Fumigation of Burrowing Rodents with Carbon Monoxide: A Comparison to Alternative Management Options

Pocket gophers and ground squirrels cause extensive damage to many crops. Pressurized exhaust injection devices are increasingly used for managing these rodents, although no data were available to support their use. Therefore, we established a study to: 1) determine the efficacy of pressurized exhaust machines for pocket gopher and ground squirrel management, 2) compare these results to other burrow fumigant options, and 3) compare their cost effectiveness. Specifically, we tested two different pressurized exhaust machines for both ground squirrels and pocket gophers: Pressurized Exhaust Rodent Controller (PERC), and 2) Cheetah rodent control machine. For California ground squirrels, efficacy for the PERC machine was greater in moist soils (mean = 100%) than in drier soils (mean = 66%). Initial treatments using the PERC machine were more expensive than other burrow fumigation options, given the large cost of the machine. However, costs quickly dropped below that of gas cartridges (~44 days), and eventually dropped below that of aluminum phosphide if used extensively (~830 days). Efficacy for the Cheetah rodent control machine was far less encouraging for California ground squirrels, with results showing increased squirrel numbers at treatment sites (mean = +115%) post-treatment. For pocket gophers, aluminum phosphide (mean = 86%) and trapping (mean = 81%) proved to be more effective than PERC applications (mean = 56%) in heavy organic soils. We observed somewhat greater PERC efficacy in mineral soils (mean = 68%), suggesting potential variability in efficacy across soil types. At this point, the use of the PERC machine appears to be a viable option for inclusion into Integrated Pest Management programs for burrowing rodents where alternative options are limited; the Cheetah rodent control machine showed no utility for ground squirrel management in our study. More extensive testing of pressurized exhaust devices in differing soil types and under variable moisture levels is needed to determine their utility across a broader spectrum of treatment situations

Utility of Visual Counts for Determining Efficacy of Management Tools for California Ground Squirrels

Visual counts are frequently used to assess efficacy of management tools for ground squirrels (Marmotini), but the effectiveness of this approach has not been assessed for many ground squirrel species including California ground squirrels (Otospermophilus spp.). As such, we used visual counts of California ground squirrels to determine the efficacy of diphacinone-treated oat groat applications in rangelands in central California, USA, and compared those results to efficacy values derived from the use of radio-collared ground squirrels in the same plots. We also used location data of radio-collared ground squirrels to explore the size of buffer zone needed around census plots to provide an accurate assessment of efficacy when using visual counts. We did not observe a difference in efficacy associated with the 2 monitoring strategies, indicating that visual counts are an effective monitoring tool for ground squirrels. We observed low efficacy in 2 treatment plots, likely due to low usage of those plots by ground squirrels. Increasing the size of buffer zones would increase the usage of treatment areas by the target population and would help to minimize reinvasion by adjacent ground squirrel populations, which could bias efficacy values low. We suggest a minimum of a 61-m buffer surrounding census plots. Increasing to 66 m or more would further benefit efficacy assessments, but increased size of the buffer zone must be balanced with greater costs and regulatory constraints

Guiding the management of an agricultural pest: Indexing abundance of California meadow voles in artichoke fields

Nearly 100% of U.S. artichoke production comes from California and is concentrated in Monterey County. California meadow voles are damaging rodent pests that can threaten the profitability of growing artichokes. A practical population monitoring method can be invaluable to integrated pest management programs for guiding when and where control is needed and assessing control efficacy. The standard method for indexing vole populations in artichoke fields has been based on observing chewing on artichoke bracts placed throughout the field. Because toxicants are delivered on artichoke bracts, bias for population indexing is potentially introduced. We therefore compared artichoke bracts to nontoxic grain-based wax bait blocks as an alternative chewing medium for eliciting chewing observations for indexing abundance. We also compared the use of binary (presence-absence) observations of chewing to continuous measures (percent chewed). We considered the effect of three sizes of observation grids (4 x 4, 5 x 5, 6 x 6) for indexing.We conducted intensive trapping to determine number of voles known to be alive (KTBA) at each site as a basis for assessing which of the 12 indexing approaches (2 chewing mediums, 2 measurement types, 3 grid sizes) best tracked population abundance. The percent chewed on artichoke bracts for all grid sizes only marginally correlated with KTBA (~0.5), whereas percent chewed on bait blocks correlated very well with KTBA for all grid sizes (~0.9). Reducing continuous data to binary observations produced indices only weakly or negatively correlated with KTBA. Available resources would probably determine whether smaller grid sizes would be used for obtaining chewing observations

Guiding the management of an agricultural pest: Indexing abundance of California meadow voles in artichoke fields

