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

Does Diphacinone Application Strategy Affect Secondary Exposure Risk Associated with Management of California Ground Squirrels?

California ground squirrels (Otospermophilus spp.) are one of the most damaging vertebrate species in California agricultural systems, including rangelands. One of the primary tools used to mitigate damage caused by ground squirrels is rodenticide application. First-generation anticoagulants, such as diphacinone, are the most commonly used rodenticide for California ground squirrels and are applied in one of three methods: 1) spot treatments, 2) broadcast applications, and 3) bait stations. Spot treatments involve spreading bait very thinly by hand around a burrow entrance. Spot treatments tend to be time consuming, so they are generally only used to treat a small number of burrow systems. Broadcast applications require the use of a calibrated seed spreader. This allows easier application of bait to much larger areas. Bait stations house bait within a centralized location. They are used to exclude non-target access to bait by animals larger than the entrance of the bait station. Bait stations are generally believed to have the lowest risk of primary non-target exposure (i.e., potential of rodenticide exposure from direct feeding on toxicant) given this exclusionary advantage, but they have been postulated to lead to higher secondary exposure risk (i.e., potential of exposure to predators or scavengers from feeding on intoxicated animals) if repeated feedings at bait stations result in higher residual concentrations in dead and dying ground squirrels. Other factors can influence secondary exposure risk as well, including the amount of bait applied, the time from bait application until death, and the proportion of carcasses that are exposed aboveground. Anticoagulant rodenticides have come under intense scrutiny over the last several decades, given the potential for secondary exposure concerns associated with their use. Therefore, we established a study in rangelands in central California using radiotransmittered individuals to address how bait application strategy (i.e., spot treatment, broadcast application, and bait station) influences diphacinone (0.005% concentration for spot treatments and bait stations, primarily 0.01% for broadcast applications, although 0.005% was used in one application period; see Baldwin et al. 2020 for additional details) secondary exposure risk in California ground squirrels. Specifically, we addressed: 1) differences in amount of bait applied across application strategies, 2) differences in residual concentrations of diphacinone across application strategies, 3) potential variability in time from application until death across application strategies, and 4) proportion of ground squirrels that died belowground. Bait stations resulted in the greatest amount of bait applied ( = 18.6 kg ha-1), followed by spot treatments ( = 3.2 kg ha-1), and broadcast applications ( = 1.0 kg ha-1). We believe much of the bait that was removed from bait stations was cached, but this was not documented, nor was the potential impact of this caching behavior on non-target species known. This could be an area of additional research in the future. Average time from bait application until death did not vary across application strategies ( = 9.1 days), suggesting that application strategy has little effect on this potential exposure risk. The vast majority of ground squirrels died belowground (82-91%), substantially reducing scavenging concerns. Likewise, most carcasses were severely decayed within three days post-mortality, further reducing scavenging risk. We detected no difference in residual concentrations of diphacinone across the three application strategies ( = 1,399 ppb), although our results from broadcast applications may have been confounded by our primary use of a higher-concentration diphacinone bait. Our use of this higher-concentration bait was in accordance with the label at the time of this study, but it is possible that the use of a lower concentration bait could yield similar efficacy as compared to the higher concentration product while yielding lower residual diphacinone concentrations. This is another area of potential exploration. We did observe substantially lower residual concentrations of diphacinone in ground squirrels that survived the bait application period ( = 112 ppb), suggesting that the greatest risk of secondary exposure lies with scavengers and predators that consume squirrels that would have died from lethal exposure. Collectively, this study helps fill in knowledge gaps into the effect of application strategy on secondary exposure risks. Following proper application protocols, combined with daily carcass searches, should substantially reduce secondary exposure concerns associated with first-generation anticoagulants

Potential impact of diphacinone application strategies on secondary exposure risk in a common rodent pest: implications for management of California ground squirrels

