Scaling up pest animal control with aerial baiting – what’s your poison?
Australasian harrier with rabbit. Image - Nga Manu Images.
The feasibility of creating large pest-free areas will always be hampered by limited resources. This makes aerial application of toxic bait a practical choice for initially reducing populations of pests such as possums and rats to low levels on a large scale. After this, other more expensive tools (such as trapping, detector dogs, shooting or ground baiting) can be used to mop up the survivors. A combination of methods will be needed, but using aerial baiting raises questions about the environmental effects of the toxins. Objective information about the toxins likely to be used is needed to underpin discussion of the benefits and risks of striving to be pest-free.
Two toxins can currently be applied aerially in New Zealand under specific circumstances – sodium fluoroacetate (1080) and brodifacoum. It is important to distinguish between them when scoping the risks and benefits of their use as they differ significantly in their mode of toxic action and pathways through which each can be transferred to, and degraded in, natural environments.
Brodifacoum is an anticoagulant, one of a ‘family’ of such compounds used worldwide as rodenticides. Bait formulations of several anticoagulants, including brodifacoum, are widely available to the New Zealand public for rodent control around houses and farms (just have a look at the label next time you buy rodent bait!) Anticoagulants act by preventing blood from clotting effectively, so that death in mammals normally eventuates through internal haemorrhage after a few days. Anticoagulant poisoning can be treated successfully by administration of Vitamin K1. Anticoagulants are not water-soluble, and persist for various times in liver tissue (less so in muscle and fat) of animals that eat a sub-lethal dose. Brodifacoum is one of the most persistent anticoagulants so it poses a secondary exposure hazard to any wildlife that scavenge carcasses or prey on rodents and possums. Anticoagulant residues have been reported in birds of prey in many countries, including Australasian harriers in New Zealand. Aerial application of brodifacoum bait is less common than ground application and has much stricter regulation. It is an important conservation tool in the eradication of introduced rodents from large off shore islands or fenced sanctuaries, where complete removal of introduced pests (and their ongoing exclusion) has major biodiversity benefits for island ecosystems and endemic wildlife.
1080 is an acute toxin and its use in New Zealand is also tightly controlled – it requires a licence to use, and its aerial application is subject to stringent regulatory controls and reporting requirements. 1080 is a metabolic energy inhibitor, causing death through cardiac or respiratory failure usually within 24 hours. Treatments for accidental 1080 poisoning have been described, but their success generally depends on early intervention. Aerial application of 1080 for possum and rat control on mainland New Zealand attracts a wide spectrum of positive and negative perceptions, even though from a toxicological and environmental risk assessment context, 1080 is the best researched and described vertebrate pesticide currently in New Zealand use. It is water-soluble, biodegradable in natural water and soil, and does not persist for more than a few days in living animals.
Both brodifacoum and 1080 can cause secondary poisoning of non-target wildlife that scavenge carcasses or prey on poisoned animals. The benefits where feral cats, ferrets or stoat numbers are reduced through secondary poisoning need to be weighed against the secondary hazard to native wildlife scavengers, such as weka, as long as carcasses retain residual concentrations of the toxin.
Penny Fisher recommends continuing to apply the food-web approach suggested by John Innes and Gary Barker to investigate toxin movement and persistence in natural environments, and to assess net outcomes at the ecological community level of using toxins for pest control. This approach is readily applicable to the concept of whether significant areas of the mainland or inhabited islands of New Zealand could feasibly be made pest-free. Penny believes public concern and media attention are likely to focus on the potential risks of using aerial baiting to achieve this, and that this will result in increased publication of both factual and inaccurate information about diff erent toxins. A trusted source of objective and clear information will be needed because 1080 and brodifacoum are already known to have diff erent eff ects on non-target wildlife, diff erent fates in the environment, and diff erent potential to contaminate human food. Such information will be essential in discussions about balancing the benefits and risks of attempting to achieve pest-free status using either toxin as part of an overall strategy.
This work was funded by the Ministry of Business, Innovation and Employment through core funding to Landcare Research.