FIELD EFFICACY EVALUATION OF PELLETED STRYCHNINE BAITS FOR CONTROL OF MOUNTAIN BEAVERS (Aplodontia rufa)

Radio-telemetry evaluation of several concentrations of pelleted strychnine bait in earlier tests indicated moderate to good bait efficacy for control of mountain beavers. Evaluation of operational baiting with 0.0%, 0.15%, 0.50%, and 0.90% strychnine bait pellets was made in 1990 and 1991 on 24 reforestation units in Washington and Oregon. Results of baiting monitored with burrow activity indicators showed there was little difference in reduction of activity among treatments. Chemical assays and pen bioassays showed baits were lethal, but ingestion was often delayed or baits were discarded. Reexamination of test plots 1 year after baiting showed no detectable change in activity among treatments. Proc. 15th Vertebrate Pest Conf. (J. E. Borrecco & R. E. Marsh, Editors) Published at University of Calif., Davis. 1992 INTRODUCTION Mountain beaver (Aplodontia rufa) cause economic losses through severe damage to reforestation on over 121,500 ha in Oregon and Washington (Northwest Forest Animal Damage Committee 1979). Most forest managers in western Oregon consider damage by mountain beaver the most serious limiting factor to successful regeneration of conifers (Campbell and Evans 1984). Damage to unprotected tree seedlings by mountain beavers over a 3 year period in coastal Oregon showed that about 75% of the study trees were damaged and about one-half were killed by mountain beavers (Campbell et al. 1988). Plantations that are 10 to 15 years old and older also are being damaged by mountain beavers (Evans 1987). Stems and roots of large trees are girdled as the tree canopies close, resulting in understocked stands. Stands that have been precommercially thinned are also girdled. Mountain beaver control methods have been reviewed (Campbell and Evans 1988) but are primarily limited to trapping and use of plastic mesh seedling protectors developed in the 1960s and 1970s (Campbell and Evans 1975). In the early 1980s a water-resistant pelleted bait was developed at Olympia for evaluation of reserpine to control mountain beavers (D. L. Campbell, G. D. Lindsey, and J. Evans, unpublished report, Denver Wildlife Research Center, 26 pp, 1983). Registration of reserpine as a pesticide appeared complex and studies required for registration were not economically feasible; however, the water-resistant bait pellets developed for this use were subsequently formulated with strychnine for mountain beaver control by Oregon Rodent Control Outfitters (ORCO). A Special Local Needs (SLN) registration of ORCO Boomer-Rid 0.31% strychnine alkaloid bait (EPA SLN OR-840029; EPA Est. No.5042-OR-1) for mountain beaver control in Oregon made this product available for use by forest managers. Field tests with Boomer-Rid indicated a 67% reduction in mountain beaver damage to conifers (W. Schaap unpublished report, Oregon State University, 10 pp., 1986). On October 10, 1986 the Environmental Protection Agency (EPA) issued a “Call in of Applications and Data for Federally Registered and Intrastate Products Containing Strychnine” in which efficacy requirements for registration of various strychnine baits were established (EPA 1986). Additional field efficacy studies were required for retaining the Oregon Boomer-Rid Registration. Radio-telemetry studies of mountain beavers exposed to Boomer-Rid baits containing various strychnine concentrations continued to show promising results (D. L. Campbell and J. Evans, unpublished report, Denver Wildlife Research Center, 22 pp., 1989). After reviewing these data, EPA requested that field efficacy tests be made at strychnine bait concentrations of 0.0%, 0.15%, 0.50% and 0.90% using operational procedures in typical field conditions. This report summarizes the operational field efficacy evaluation of 4 strychnine alkaloid concentrations of greendyed ORCO Boomer-Rid bait pellets on a variety of damaged or recently planted reforestation units in western Washington and Oregon. Use of trade names in this report is for purposes of identification and does not indicate endorsement of commercial products by the U. S. Department of Agriculture.