Repellents to protect native birds from 1080 baits
Kea. Image - John Hunt.
New research on bird repellents may provide a means to further reduce the risk of deaths of native birds from aerially sown baits for possums and rats. Addressing the issue of non-target risk will be a major challenge if control of possums and rats becomes more widespread as part of the Predator-Free New Zealand initiative.
Research into optimising aerial 1080 poisoning for possum and rat control has enabled >50% reductions in the amount of poison bait used (see Kararehe Kino Issue 14). This has reduced the potential risk that native birds will be killed by primary or secondary poisoning. However, adverse incidents still occasionally occur, such as the deaths of seven kea after 1080 possum control operations in Westland in 2008 and 2011. In response to these deaths the Department of Conservation (DOC) and the Animal Health Board (AHB) have worked with the Kea Conservation Trust, Unitec and Landcare Research to evaluate chemicals that could be added to cereal baits sown for possums and rats to deter kea and other native birds from eating the baits.
A review of previous research identified d-pulegone and anthraquinone as the most promising candidates. D-pulegone is a primary repellent and is avoided due to its unpleasant taste, smell or irritancy. Anthraquinone is a secondary repellent, inducing an unpleasant feeling shortly after ingestion that results in subsequent avoidance of the bait.
Although both chemicals are effective repellents by themselves, previous research suggested the use of these repellents in combination might produce greater repellency than the use of either alone. If both were present in non-toxic prefeed bait, the primary repellent in toxic bait could remind birds that ate prefeed of their previous unpleasant experience and so increase repellency.
Before proceeding to more trials, DOC, AHB and Landcare Research agreed on further evaluation of the responses of possums and rats to bird-repellent baits, primarily because there was little information about their responses to different concentrations of anthraquinone and most previous trials had used only carrot baits rather than the more commonly used cereal baits.
Landcare Research therefore undertook trials with individually caged possums and ship rats to assess the palatability and efficacy of different combinations of RS5 cereal pellets containing anthraquinone and d-pulegone (Table 1). To simulate aerial operational practice, animals were offered a choice between their normal food pellets and test pellets for 3 days (when most prefeed in an aerial operation will be eaten), given normal pellets only for 5 days, and then given a choice between normal pellets and test pellets containing 0.15% 1080 for 2 days. Baits were manufactured by Animal Control Products and quality assurance testing done by the Landcare Research toxicology laboratory.
Table 1. Levels of bird repellents in non-toxic prefeed and toxic pellets available to six test groups of possums (n = 15) and ship rats (n = 20).
| Test group | Prefeed pellets | Toxic pellets (0.15% 1080) | ||
|---|---|---|---|---|
| d-pulegone +anthraquinone | d-pulegone +anthraquinone | |||
| 1 | 0 | 0 | 0 | 0 |
| 2 | 17% | 0 | 17% | 0 |
| 3 | 0 | 0.25% | 0 | 0.25% |
| 4 | 0.17% | 0.1% | 0.17% | 0.1% |
| 5 | 0.17% | 0.25% | 0.17% | 0.25% |
| 6 | 0 | 0.25% | 0.17% | 0.25% |
Table 2. Mortality of possums and rats in each of the six test groups when offered toxic pellets.
| Test group mortality (%) | ||||||
|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 5 | 6 | |
| Possums | 87 | 87 | 53 | 93 | 83 | 73 |
| Rats | 71 | 75 | 10 | 15 | 35 | 0 |
For possums, overall palatability of the test pellets and the weight of test pellets eaten during the 3 days of prefeeding were lower for groups 3 and 6 than the other groups. Groups 3 and 6, unlike the other groups, showed a progressive decline in palatability of test pellets over the 3 days. Both groups ate less of the toxic test pellets than the other groups and this was reflected in mortality, which was lower for group 6 and significantly lower for group 3 than for the other groups (Table 2). These results suggest possums developed an aversion to pellets after exposure to non-toxic pellets containing 0.25% anthraquinone. The inclusion of d-pulegone with anthraquinone in the prefeed pellets appeared to mitigate development of this aversion in terms of mortality, but consumption of toxic pellets was somewhat less in groups 4 and 5 than groups 1 and 2 (which had no exposure to anthraquinone).
For ship rats, aversion to pellets containing anthraquinone was more pronounced. Groups 3–6 all had palatability, weight of test and toxic baits eaten, and mortality significantly less than groups 1 and 2, which did not differ. Consumption of test pellets by groups 3–6 declined significantly over the 3 days of prefeeding (unlike groups 1 and 2), and only one or two rats in each of groups 3–6 ate any toxic pellets. The inclusion of d-pulegone with anthraquinone (groups 4 and 5) appeared to have a mitigating effect on the aversion, although it was not as pronounced for rats as for possums.
Based on these results, DOC and AHB have agreed to proceed to observational trials with free-living kea at carparks to test their response to non-toxic RS5 baits containing 0.17% d-pulegone and to a case study operation where kea survival will be monitored. If those trials indicate sufficient repellency, operational field trials will be undertaken later in 2013 to test for efficacy against rats and possums. Based partly on the apparent mitigating effect of d-pulegone on anthraquinone aversion, field testing may follow on rats of the repellent combination used in the original Kea Conservation Trust tests, namely prefeed with 0.17% d-pulegone and 0.1% anthraquinone followed by toxic bait with 0.17% d-pulegone. Results from an earlier DOC field trial of that combination did not show the strong aversion found in the present cage trials.
If the observational and operational trials are successful, then DOC and AHB will gather further information to underpin an application for registration of RS5 bait containing bird repellents for possum and rat control.
This work is funded by the Ministry of Business, Innovation and Employment, Science and Innovation Group (contract no. C09X1007), the Animal Health Board and the Department of Conservation.