Longevity of zinc phosphide on wheat bait in tropical Australia

Abstract

The longevity of zinc phosphide (ZP) on whole wheat bait (2.5% A.I.) was determined at the end of the ‘dry’ and ‘wet’ seasons at Kununurra, Western Australia, the time when rats can damage agricultural crops. While the total rainfall during the two trials was 74 mm and 155 mm, substantial loss of ZP was recorded only after significant rainfall events. Irrespective of season, the loss of ZP from bait applied in bait stations was minimal. The maximum recorded loss was 17%, and this occurred after 21 days’ exposure during the wet season where the bait stations were placed in-crop. Bait stations on the adjacent ‘hilled’ fallow recorded only an 8% loss of A.I. during the same period. However, the loss of ZP from exposed, simulated broadcast bait was much greater: 43% and 91% of ZP was lost from the wheat bait within 21 days during the ‘dry’ and ‘wet’ seasons, respectively. Nevertheless, regardless of the application method, sufficient ZP always remained on the wheat bait for it to be theoretically lethal to rats for at least 8–14 days. The potential of ZP bait for controlling rodent pests in tropical environments, and possible associated ‘problems’ of such an approach, are discussed.