Cholecalciferol plus diphacinone baits for vole control: a novel approach to a historic problem

Abstract

Combination baits containing cholecalciferol plus an anticoagulant are effective against commensal rodents resistant to anticoagulants, and they likely pose less risk than anticoagulant-only rodenticides due to lower concentrations of active ingredients and shorter time to death. However, these combination baits have not been tested for agricultural rodent pests. Therefore, we established a study to test the efficacy of cholecalciferol plus diphacinone artichoke bract and pellet baits to determine their ability to manage California voles Microtus californicus in artichokes, where resistance to anticoagulants is known to occur. Field tests using radiocollared voles indicated that bract baits were highly efficacious (85 %), although pellet baits were less effective (60 %). Low observed efficacy of pellet baits may have resulted from poor weather following application during the second sampling period; further testing may yield more positive results. We observed a bimodal distribution in timing of death, with one group of voles dying between 4.3 and 5.8 days post-consumption; the other group died between 9.0 and 14.5 days post-consumption. Deaths in the first group were attributed to cholecalciferol, while deaths in the second group were likely due to chronic anticoagulant exposure. Almost double the proportion of voles that died from bract consumption did so during the early period when compared to their pellet plot counterparts. This suggests that voles were consuming greater quantities of bract baits over a shorter period of time when compared to the pellet bait. Collectively, these findings indicate that baiting with cholecalciferol plus diphacinone coated bracts is an effective method for controlling vole populations in artichokes. Key message• Cholecalciferol plus diphacinone could be an effective alternative to chlorophacinone for managing California vole populations but has not been field tested • Our findings indicate that cholecalciferol plus diphacinone bract baits were very effective at reducing vole populations in artichoke fields; pellet baits were less effective • Time to death is quicker with cholecalciferol plus anticoagulant baits than with anticoagulants alone, thereby reducing secondary poisoning hazards • This combination bait shows promise for use in field applications.

Perspectives on Existing and Potential New Alternatives to Anticoagulant Rodenticides and the Implications for Integrated Pest Management

“…Some researchers are re-visiting formerly registered active ingredients such as norbormide (Campbell et al 2015). Some of the research efforts with potential new rodenticides have been reported by Baldwin et al (2016Baldwin et al ( , 2017, Blackie et al (2014), Campbell et al (2015), Eason et al (2010), Morgan et al (2013), Witmer and Moulton (2014), and Witmer et al (2014a). Another recent research area that is showing good promise is the development and testing of longterm, re-setting toxin delivery systems (Blackie et al 2014;Murphy et al 2014;Witmer and Moulton 2016).…”

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

“…That said, rodents sometimes avoid zinc phosphide baits given an unattractive odor and taste, and through a learned avoidance through sublethal exposure . Green baits such as cabbage can often overcome these concerns by providing a preferred food source for target species, although past experience has not always yielded positive results against ground squirrel species . Substantial variability in efficacy has often been the expectation when using zinc phosphide baits …”

Comparison of mixing methods and associated residual levels of zinc phosphide on cabbage bait for rodent management

“…This is particularly important for certain species, such as Belding's ground squirrels (Urocitellus beldingi), which do not readily consume grain baits, but instead prefer green vegetation (O'Brien, 1978). Applicators generally prefer using grain or pelletized baits rather than green baits (e.g., cabbage leaves and artichoke bracts; O'Brien, 1978;Baldwin et al, 2016) given their longer-term stability and the fact that they can be premixed by certified mixing facilities. The use of green baits requires daily mixing at local sites, potentially leading to greater risk of phosphine exposure for the mixer if improperly trained, and could lead to lower efficacy or greater non-target risk if the resultant concentration is too low or too high.…”

“…The label allows for the mixing of the cabbage bait with vegetable oil and zinc phosphide either in a bucket or via a mechanical mixer. The use of a commercial-style mechanical mixer has proven effective at thoroughly mixing rodenticide-coated artichoke bract baits for vole control (Salmon and Lawrence, 2006;Baldwin et al, 2016), but there is some concern that hand mixing in a bucket may result in an uneven distribution of the active ingredient. If this did occur, the bait would be both less effective and potentially more hazardous to non-target species.…”