Laboratory and Field Assessment of a Carbon Monoxide Producing Fumigant Cartridge for Use in the Control of Rabbits (oryctolagus Cuniculus)

Fumigation is the most effective method of rabbit (Oryctolagus cuniculus) control available in the United Kingdom. Use of current methods, involving formulations that generate hydrogen cyanide (HCN) or phosphine (PH3) on exposure to moisture, is limited by weather conditions. A carbon monoxide producing cartridge has been developed which can be used independent of weather conditions. The cartridge is similar in size to the smaller carbon monoxide (CO) cartridge used in the United States, but produces up to 70% more CO. High concentrations of CO were measured at the entrances of an unoccupied artificial warren fumigated with CO-cartridges, but these declined quickly. There was relatively little movement of CO through the warren but in most parts concentrations of _>.l % were maintained for one hour or more. Wind speed and direction were shown to have significant effects on CO concentration and distribution. Proc. 16th Vertebr. Pest Conf. (W.S. Halverson& A.C. Crabb, Eds.) Published at Univ. of Calif., Davis. 1994. INTRODUCTION The most effective method available for reducing rabbit (Oryctolagus cuniculus) numbers in Great Britain is fumigation of rabbit warrens. Formulations which release hydrogen cyanide (HCN) or phosphine (PH3) in response to moisture are currently approved for such use, but all such formulations are potentially hazardous and their use is limited by weather conditions (Roe and McKillop 1982). PH3-generating formulations are more convenient to use (Ross 1986) for rabbit control than those releasing HCN. In mammals, carbon monoxide (CO) replaces oxygen in the hemoglobin of arterial blood leading eventually to a depletion of oxygen to the brain, resulting in collapse, unconsciousness, and death. A recent review of the toxicology of CO to mammals (Ross unpublished) suggests that carbon monoxide could be a humane fumigant for use in the control of wild rabbits, provided that animals are exposed to concentrations of 1% or greater. Carbon monoxide has been used successfully for vertebrate pest control in a number of countries (Savarie et al. 1980, Deng and Zhao 1986, Pelz and Gemmeke 1988) against a variety of species including many species of burrowing rodents, coyotes (Canis latrans), rats (Rattus sp), and water voles (Arvicola terrestris). CO used for pest control has been produced by charcoal-burning in a stove mounted on a vehicle, from motor exhaust (Oliver and Blackshaw 1979), or by combustion of a cartridge containing a source of carbon (charcoal or sawdust) and a source of oxygen (potassium or sodium nitrate): 4C + 2NaNO3 -» 3CO + Na2CO3 + N2 The last method was used by Savarie et al. (1980), who developed two different cartridges for the control of coyotes and small burrowing animals. These cartridges are available commercially in the United States (USA), but are unsuitable for use against rabbits in Great Britain (UK); the larger (240 g) cartridge is too large for rabbit burrows and the other (65 g) cartridge, although of suitable size, contains a number of ingredients, apart from sodium nitrate and charcoal, which generate a number of other toxic and irritant gases. The use of a CO cartridge as a fumigant has several potential advantages over the currently available fumigants. It would be unaffected by weather conditions, the ingredients of the cartridge are inert until ignited and the CO is generated within the warren. The aim of the present work was to develop a cartridge producing only CO and carbon dioxide (CO2) at least as efficiently as the larger USA cartridge and to investigate the ability of the cartridges to produce concentrations of _>_1 % of CO in an artificial rabbit warren. Weinhouse (1948) used an alternative method of CO production by igniting a mixture of magnesite (MgCO3) and zinc dust, according to the equation: MgCO3 + Zn -» MgO + ZnO + CO Therefore, MgCO3 and zinc dust, or zinc oxide, were added to sodium nitrate and charcoal in attempts to increase CO production.