Rodenticides are an essential tool in the integrated pest management (IPM) of infestations of commensal rodents. With the introduction of Assembly Bill 2422 California Natural Predator Protection Act, the State of California is potentially facing a future with serious restrictions on the use of anticoagulant rodenticides to manage commensal rodents in urban areas. Assembly Bill 2422 has been proposed to protect predators from rodenticide poisoning and seeks to significantly restrict the application of first and second generation anticoagulant rodenticides for use in many urban and no-urban areas of California. Exclusion and cultural practices, such as landscape management and sanitation, are important and successful tools for managing rodent populations. However, quick and efficient control of commensal rodent infestations often necessitates the use of rodenticides. While rodenticide is an important tool, exposure of wildlife to anticoagulant rodenticides has been evident for many years in the state of California. When rodents are consumed by predators, second generation anticoagulant rodenticides can be detected as residues in the livers of predators. Many species of animals are documented as having succumbed to rodenticide toxicosis, however the effects of chronic, sub-lethal exposure to predators are not well understood. This paper will discuss the current and proposed changes to rodenticide legislation in California, impacts of the legislation on communities across California, and gaps in research preventing the adoption of evidence-based best management strategies for rodent control. In order to improve the success of commensal rodent control programs in California, effective strategies for rodent management must be developed.

Roof rats (Rattus rattus) and deer mice (Peromyscus maniculatus) are occasional pests of nut and tree fruit orchards throughout California and in many other parts of the USA and beyond. In general, the most practical and costeffective control method for rodents in many agricultural environments is the use of rodenticides (toxic baits), but little or no information exists on the efficacy of current rodenticides in controlling roof rats and deer mice in orchards. Therefore, our goals were to develop an index of rodent activity to monitor efficacy of rodenticides and to subsequently test the efficacy of three California Department of Food and Agriculture rodenticide baits (0.005 % chlorophacinone treated oats, 0.005 % diphacinone treated oats, and 0.005 % diphacinone wax block) to determine their utility for controlling roof rats and deer mice in agricultural orchards. We determined that a general index using the number of roof rat photos taken at a minimum of a 5-min interval was strongly correlated to the minimum number known estimate of roof rats; this approach was used to monitor roof rat and deer mouse populations preand post-treatment. Of the baits tested, the 0.005 % diphacinone treated oats was most effective for both species; 0.005 % chlorophacinone grain was completely ineffective against roof rats. Our use of elevated bait stations proved effective at providing bait to target species and should substantially limit access to rodenticides by many non-target species.

Anticoagulant rodenticides have been detected in many species of wildlife worldwide; yet the origins, exposure pathways, and effects of this exposure are not well understood. Furthermore, to accurately characterize the risks from rodenticide use, information is needed on what proportion of populations are being exposed, what proportion of the exposed individuals are affected, and in what ways. The relationship between anticoagulant rodenticide concentrations found in wildlife and the rate of mortality or illness is the subject of much current research. Residue levels observed in liver and whole body analyses vary, and overlap extensively among apparently healthy asymptomatic individuals and sublethal and lethal cases. Results from laboratory studies also show there can be wide variability in lethal and sublethal effects among and within taxonomic groups. Correlating the sublethal and reproductive effects observed in laboratory studies with realistic exposure scenarios and effects in the wild is needed to improve risk assessments. For species with limited numbers/declining populations, a critical question is whether the rodenticide exposure documented in individual animals inhibit population growth or contribute to population declines by lowering survival and reproductive success. This information is essential to the regulatory agencies that must weigh the risks and benefits of rodenticide uses and identify restrictions that are effective in reducing risks to wildlife. A primary objective of this symposium was to facilitate communication between regulators and researchers. Current research on many of these topics was presented, and was followed by discussions on how to improve our understanding of what factors lead to wildlife exposure and improve our ability to assess the effects of exposure on individuals and populations. A collaborative approach will be developed to design studies that provide regulatory and wildlife management agencies with additional science on which to base their decisions.

Rodenticides are often used to control burrowing rodents but have not been overly efficacious for Belding's ground squirrels due to poor bait acceptance. This has left alfalfa growers searching for alternative options for controlling this rodent species. As such, we tested aluminum phosphide and gas cartridge burrow fumigation in an alfalfa field in Butte Valley, CA, to determine if either of these approaches were efficacious and cost effective for controlling Belding's ground squirrels. A comparison of the number of burrows treated and the number of burrows reopened 48-hours post-treatment indicated that both burrow fumigants were highly effective (aluminum phosphide = 94-98%, gas cartridges = 100%). The average cost per application was $1.05 and $2.92 for aluminum phosphide and gas cartridges, respectively. Given the almost 3-fold difference in cost of application between the 2 approaches, aluminum phosphide appears to be the more practical approach for Belding's ground squirrel control. Although we currently lack an approach for estimating the amount of damage that Belding's ground squirrels are likely to cause to an alfalfa field, it seems plausible that burrow fumigation could be a cost effective approach to reduce damage caused by this species, particularly if long-term control can be obtained. Possible long-term management options for Belding's ground squirrels are discussed.

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