Nearly 100% of U.S. artichoke production comes from California and is concentrated in Monterey County. California meadow voles are damaging rodent pests that can threaten the profitability of growing artichokes. A practical population monitoring method can be invaluable to integrated pest management programs for guiding when and where control is needed and assessing control efficacy. The standard method for indexing vole populations in artichoke fields has been based on observing chewing on artichoke bracts placed throughout the field. Because toxicants are delivered on artichoke bracts, bias for population indexing is potentially introduced. We therefore compared artichoke bracts to nontoxic grain-based wax bait blocks as an alternative chewing medium for eliciting chewing observations for indexing abundance. We also compared the use of binary (presence-absence) observations of chewing to continuous measures (percent chewed). We considered the effect of three sizes of observation grids (4 x 4, 5 x 5, 6 x 6) for indexing.We conducted intensive trapping to determine number of voles known to be alive (KTBA) at each site as a basis for assessing which of the 12 indexing approaches (2 chewing mediums, 2 measurement types, 3 grid sizes) best tracked population abundance. The percent chewed on artichoke bracts for all grid sizes only marginally correlated with KTBA (~0.5), whereas percent chewed on bait blocks correlated very well with KTBA for all grid sizes (~0.9). Reducing continuous data to binary observations produced indices only weakly or negatively correlated with KTBA. Available resources would probably determine whether smaller grid sizes would be used for obtaining chewing observations

Managing Roof Rats in Citrus Orchards: Initial Efforts toward Building an Integrated Pest Management Program

Roof rats cause extensive damage in orchards throughout the world. Integrated Pest Management (IPM) systems are the best option for managing rodents, yet few management systems have been developed and tested to control roof rats in agricultural settings. We initiated a study in 2020 to provide the foundation for an IPM program to manage roof rats in California citrus orchards. Our initial efforts centered on developing effective monitoring strategies for roof rats to determine when management actions are needed, assessing rat movement patterns to determine proper placement of management tools, and conducting initial tests of Goodnature A24 self-resetting traps and elevated bait stations containing 0.005% diphacinone-treated oats. We determined that the use of both tracking tunnels and remote-triggered cameras served as effective monitoring tools for roof rats in citrus orchards, and a smaller 3 × 3 grid placement of these monitoring tools was as effective as a 5 × 5 grid, indicating that substantial material and labor costs could be saved by using the smaller grid size. Placement of the monitoring tools on the ground or up in trees did not influence the effectiveness of this approach. We determined that roof rats exclusively used orchard habitats rather than surrounding fields, so control efforts should be focused in these areas. Roof rats moved substantial distances daily (~170-190 m), but results from bait station trials suggest that spacing of bait stations and traps should be closer together to increase roof rat encounter rates of these devices. Our trial with A24 traps elevated in orchard canopies suggest that there is an advantage to including a platform underneath the trap to increase trap activation. Collectively, this information provides a baseline for future research targeted at developing an effective IPM program for managing roof rats in citrus orchards

Wild Horses, Livestock, and Wildlife Use of Springs and Riparian Areas on the Devil\u27s Garden

In Modoc County, located in northeastern California there is a unique rangeland area heavily populated by wild horses and managed primarily by United States Forest Service known as the Devil\u27s Garden. Wild horses have significantly exceeded (4000 horses) appropriate management levels (206-402 horses) in recent years and expanded their range outside of the wild horse territory (258,000 acres) and on to private and tribal lands (over 450,000 acres). This increase has prompted concern about resource degradation particularly associated with riparian areas. In otherwise arid sage steppe rangelands, springs provide critical watering sources as well as wildlife habitat for sage grouse, deer, elk, pronghorn, and other wildlife. Our objective is to quantify the relative frequency, duration, and timing of use by horses, permitted livestock, and wildlife at spring locations. We correlate how varying levels of horse and/or livestock use affects spring site vegetation and riparian health standards. Ten representative study locations were selected on the Devil’s Garden and motion sensitive cameras were deployed at each location for 14-day sampling periods during the spring, summer and fall of 2015-2017. All photos were visually assessed to record species present, number of each species, and the time, date, and location of the observation. We present preliminary occupancy data, as well as results of corresponding vegetative cover, plant community, and bank alteration sampling. Implications for management and on-going research are discussed

The elusive search for an effective repellent against voles: an assessment of anthraquinone for citrus crops