Anticoagulant rodenticides are a common tool used to manage rodents in agricultural systems, but they have received increased scrutiny given concerns about secondary exposure in non-target wildlife. Rodenticide application strategy is one factor that influences exposure risk. To understand the impact of application strategy, we tested residues of a first-generation anticoagulant (diphacinone) in liver tissue of radiotransmittered California ground squirrels (Otospermophilus beecheyi) following spot treatments, broadcast applications, and bait station applications in rangelands in central California during summer and autumn 2018-2019. We also documented the amount of bait applied, the mean time from bait application until death, and the proportion of ground squirrels that died belowground. We documented the greatest amount of bait applied via bait stations and the least by broadcast applications. We did not document a difference in diphacinone residues across any application strategy, although survivors had an order of magnitude lower concentration of diphacinone than mortalities, potentially lowering secondary exposure risk. We did not observe any difference among bait delivery methods in time from bait application to death, nor did we identify any impact of seasonality on any of the factors we tested. The vast majority of mortalities occurred belowground (82-91%), likely reducing secondary exposure. Secondary exposure could be further reduced by daily carcass searches. Results from this study better define risk associated with first-generation anticoagulant rodenticide applications, ultimately assisting in development of management programs that minimize non-target exposure

Estimating the Cost of Replacing Forage Losses on Annual Rangeland

Wildfires and natural events can drastically reduce or eliminate the useable forage on grazing rangeland. This publication will help you assess your losses and set a reasonable budget for restoring the rangeland to a productive condition

Estimating the Cost of Replacing Forage Losses on Annual Rangeland

Wildfires and natural events can drastically reduce or eliminate the useable forage on grazing rangeland. This publication will help you assess your losses and set a reasonable budget for restoring the rangeland to a productive condition

Barb Goatgrass and Medusahead: Timing of Grazing and Mowing Treatments

Barb goatgrass and medusahead are invasive annual grasses that have spread or have the potential to spread throughout much of California's annual grasslands. Barb goatgrass is a B-rated noxious weed and medusahead is a C-rated noxious weed in the State of California, meaning that they both cause economic or environmental detriment. Both are prolific seed producers, making management to reduce their abundance possible but eradication difficult and unlikely. This publication gives a thorough description of growth stages of these invasive plants so you can employ the optimum timing of grazing and mowing treatments for management in your situation. Includes color photographs of key growth stages

Cattle Management Strategies To Minimize Foothill Abortion

A tick-borne bacterial disease, commonly known as foothill abortion, kills cow fetuses and has had a devastating effect on herds grazing in parts of the intermountain West. This publication will help you develop management strategies that minimize the impacts of the disease including whether ticks are present in a pasture (identified by dry ice trapping), the time when ticks are active, and the stage of pregnancy of a heifer or cow when grazing pasture where ticks may be present

Cattle Management Strategies To Minimize Foothill Abortion

A tick-borne bacterial disease, commonly known as foothill abortion, kills cow fetuses and has had a devastating effect on herds grazing in parts of the intermountain West. This publication will help you develop management strategies that minimize the impacts of the disease including whether ticks are present in a pasture (identified by dry ice trapping), the time when ticks are active, and the stage of pregnancy of a heifer or cow when grazing pasture where ticks may be present

Estimating Forage Loss from California Ground Squirrels in Central California Rangelands (Abstract)

