundance of deer mice on the treated plots remained similar from before to 30-44 d after treatment, perhaps due to juvenile recruitment and/or immigration. In a laboratory experiment, 33 deer mice offered one bait pellet (0.86 mg fipronil/bait) consumed 27% of their bait, on average (range = 0-100%). Over 3 d, deer mouse mortality was estimated at 53%; mortality increased with fipronil dose, which averaged 11 mg fipronil/kg body mass (range = 3-46 mg/kg). Brain samples were available from 31 deer mice; all tested positive for fipronil sulfone, the primary mammalian metabolite of fipronil, at 19 to 61,205 ng fipronil sulfone/g. Additional experiments could determine if these findings scale up to larger landscapes.</span></span></p> application/pdf 10.1016/j.ijppaw.2026.101239 en Elsevier Effects of fipronil bait pellets on two cricetid species: Potential implications for plague mitigation and wildlife conservation article