New Zealand Journal of Zoology abstracts
Field efficacy of cholecalciferol gel baits for possum (Trichosurus vulpecula) control
David R. Morgan
Landcare Research
P.O. Box 69
Lincoln 8152
New Zealand
morgand@landcareresearch.co.nz
Abstract The efficacy of a cholecalciferol gel
bait against high-density possum populations was assessed in two
replicate field trials conducted in beech forest in the Hopkins Valley.
The possum populations were monitored by (i) trap-catch before and
after control at both treated and non-treated sites and (ii) recapture
of possums tagged during pre-control trap-catch. Control was conducted
at each treatment site by presenting a non-toxic gel as a prefeed in
mid April, followed a fortnight later by gel bait containing 0.9%
cholecalciferol; these baits were checked and replenished at intervals
of decreasing frequency (i.e., 1–8 weeks) until late July.
Monitoring of interference at bait stations suggested that the
population was greatly reduced within the first fortnight of toxic
baiting (i.e., by early May). The mean corrected trap-catch reduction
at the two treatment sites was 81%, while there was a 100% reduction in
the recapture of ear-tagged possums at both treatment sites. This
difference was due to an average 68.5% drop in trap-catch at the
non-treatment sites, which reduced the effect of the otherwise very
large declines (average of 94.1%) in the treatment sites, while the
mark/recapture-based estimate was not affected because no tagged
possums were caught after control at the treatment sites. The
unexpectedly large population reductions in the non-treatment sites are
believed to be mainly (possibly entirely) because possums were removed
by unauthorised hunters during June, as verified by Department of
Conservation staff. Therefore, the effectiveness of the cholecalciferol
gel was markedly underestimated by the trap-catch monitoring.
Conversely, the 100% reduction assessed by mark/recapture was
overestimated because some possums were caught in the treatment sites
after control. Consequently, the true reduction achieved by the gel
bait was between 81 and 100%, at a cost of about $35/ha. This figure is
similar to the cost of using other ground-based control methods, but
there is potential for the development of more efficient use of
cholecalciferol gel baits, particularly where sustained control at low
density is the aim.
Keywords possum; Trichosurus vulpecula; pest control; baits; bait palatability; cholecalciferol; trap-catch
Z05044; Received 12 December 2005; accepted 29 May 2006; Online publication date 27 July 2006
New Zealand Journal of Zoology, 2006, Vol. 33: 221–228
0301–4223/06/3303–0221 © The Royal Society of New Zealand 2006
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