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Why 0.02%? A review of the lethality of carrot baits used in aerial 1080 poisoning of rabbits

Why 0.02%? A review of the lethality of carrot baits used in aerial 1080 poisoning of rabbits. G. Nugent, B. Warburton, A. Gormley, P. Fisher, A. McGlinchy, J. Parkes Landcare Research, P.O. Box 40, Lincoln, New Zealand L. Twigg

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Why 0.02%? A review of the lethality of carrot baits used in aerial 1080 poisoning of rabbits

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  1. Why 0.02%? A review of the lethality of carrot baits used in aerial 1080 poisoning of rabbits G. Nugent, B. Warburton, A. Gormley, P. Fisher, A. McGlinchy, J. Parkes Landcare Research, P.O. Box 40, Lincoln, New Zealand L. Twigg Biol. Science and Biotechnology, Murdoch Univ, South St, Murdoch, 6150 Australia Photo: Don Robson, G Nugent

  2. Outline • Renewed need for rabbit control • Aerial 1080 poisoning of rabbits • Current best practice • Review findings • assessment of bait lethality • Potential for improvement Photo: Anon

  3. Renewed need for rabbit control • Rabbit control crucial to economic viability of farming in some ‘rabbit prone’ areas • But 1997 illegal release of RHD resulted in a decade of free control • Impact of RHD now waning => Increased need for conventional control Photo: Don Robson

  4. Need for rabbit control (2) • Land managers have resumed aerial 1080 baiting of rabbits • But still using practices established before 1997 • cf aerial 1080 poisoning of possums where baiting rates have declined dramatically Photo: G Nugent

  5. Aerial 1080 poisoning of rabbits ERMA 2009 summary: • 20 aerial 1080 poisoning rabbit control operations • All carrot bait (cf cereal) • 1080 loading 0.02% • sown at 8-35kg/ha. • c. 4 grams of 1080 per ha . Photo: Don Robson

  6. Aerial 1080 poisoning of possums ERMA 2009 summary: • 44 aerial1080 poisoning possum and rat control ops • Mostly cereal • 1080 loading usually 0.15% • sown at 1-5kg/ha (usu. 2kg) • c. 3 grams of 1080 per ha . Photo: G. Morris

  7. Current best practice for rabbits • On-site cutting of carrot into baits • Screened to remove ‘chaff’ • Sprayed with 1080 solution • to produce 0.02% loading Photo: Don Robson

  8. Current best practice for rabbits (2) • Sown by fixed wing aircraft • Two non-toxic pre-feeds (unscreened) sown first • 10-20kg/ha each • 1080 bait sown along GPS-guided flight paths • 15-35kg/ha Photo: G Nugent

  9. Current best practice for rabbits (3) Key Questions: • Why 0.02% loading? cf 0.15% for possums • Belief: Rabbits may detect and avoid higher rates • Why such high sowing rates? • Belief: Required to ensure there is enough bait for all rabbits at highest local densities • Why two prefeeds? cf one for possums • Belief: Some rabbits highly neophobic so need multiple exposures to increase acceptance

  10. Review findings (1) Why 0.02%? • Not optimised for efficacy against rabbits • Rabbits may in fact be attracted (not repelled) by 1080 • Low toxic loading favoured to reduce the risk to livestock ‘0.20% or 0.02% equally effective for rabbits but 0.02% will kill less sheep’ Rowley 1958 Photo: Anon ,Landcare Research

  11. Review findings (2) Why high sowing rates? • Local rabbit densities can exceed 100 per ha • Rabbits can eat more than 200g of carrot per day • At 0.02% loading, there is only moderate suppression of bait consumption BUT • The number of lethal doses sown is far higher than needed even for 100 rabbits/ha • Prompted re-assessment of lethality of rabbit bait Photo: G Nugent

  12. Lethality of rabbit baitsQ1: How sensitive to 1080 are rabbits?

  13. Lethality of rabbit baitsQ2: How big are the rabbits to be killed?

  14. Lethality of rabbit baitsQ3: How much 1080 to kill each rabbit? Minimum lethal dose (mg 1080/ rabbit) => For unadapted rabbits, max of 4 mg of 1080 required

  15. Lethality of rabbit baitsQ4: How big are the baits? => Despite screening, many small baits - mean only 1.7g

  16. Lethality of rabbit baitsQ5: How much 1080 per bait? => Few baits contain a minimum lethal dose of 1080

  17. But is all the 1080 available?. 1080 in carrot far less lethal than in aqueous solution Photo: G Nugent Henderson, R.J.; Frampton, C.M.; Morgan, D.R.; Hickling, G.J. 1999: The efficacy of baits containing 1080 for control of brushtail possums. Journal of Wildlife Management 63: 1138–1151

  18. Lethality of rabbit baitsQ6: How many baits to kill each rabbit? • NB: Assuming 50% bioavailability and actual 0.02% concentration • At best, under current practice, the largest least 1080-sensitive rabbits must encounter > 15 baits

  19. Review findings (3) Why high sowing rates? • New hypothesis: High rates are needed to ensure all rabbits are able to encounter enough baits before they become ill and stop feeding • Most bait left uneaten (cf current beliefs of some practitioners) • Increasing the toxic loading per bait to 4mg should ensure each rabbit had to encounter only one bait Photo: G Nugent

  20. Review findings (4) Why two prefeeds? • New hypothesis: Rabbits vary in the readiness with which they accept bait. Prefeeding teaches rabbits to look for and eat large numbers of baits – i.e. Prefeeding increases encounter rate (as well as increasing familiarity) • But if each bait (encounter) was lethal, then a high encounter rate would be unnecessary Photo: Don Robson

  21. Potential for improvement Conclusions: • Current ‘best’ practice works • by overcoming built-in constraints of low toxic loading and small highly variable bait size • But there is potential for alternative solutions • Larger uniform baits and/or incr. 1080 loading • Should reduce the sowing rate required and the need for two prefeeds => >50% reduction in cost and 1080 use appear feasible Photo: Don Robson

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