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 ZealandL. TwiggBiol. Science and Biotechnology, Murdoch Univ, South St, Murdoch, 6150 AustraliaPhoto: Don Robson, G Nugent

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

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

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

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

Aerial 1080 poisoning of possumsERMA 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

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

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

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

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

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 consumptionBUT

– The number of lethal doses sown is far higher than needed even for 100 rabbits/ha

Þ Prompted re-assessment of lethality of rabbit bait

Photo: G Nugent

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

Freq

uenc

y

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

0 500 1000 1500 2000 2500 30000%

5%

10%

15%

20% FemaleMaleAll

Rabbit weight class (±50g)

% of rabbits

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)

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 bai

ts

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 bai

ts

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)

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

Lethality of rabbit baitsQ6: How many baits to kill each rabbit?

NB: Assuming 50% bioavailability and actual 0.02% concentration

Þ At 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

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

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

Potential for improvementConclusions:• 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