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.

Efficacy and nontarget impact of zinc phosphide-coated cabbage as a ground squirrel management tool.

BACKGROUND: Effective management of ground squirrels relies on an integrated pest management (IPM) approach. Rodenticides may be included in an IPM program, but they must be efficacious with minimal impact on nontarget species. A zinc phosphide-coated green bait may meet these requirements. We established a study in northeastern California to test zinc phosphide-coated cabbage as a management tool for Belding's ground squirrels (Urocitellus beldingi). We specifically addressed factors that would influence the efficacy of a baiting program, as well as potential exposure risk to nontarget species. RESULTS: We found that prebaiting was an important application strategy, and efficacy increased as ground squirrel abundance increased. Efficacy was also greater in western portions of the study area, likely due to greater bait consumption at western sites. Belding's ground squirrels fed most heavily on cabbage during mid-morning and late afternoon; bait applications shortly before these time periods would increase bait consumption while minimizing nontarget risk. Bait uptake was greatest around burrow entrances. The only nontarget species observed feeding on cabbage was the California kangaroo rat (Dipodomys californicus), although they were never observed feeding on treated cabbage. CONCLUSION: Zinc phosphide-coated cabbage can be an efficacious tool for managing ground squirrels, but there will be limitations on where and how it can be used effectively. It posed a low risk to nontarget species present in our study area, but nontarget risk could vary regionally. The use of a zinc phosphide-coated green bait should only be one part of an IPM strategy for managing ground squirrels. © 2019 Society of Chemical Industry.

Novel and current rodenticides for pocket gopher Thomomys spp. management in vineyards: what works?

BACKGROUND: Rodenticides are often included as part of an integrated pest management approach for managing pocket gophers (Thomomys spp.) given that they are relatively quick and inexpensive to apply. Strychnine has historically been the most effective toxicant for pocket gophers, but its use is currently limited in the United States; alternative registered toxicants have not proven effective. Recent research with baits containing cholecalciferol plus anticoagulant toxicants proved effective against pocket gophers in a lab setting. Therefore, we established a field study to compare cholecalciferol plus anticoagulant combinations [0.03% cholecalciferol plus 0.005% diphacinone (C + D), 0.015% cholecalciferol plus 0.0025% brodifacoum (C + B1), 0.03% cholecalciferol plus 0.0025% brodifacoum (C + B2)] with strychnine (0.5%) for pocket gopher management. RESULTS: Strychnine treatments resulted in 100% efficacy after two treatment periods. Both C + D and C + B2 resulted in efficacy significantly greater than 70% after two treatment periods (83 and 75% respectively). Efficacy from C + B1 (85%) was not significantly greater than 70%, but did yield high overall efficacy as well. CONCLUSION: Although strychnine remains the most effective rodenticide for pocket gopher control, the cholecalciferol plus anticoagulant baits tested would be a good alternative when strychnine is unavailable. C + D may be the best option given that it uses a first-generation anticoagulant as the synergist. © 2016 Society of Chemical Industry.