Vole (Cricetidae) girdling of tree trunks is a common form of damage experienced by tree and vine growers throughout much of the Northern Hemisphere. Management programs that effectively incorporate chemical repellents and vegetation management would be of substantial assistance to growers that experience such damage. Anthraquinone has proven effective as a repellent against voles in lab trials, yet controlled field tests of combined anthraquinone and vegetation management programs are lacking. Therefore, we established a mesocosm-based study in central California, USA, to test the efficacy of anthraquinone and vegetation management for reducing girdling damage caused by California voles Microtus californicus to Clementine citrus trees Citrus clementine under semi-field conditions. We observed a 90–100% reduction in girdling damage for trees following a single application of anthraquinone during two trials in summer and spring, respectively. Removal of vegetation around the base of trees further reduced damage during the summer sampling period, with no girdling observed on anthraquinone-treated trees that were surrounded by bare soil. We did not observe this same relationship during spring, and we observed no relationship between vegetation management in the absence of anthraquinone treatments in either seasonal trial, suggesting that vegetation management had a lesser impact on vole girdling than anthraquinone applications. We observed no decrease in efficacy of anthraquinone across the duration of both sampling periods (5–6 weeks), indicating substantial longevity for anthraquinone. Anthraquinone appears to have substantial utility for minimizing vole girdling damage. Field testing is warranted for additional mammalian species to determine potential uses for other taxa

Repellent application strategy for wild rodents and cottontail rabbits

Effective chemical repellents and repellent application strategies are needed to manage damages caused by wild rodents and rabbits to agricultural resources. For the purpose of comparatively investigating the behavioral response of wild rodents and rabbits to a chemical repellent, we experimentally evaluated the concentration-response relationship of an anthraquinone-based repellent in California voles (Microtus californicus Peale), Richardson’s ground squirrels (Urocitellus richardsonii Sabine), deer mice (Peromyscus maniculatus Wagner) and cottontail rabbits (Sylvilagus audubonii Baird) in captivity. We observed 52–56% feeding repellency for whole oats treated with 10,800ppm anthraquinone or 18,500ppmanthraquinone in mice and squirrels, and 84–85% repellency for oats treated with 18,300ppm anthraquinone or 19,600ppm anthraquinone in voles and rabbits, respectively. In addition to providing the negative postingestive consequences necessary for conditioned food avoidance, the anthraquinonebased repellent also absorbs ultraviolet (UV) wavelengths that are visible to most wild birds. For the purpose of developing a repellent application strategy to modify the behavior of vertebrate pests, we therefore conducted a conditioned avoidance experiment by offering repellent- and UV-treated food to California voles in a subsequent behavioral assay. Relative to unconditioned test subjects (P = 0.3161), voles conditioned with the UV, postingestive repellent subsequently avoided whole oats treated only with an UV cue (P = 0.0109). These behavioral responses to anthraquinone-based repellents and UV feeding cues can be exploited as a repellent application strategy for wild mammals. We discuss potential applications of preplant seed treatments and surface treatments that include postingestive repellents and related visual cues for the protection of agricultural resources associated with mammalian depredation

Repellent application strategy for wild rodents and cottontail rabbits

Effective chemical repellents and repellent application strategies are needed to manage damages caused by wild rodents and rabbits to agricultural resources. For the purpose of comparatively investigating the behavioral response of wild rodents and rabbits to a chemical repellent, we experimentally evaluated the concentration-response relationship of an anthraquinone-based repellent in California voles (Microtus californicus Peale), Richardson’s ground squirrels (Urocitellus richardsonii Sabine), deer mice (Peromyscus maniculatus Wagner) and cottontail rabbits (Sylvilagus audubonii Baird) in captivity. We observed 52–56% feeding repellency for whole oats treated with 10,800ppm anthraquinone or 18,500ppmanthraquinone in mice and squirrels, and 84–85% repellency for oats treated with 18,300ppm anthraquinone or 19,600ppm anthraquinone in voles and rabbits, respectively. In addition to providing the negative postingestive consequences necessary for conditioned food avoidance, the anthraquinonebased repellent also absorbs ultraviolet (UV) wavelengths that are visible to most wild birds. For the purpose of developing a repellent application strategy to modify the behavior of vertebrate pests, we therefore conducted a conditioned avoidance experiment by offering repellent- and UV-treated food to California voles in a subsequent behavioral assay. Relative to unconditioned test subjects (P = 0.3161), voles conditioned with the UV, postingestive repellent subsequently avoided whole oats treated only with an UV cue (P = 0.0109). These behavioral responses to anthraquinone-based repellents and UV feeding cues can be exploited as a repellent application strategy for wild mammals. We discuss potential applications of preplant seed treatments and surface treatments that include postingestive repellents and related visual cues for the protection of agricultural resources associated with mammalian depredation