A STUDY ON USING CARBON MONOXIDE FOR GOPHER CONTROL TITLED: Assessing the efficacy of carbon monoxide producing machines at controlling burrowing rodents.
PROJECT LEADER: Roger A. Baldwin Department of Wildlife, Fish, and Conservation Biology University of California, Davis One Shields Ave., Davis, CA 95616
COLLABORATORS: Ryan Meinerz Department of Wildlife, Fish, and Conservation Biology University of California, Davis One Shields Ave., Davis, CA 95616
June 2016
Pocket gophers
Efficacy of PERC machine. —We used the open-hole method to monitor the efficacy of PERC applications (Engeman et al. 1993, 1999) in paired treatment and control plots (each approximately 4 ha in size) in each field. For this approach, we established twenty 9.2 × 9.2-m monitoring plots that were focused on areas with fresh pocket gopher mounding activity. Each monitoring plot was at least 18.3 m from adjacent monitoring plots to maintain independence. Also, no monitoring plots were placed within 9.2 m of the border of the treatment plots to reduce the likelihood that individuals would reinvade treatment plots from outlying areas post-treatment. Within each monitoring plot, we opened two holes into pocket gopher burrow systems. Pocket gophers do not tolerate openings into their burrow systems; they plug holes with soil when encountered thereby rendering this approach very sensitive to pocket gopher presence (Engeman et al. 1993, 1999). We checked breached burrows two days after opening holes to determine if they were plugged by pocket gophers. If either of the holes were plugged, we considered the plot occupied. This approach allowed us to determine efficacy by comparing the number of plots occupied before and after treatment.
For application, we used the PERC 412 machine to inject exhaust into pocket gopher burrow systems for approximately three min. We attempted to treat all active burrow systems at least.
once within each of the two treatment plots. Once injections were complete, we levelled all mounds associated with that burrow system so that we could rapidly identify new activity for subsequent treatments. One to two days post-treatment, we initiated another assessment of pocket gopher activity using the open-hole method. This second assessment allowed us to determine efficacy for the first treatment period. We then conducted a second round of PERC treatments three days after completion of the second open-hole assessment given that 20–30% of individuals are missed each treatment session due to variable mounding activity (Richens 1965, Baldwin et al. 2016). The final assessment of pocket gopher activity was initiated the day following completion of the second PERC treatment. This quick turn-around time for assessing post-treatment activity almost completely eliminated the possibility of reinvasion by pocket gophers from adjacent burrow systems. No activities occurred in the control plots other than the use of the open-hole method for monitoring pocket gopher occupancy.
Pocket gophers
Efficacy. —Efficacy associated with the initial PERC treatment ranged from 40–55% across the two treatment plots (Table 2). Efficacy values increased to a mean of 68% (SE = 2.5) after a second treatment (Table 2). Although mean values never attained the desired 70% threshold, the resultant values were close to this threshold and indicate that this approach likely has some utility for pocket gopher management. Control plots exhibited little variability in occupancy across treatment periods (Table 2), thereby indicating that the observed reductions in pocket gopher activity in treatment plots was due to the PERC applications.
Based on our findings, the PERC machine Does Not Appear to be as effective for pocket gophers. nor is the PERC machine as efficacious as aluminum phosphide. We are uncertain as to why, although there are several likely factors worth considering. First, pocket gopher tunnel systems are longer and narrower than ground squirrel burrow systems, and they have more branches to their tunnel systems (Witmer et al. 2012). As such, it may be more difficult for the exhaust to completely fill the pocket gopher tunnel system, thereby rendering it less effective. Phosphine gas is heavy compared to the air in a tunnel system (Salmon et al. 1982) and may be better able to fill the entire burrow system more completely than carbon monoxide. Alternatively, pocket gophers may be more sensitive to carbon monoxide than ground squirrels. Gas cartridges produce high concentrations of carbon monoxide and are not typically reported effective for pocket gophers (Matschke et al. 1995, Nolte et al. 2000). Pocket gophers apparently detect the gas and smoke and quickly build a wall in their tunnel system that blocks the gas from reaching them. A similar situation may occur in some pocket gopher tunnel systems when using the PERC machine. Pocket gophers do not seem to have an adverse reaction to aluminum phosphide. In fact, pocket gophers may be attracted to aluminum phosphide, as tablets have been identified in nesting chambers following application (R. O. Baker, California State Polytechnic University, Pomona, personal communication For our studies, approximately 3-min injection times were recommended by the manufacturer. However, duration of injection deserves additional scrutiny to determine the optimal time for injection so as to maximize efficacy while minimizing application time and treatment costs.
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