Quantifying Bait Quality from Number of Random Encounters Required to Kill a Pest

Information on toxic loading and piece-weight distribution are used to calculate an index of the number of baits which an animal must encounter and eat before the probability of death exceeds 99%. It is termed the 99% bait quality index (99BQI).The index shows that, on average, rabbits of median susceptibility to 1080 must find 4.2-5.4 unscreened carrot baits; possums of median susceptibility must find 6.5-8.4. Indices for 1080-tolerant rabbits and pos- sums ranged from 6.2-15.6 baits. Indices for machine-screened baits range from 1.9-3.2 for rabbits of median susceptibility, and 3.2-4.4 for tolerant individuals. Corresponding figures for possums are 2.4-3.8 and 3.2-5.7. Indices for hand-screened bait of appropriate toxic loading are 1-2 baits.
It is argued that when poorly prepared baits, containing many sub-lethal fragments, are used, large amounts of bait must be distributed to enable an intended victim to find many pieces before poison-induced anorexia suppresses further feeding. If bait is scarce, the chances of survival are high, and this is suggested as a possible cause of failure of some control operations. Ways by which the use of large quantities of poorly prepared bait increase the risk of incidental poisoning of birds are also discussed.