Understanding infectious disease risks of dung beetle releases into New Zealand
Eleven exotic species of dung-burying beetles were approved for release into New Zealand’s agricultural pastures by the Environmental Protection Agency (EPA) in 2011, to join four previously introduced species. These beetles are part of the natural nutrient recycling mechanism for dung produced by wild ruminants. They are expected to bring multiple ecosystem service benefits, including reduced nutrient runoff and waterway pollution from agricultural pastures and reduced greenhouse gas emissions and parasitism of livestock, due to the rapid transport of cattle dung underground as the beetles create brood balls.
The decision to apply for permission to release dung beetles was led by the Dung Beetle Release Strategy Group (DBRSG), a landowner-driven organisation with support from the Ministry of Business, Innovation and Employment (MBIE), the Ministry of Primary Industries (MPI), DairyNZ and Environment Southland. Despite the formal risk assessment process conducted by the EPA, the decision to grant approval for unconditional release was publicly questioned. Concerns included the spread of infectious diseases affecting both livestock and public health. Landcare Research worked with DBRSG Project Manager Andrew Barber (Agrilink NZ) to address these concerns.
A team led by Dan Tompkins sought to improve our knowledge in regard to three perceived disease risks: (1) that contaminated dung beetles may potentially transport Mycobacterium bovis (the causative agent of bovine TB) away from either cattle or possum dung; (2) dung beetles on pasture could be a food source for possums or encourage their bush-to-pasture movements and, potentially increase rates of TB transmission between wildlife and cattle; and (3) dung beetle tunnelling activity could potentially increase freshwater microbiological loading via increased groundwater contamination.
To understand the risk of dung beetles disseminating M. bovis away from either cattle or possum dung, the team investigated two key components – first whether TB-infected cattle produce M. bovis-contaminated dung to which dung beetles could be exposed, and second whether dung beetles utilise possum dung (the possum being the primary wildlife host of TB in New Zealand). For the first component, the team showed that dung samples collected from 12 tuberculous cattle (at least three of which had sufficiently generalised TB for their carcass to be condemned) failed to yield any positives upon gold-standard bacteriological culture for M. bovis. For the second component, no-choice host range tests showed that possum dung is rarely explored, let alone used, by dung beetles (Fig.). The team concluded that there is a negligible current risk of dung beetles acting as TB transport hosts in New Zealand.
To understand the risk of possums increasing their bush-to-pasture movements, the team first used captive feeding trials with nine possums to investigate whether they would forage for and eat dung beetles. With all of the dung beetles included in cages with possums being accounted for after 2 days’ exposure, and no evidence of any possum foraging for the beetles, these trials demonstrated that possums are unlikely to forage for and eat dung beetles. To further understand this risk, the team conducted a diet survey of free-living possums in an area of high dung beetle availability in Northland. No dung beetle remains were found in the stomach of any of 30 possums examined. With the possums clearly foraging on pasture (evident from high stomach grass contents), the team concluded that there was negligible risk of altering possum foraging behaviour and hence negligible risk of additional dung beetle species potentially increasing rates of TB transmission between wildlife and cattle.
Finally, to understand the risk of dung beetle activity increasing microbial percolation through soil, the team conducted leaching experiments with soil cores in six undisturbed barrel lysimeters (three containing fresh cow dung and dung beetles, and the other three as dung-only controls). After leaving them for 11 days to allow beetles sufficient time to construct burrows and brood chambers, the soil cores were irrigated for 2 days using a drip-type rainfall simulator during which leachate was collected hourly. Testing the leachate for the indicator microbe Escherichia coli (a highly labile microbe expected to be influenced by any changes in microbial-bypass-flow process in soil) showed no pattern of greater microbial loading in the leachate from the soil cores containing dung beetles. The team concluded that the risk is negligible of dung beetles increasing freshwater microbiological loading via increased groundwater contamination.
On the basis of this work, and on evidence addressing other perceived risks, the Technical Advisory Group to the DBRSG recommended that planned releases of new dung beetle species onto New Zealand pastures should proceed. The first release was subsequently conducted in Southland in late 2013.
This work was funded by Core funding from the Ministry of Business, Innovation and Employment to Landcare Research, and Northland Regional Council Envirolink Advice Grant #1296-NLRC161.