why 0.02%? a review of the lethality of carrot baits used in aerial 1080 poisoning of rabbits g....
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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. ParkesLandcare Research, P.O. Box 40, Lincoln, New Zealand
L. TwiggBiol. Science and Biotechnology, Murdoch Univ, South St, Murdoch, 6150 AustraliaPhoto: Don Robson, G Nugent
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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
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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
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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
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Aerial 1080 poisoning of rabbitsERMA 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
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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
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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
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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
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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
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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
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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/haÞ Prompted re-assessment of lethality of rabbit bait
Photo: G Nugent
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Lethality of rabbit baitsQ1: How sensitive to 1080 are rabbits?
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20
Unadapted
Moderately adapted
Tolerant
Minimum lethal dose (mg/kg) 1080
Fre
qu
en
cy
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Lethality of rabbit baitsQ2: How big are the rabbits to be killed?
0 500 1000 1500 2000 2500 30000%
5%
10%
15%
20% Female
Male
All
Rabbit weight class (±50g)
% of rabbits
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Lethality of rabbit baitsQ3: How much 1080 to kill each rabbit?
=> For unadapted rabbits, max of 4 mg of 1080 required
Minimum lethal dose (mg 1080/rabbit)
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Lethality of rabbit baitsQ4: How big are the baits?
=> Despite screening, many small baits - mean only 1.7g
0.5
2.5
4.5
6.5
8.5
10.5
0%
10%
20%
30%
40%
(a) by weight
Bait size class (± 0.5g)
% o
f ba
its
0.5
2.5
4.5
6.5
8.5
10.5
0%
20%
40%
(b) by number Before sowingAfter sowing
Bait size class (± 0.5g)
% o
f ba
its
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Lethality of rabbit baitsQ5: How much 1080 per bait?
=> Few baits contain a minimum lethal dose of 1080
0 2 4 6 8 100
0.2
0.4
0.6
f(x) = 0.0737359607330126 x + 0.0165840167468801R² = 0.658436581360025
Morven Hills, 2010, after sowing
Bait weight (g)
1080 load-ing
(mg/ bait)
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But is all the 1080 available?.
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
Photo: G Nugent
1080 in carrot far less lethal than in aqueous solution
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Lethality of rabbit baitsQ6: How many baits to kill each rabbit?
NB: Assuming 50% bioavailability and actual 0.02% concentration
ÞAt best, under current practice, the largest least 1080-sensitive rabbits must encounter > 15 baits
Rabbit Sensitivity to 1080
Median(50th percentile)
Upper 95% CI(97.5th percentile)
Unadapted 6 13
Moderate 9 19Tolerant 12 24
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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)
Þ Increasing the toxic loading per bait to 4mg should ensure each rabbit had to encounter only one bait
Photo: G Nugent
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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)
Þ But if each bait (encounter) was lethal, then a high encounter rate would be unnecessary
Photo: Don Robson
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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