Controlling rodents near municipal areas requires bait that must be housed in purpose-built bait stations to prevent interference from curious people, companion animals, and non-target species. Bait station design is important, but so are the baits themselves.
Introduced mammalian predators, in particular rats (Rattus spp.), are a major threat to New Zealand bat populations. Aerial application of the toxin sodium monofluoroacetate (1080) is currently the most costeffective method of controlling rats across large spatial extents. Lesser short-tailed bats (Mystacina tuberculata) may be vulnerable to secondary poisoning from 1080 because they feed on invertebrate prey on the ground that may have consumed toxic bait.
Non-target risk from the use of vertebrate toxins for pest control is an ongoing issue globally. In New Zealand, toxic bait for aerial control of possums and rodents is dyed green and contains cinnamon oil, both of which are thought to reduce the risk that birds will eat the bait. It has been suggested for some time that blue dye may be a more effective feeding deterrent than green dye.
New Zealand has just passed half a century of rodent eradications on islands. Confirmation of the first rat eradication in New Zealand on Maria Island/Ruapuke coincided with the devastating rat invasion on Big South Cape Island/Taukihepa. We review the early history of rodent management in New Zealand leading up to and including the Big South Cape Island/Taukihepa ship rat invasion, and document the development and implementation of rodent eradication technologies on New Zealand islands up to the present day.
Norway rats (Rattus norvegicus) were present on Whale Island (Moutohora), Bay of Plenty, New Zealand between about 1920 and 1987. During 1969-1971 they reduced by less than 10-35 % the breeding success of grey-faced petrels (Pterodroma macroptera gouldi), by eating unattended eggs and killing young or weak chicks. Rabbits (Oryctolagus cuniculus), introduced to Moutohora (240 ha surface area) in about 1968, multiplied rapidly to reach a density of up to 375 individuals/ha by early 1973.
We monitored 16 radio-tagged moreporks (Ninox novaeseelandiae) on Mokoia Island after a brodifacoum poison drop to eradicate mice (Mus musculus), normally included in the owls' diet. All 16 moreporks were alive after 13 days. One bird was found dead on day 22, and corpses of two radio-tagged birds were located on day 51. The bird found on day 22 contained 0.97 mg kg(-1) of brodifacoum in its liver. The other two carcasses were not analysed, but they probably died as a result of brodifacoum poisoning. Thus, three out of 14 birds died (21% mortality).
In Britain, the use of "second-generation'' rodenticides has become widespread on agricultural premises. The high toxicity and relatively long half-lives of these compounds has raised concerns over potential secondary exposure and poisoning of non-target predators. Over the last 15 years, exposure has been extensively documented in the barn owl Tyto alba but relatively little is known about mammalian terrestrial predators.
Stoats were monitored by three methods through an aerial 1080 poisoning operation at Waimanoa, Pureora Forest in August 1997. Tracking rates and number of live captures were used as indices of abundance, and radio-transmitters were used to follow individual animals. All 13 stoats with radio-transmitters within the poisoned area died between 2-18 days after the operation. No mustelids were tracked or live-trapped after the operation for three months. Of the radio-tracked stoats that died, rat remains occurred in 67%, passerine birds in 17%, cave weta in 17% and possum in 8%.
Large scale aerial poison operations with 1080-carrot baits are used extensively to control possum (Trichosurus vulpecula) populations in New Zealand forests for ecosystem conservation purposes and to stop the spread of bovine tuberculosis. Although various procedures have been implemented to reduce the incidence of bird kills, dead birds continue to be found after poison operations.
Toxins, especially sodium monofluoroacetate (1080) and brodifacoum, are widely used throughout New Zealand for control of introduced mammals that are considered pests. This level of toxin use (not necessarily with these toxins) is unlikely to decline for at least 5-10 years. Ecological consequences derive both from mammal population reduction or eradication, and from using toxins as the control method.