California ground squirrels (Otospermophilus spp.) cause more economic damage to California rangelands than any other rodent (Baldwin et al. 2022). Burrow systems can undermine stock pond dams, ranch roads, and even hillslopes. However, forage loss is generally the primary concern. When asked, ranchers offer detailed qualitative descriptions of the damage ground squirrels cause and how it impacts the economic viability of their operations. Damage by ground squirrels can be particularly concerning because ranching operations are often on the margins of profitability. Unfortunately, few studies have quantified economic losses from ground squirrels. Additionally, ranchers commonly lease grazing land from public agencies. In many cases, agency employees have a limited understanding of ranching operations and generally do not allow for control of ground squirrels. Quantitative data on ground squirrel impacts to rangelands and ranching operations may help land management agencies better understand the challenges faced by their lessees and justify targeted management actions in the future. Therefore, we tested the amount of standing crop removed by ground squirrels across 16 sites on the Central Coast and interior central California 2019 and 2020. Sampling was conducted during a 4 to 6-week period in May and early-June. This timeframe coincided with the period after juvenile squirrels emerged from natal dens, which allowed us to quantify the collective impact that the entire ground squirrel population had on the landscape. We included four different ground squirrel density categories per site: minimal (0-1 squirrel), low (2-6 squirrels), medium (7-15 squirrels), and high (more than 15 squirrels). To quantify ground squirrels, we counted individuals in 0.4-ha plots using binoculars from an observation point outside the plots. We conducted five ground squirrel counts, repeated twice a day over three days for a total of 30 counts per plot, and used the highest number observed in analyses. We estimated standing crop (biomass of herbaceous vegetation) using the comparative yield method. We also evaluated precipitation and livestock grazing intensity in relation to forage production. We found that each ground squirrel reduced standing crop by 27.2 kg per ha. Precipitation also influenced forage production: each cm of precipitation yielded 16.6 kg per ha of additional available forage. In our model, identified effects of livestock grazing intensity; interaction between livestock grazing intensity and ground squirrel abundance; and interaction between precipitation and ground squirrel abundance on residual standing crop were not statistically significant. Although grazing intensity does influence vegetation biomass, we did not identify a significant relationship between the two, likely because we were limited to collecting grazing intensity data at the scale of the field, not at the scale of our survey plots. Despite their economic impacts, ground squirrels are critically important to California rangeland ecosystems. Ground squirrel burrows are documented to provide habitat for native wildlife including burrowing owls (Athene cunicularia) and California tiger salamanders (Ambystoma californiense). Ground squirrel colonies are also known to be associated with increased native bird species richness, diversity, and abundance (Lenihan 2007). Because of these potential benefits to supporting native wildlife, active ground squirrel management should only be conducted on rangelands when ground squirrel damage exceeds levels considered tolerable by ranch managers. The level of damage considered intolerable will likely vary among managers of different sites. Results from this study could help ranchers and other land managers identify when such a threshold is exceede

Statewide Cross-Sectional Survey of Cryptosporidium and Giardia in California Cow-Calf Herds

Cryptosporidium spp. and Giardia duodenalis are common protozoal parasites in livestock including beef cattle on rangeland and irrigated pasture. A statewide cross-sectional study was conducted to determine the prevalence, species or genotype, and risk factors for fecal shedding of Cryptosporidium and Giardia by cattle from California cow-calf operations. Species and genotypes of Cryptosporidium and Giardia were determined by molecular fingerprinting. Prevalence of Cryptosporidium (19.8%) and Giardia (41.7%) in fecal samples from calves were approximately twice as high as fecal samples from cows (9.2% and 23.1%, respectively). In addition to age, multivariable logistic regression showed that higher stocking density and a higher number of replacement heifers were positively associated with fecal shedding of Cryptosporidium while longer calving interval, a winter/spring calving season, and higher numbers of replacement heifers were positively associated with shedding of Giardia. The dominant species and genotypes of Cryptosporidium and Giardia in feces from these cow-calf herds were Cryptosporidium ryanae (75%) and assemblage E for Giardia duodenalis (90%), which have low impact on public health compared with other zoonotic species/genotypes of these two parasites. We identified host and potential management practices that can be used to protect cattle health and reduce the risk of surface water contamination with protozoal parasites from cow-calf operations. In addition, this work updated the scientific data regarding the predominance of low zoonotic genotypes of Cryptosporidium and Giardia shed in the feces of commercial cow-calf herds on California rangeland and irrigated pasture