Evaluating Norway Rat Response to Attractant and Repellent Odors to Improve Rodenticide Baiting Effectiveness

Control of rodent infestations using acute-rodenticide baiting is frequently fraught with difficulties involving bait-shyness and neophobic effects. To simulate some of the parameters encountered in baiting situations, pre-weighed quantities of Environmental Protection Agency (EPA) standard challenge bait mixture were used as a highly palatable bait base to induce feeding without the need for food deprivation in Wistar-strain laboratory albino rats. Individual animals were tested for attractant effects of 3 odors (rat urine, preputial gland extract, and carbon disulfide) and a natural repellent odor (coyote urine) in an observation area constructed of clear plastic panels containing a central alley and 2 choice compartments. Three measures of rat preference -- the bait intake levels of challenge bait, arena compartment choice, and time expended in each compartment -- suggested that the carbon disulfide at IO ppm had an attractant effect. None of the other tested odors produced consistent effects on these measures. A low level of 0.20% zinc phosphide (ZP) rodenticide added to the EPA challenge bait was then used to examine effects of the carbon disulfide attractant odor when compared to deionized (DI) water odor in separate groups of Wistar albino rats. This was accomplished by presenting the foods in containers that incorporated an odor-dispensing wick (filter paper) that surrounded the ZP baits. Consumption levels of EPA challenge bait were elevated in the presence of the carbon disulfide odor when compared with DI water odor during a 3-day pre-baiting period. The effect appeared in both males and females, indicating the potential for improved baiting efficacy. Although mortality was 11% higher when carbon disulfide was present as an odor additive to the ZP bait, the mg/kg dosages of ZP ingested were not significantly higher when compared to rats in the DI water group. Thus, the rate of bait feeding may have been increased by the addition of the attractant odor and this then led to increased mortality with no measurable change in rodenticide dosage consumed. This increase in efficacy, if confirmed in field tests, could indicate improved economic and safety features afforded by addition of a carbon disulfide odor ingredient to rodenticide baits