Developing fertility control for rodents: a framework for researchers and practitioners

Fertility control is often heralded as a humane and effective technique for management of overabundant wildlife, including rodents. The intention is to reduce the use of lethal and inhumane methods, increase farm productivity and food security as well as reduce disease transmission, particularly of zoonoses. We developed a framework to guide researchers and stakeholders planning to assess the effectiveness of a potential contraceptive agent for a particular species. Our guidelines describe the overarching research questions which must be sequentially addressed to ensure adequate data are collected so that a contraceptive can be registered for use in broad-scale rodent management. The framework indicates that studies should be undertaken iteratively and, at times, in parallel, with initial research being conducted on (1) laboratory-based captive assessments of contraceptive effects in individuals; (2) simulation of contraceptive delivery using bait markers and/or surgical sterilization of different proportions of a field-based or enclosure population to determine how population dynamics are affected; (3) development of mathematical models which predict the outcomes of different fertility control scenarios; and (4) implementation of large-scale, replicated trials to validate contraceptive efficacy under various management-scale field situations. In some circumstances, fertility control may be most effective when integrated with other methods (e.g. some culling). Assessment of non-target effects, direct and indirect, and the environmental fate of the contraceptive must also be determined. Developing fertility control for a species is a resource-intensive commitment but will likely be less costly than the ongoing environmental and economic impacts by rodents and rodenticides in many contexts.

Ethyl-iophenoxic acid as a quantitative bait marker for small mammals

Bait markers are indispensable for ecological research but in small mammals, most markers are invasive, expensive and do not enable quantitative analyses of consumption. Ethyl-iophenoxic acid (Et-IPA) is a non-toxic, quantitative bait marker, which has been used for studying bait uptake in several carnivores and ungulates. We developed a bait with Et-IPA, assessed its palatability to common voles (Microtus arvalis), and determined the dose-residue-relation for this important agricultural pest rodent species. Et-IPA concentrations of 40 to 1280 μg Et-IPA per g bait were applied to wheat using sunflower oil or polyethylene glycol 300 as potential carriers. In a laboratory study, common voles were offered the bait and blood samples were collected 1, 7, and 14 days after consumption. The samples were analyzed with LC-ESI-MS/MS for blood residues of Et-IPA. Sunflower-oil was the most suitable bait carrier. Et-IPA seemed to be palatable to common voles at all test concentrations. Dose-dependent residues could be detected in blood samples in a dose-dependent manner and up to 14 days after uptake enabling generation of a calibration curve of the dose-residue relationship. Et-IPA was present in common vole blood for at least 14 days, but there was dissipation by 33-37% depending on dose. Et-IPA meets many criteria for an "ideal" quantitative bait marker for use in future field studies on common voles and possibly other small mammal species.

A vertical bait station for black rats (Rattus rattus) that reduces bait take by a sympatric native rodent

Novel bait stations can be used as a targeted method of delivering bait by exploiting behavioural traits of the target species. On Muttonbird Island, New South Wales, the black rat (Rattus rattus) has been baited to aid the conservation of the island’s wedge-tailed shearwater (Ardenna pacifica) colony, which may result in poisoning of the sympatric swamp rat (Rattus lutreolus). We aimed to design a bait station that R. rattus could reach, but that R. lutreolus could not. We found that 11 (92%) of 12 captive R. rattus reached the bait chambers by climbing a 50-cm vertical pipe, whereas only four (18%) of 22 R. lutreolus reached these bait stations. In a field trial on Muttonbird Island R. rattus entered the bait chamber on an average of 5.3 events per night of vertical bait station deployment, but R. lutreolus did not enter the stations. In a field trial on the mainland at a site with a high density of R. lutreolus, this species was detected in one vertical bait station five times, equating to an average of 0.017 events per night of vertical bait station deployment. We conclude that R. rattus readily climbs a 50-cm pipe to enter the bait station, whereas R. lutreolus rarely or never does on Muttonbird Island or at the mainland site.