Combining aspirin with cholecalciferol (vitamin D3)--a potential new tool for controlling possum populations

A strategy to improve rodent control while reducing rodenticide release into the environment

In addition to having a negative impact on the health of people and domestic animals, rodents often cause enormous damage to the environment by disrupting natural biodiversity. The negative impacts of rodents in urban and rural areas have required intensive use of rodentcides in spite of the proven risk of secondary poisoning of non-target predators and scavengers. Continuous and intensive use of rodenticides has led to environmental pollution through their retention in the environment. Commensal rodents are predominantly managed with anticoagulant rodenticides, which are very persistent in the environment and move up the food chain and accumulate in the bodies of predators and scavengers. Generally, the use of anticoagulant rodenticides continues, and there is a need to take appropriate measures to reduce their harmful impact. The efficacy of second generation anticoagulants (bromadiolone, difenacoum and brodifacoum), combined either mutually or with chlorophacinone at reduced doses (0.001 % and 0.0008 %), in controlling brown rats (Rattus norvegicus) was tested in a four-day no-choice feeding test. Combinations of second generation anticoagulants were more effective than the combination of chlorophacinone and second generation anticoagulants. The results indicate that combinations of different anticoagulants at multifold lower doses than the standard may provide a successful tool for brown rat control and a more environment-friendly method of rodent control and protection of non-target animals.

Aerial Prefeeding Followed by Ground Based Toxic Baiting for More Efficient and Acceptable Poisoning of Invasive Small Mammalian Pests

Introduced brushtail possums (Trichosurus vulpecula) and rat species (Rattus spp.) are major vertebrate pests in New Zealand, with impacts on conservation and agriculture being managed largely through poisoning operations. Aerial distribution of baits containing sodium fluoroacetate (1080) has been refined to maximise cost effectiveness and minimise environmental impact, but this method is strongly opposed by some as it is perceived as being indiscriminate. Although ground based control enables precise placement of baits, operations are often more than twice as costly as aerial control, mainly due to the high labour costs. We investigated a new approach to ground based control that combined aerial distribution of non-toxic ‘prefeed’ baits followed by sparse distribution of toxic baits at regular intervals along the GPS tracked prefeeding flight paths. This approach was tested in two field trials in which both 1080 baits and cholecalciferol baits were used in separate areas. Effectiveness of the approach, assessed primarily using ‘chewcards’, was compared with that of scheduled aerial 1080 operations that were conducted in outlying areas of both trials. Contractors carrying out ground based control were able to follow the GPS tracks of aerial prefeeding flight lines very accurately, and with 1080 baits achieved very high levels of kill of possums and rats similar to those achieved by aerial 1080 baiting. Cholecalciferol was less effective in the first trial, but by doubling the amount of cholecalciferol bait used in the second trial, few possums or rats survived. By measuring the time taken to complete ground baiting from GPS tracks, we predicted that the method (using 1080 baits) would be similarly cost effective to aerial 1080 operations for controlling possums and rats, and considerably less expensive than typical current costs of ground based control. The main limitations to the use of the method will be access to, and size of, the operational site, along with topography and vegetation density.