The method chosen for the LHI REP was to eradicate rats and mice by distribution of the toxin brodifacoum at a concentration of 20 ppm in the cereal based product Pestoff 20R. Justification for this methodology, toxin and bait is provided.
The bait has been distributed at a nominal dose rate of 20 kg (12kg first application + 8kg second application of bait, or 0.4 g of toxin) per hectare on average over the island. Bait has been distributed by a combination of aerial and hand broadcast and through the use of bait stations/trays.
Less than 1.2kg of the active ingredient (toxin) brodifacoum has been used in total, over 2 baiting applications.
Each pellet has a brodificoum concentration of 20 parts per million, 2 ½ times lower than the bait that can be bought in shops.
Area to be Baited
Rats and mice occurred across LHI, including the settlement. LHI has been the only island in the LHIG that was known to contain rodents. However, ship rats are able to swim over 500 m and both rats and mice are difficult to detect at low densities. It was therefore possible that either species may occur on offshore islands and islets close to the main island or may invade those islands prior to the implementation of the operation. To minimise the risks of operational failure, the main island and all nearby islands and islets, other than Balls Pyramid and its associated islets, have been baited. The 23 km distance between Balls Pyramid and the main island renders the chances of invasion by rodents very low.
Number of Bait Drops
Aerial and hand broadcast baiting have been carried out twice, the applications separated by about 14-21 days (depending on the weather). The number of application rounds in bait stations in and around dwellings is approximately 18, however the duration of each round has been dependent on the rate of bait removal by rodents. The application rate for the first bait drop was 12 kg of bait per hectare, and 8 kg per hectare for the second drop. These application rates relate to the actual surface area of the islands. Most rodents were likely killed by bait from the first bait drop. However, it was beneficial to carry out a second bait drop to eliminate the likelihood of any gaps in the distribution of baits, ensure bait is available long enough to ensure that all individuals receive a lethal dose and to target:
- individuals that may have been denied access to bait distributed in the first application (by more dominant individuals that are now dead), and
- any surviving young that had recently emerged from the nest.
The operation has been primarily implemented over winter 2019 (June-August), when the availability of natural food for rodents is low, rodent breeding is greatly reduced and the rodent populations are at their seasonal lowest. Winter is also a period when most non-target seabirds are absent from the LHIG. Bait drops were timed to avoid periods of predicted heavy rainfall (as could have prematurely dissolved the bait) and could not take place in high winds or in the presence of low cloud. Therefore weather influenced the actual timing of the two bait drops. Weather forecasts of rainfall and wind speeds were obtained from the Bureau of Meteorology station on LHI.
The initial bait drop commenced on Saturday June 8 in fine weather and was finished on Monday 10 June. The second drop commenced on June 30 and was completed on Monday 8 July.
Aerial baiting was conducted throughout the LHI PPP and other areas of the main island excluding the settlement area and identified buffer zones. In all areas baited aerially, 10 mm baits (approximately 2 g each) were broadcast at a density of 12 kg/ha (one bait every two square metres) for the first drop and 8kg/ha for the second drop on average over the island.
The bait was dispersed using a purpose built spreader bucket (see Figure 15) slung below a helicopter. A rotating disc typically throws the bait 360o to 35 m (note outlier pellets could be thrown to 45 m); enabling a swathe of up to 70 m to be baited in a single pass.
Overlapping (50%) each swathe ensured that there were no gaps in the distribution of baits (see Figure 16). Application rates out of the bucket were calculated to account for the 50% overlap (i.e. for the first drop 6 kg/ha on each swathe with 50% overlap was applied to achieve a 12kg/ha application rate on the ground). Each bait drop was estimated to take approximately two full days (or several half days) to complete dependent on weather.
Figure 15 Custom built spreader bucket being prepared and in use on LHI during 2007 trials.
Figure 16 Aerial Application Method
In order to achieve the required baiting density on the cliffs and steep slopes (particularly around Mt Gower and Mt Lidgbird) several horizontal flight lines were flown at approximately 50 m vertical spacing along these areas to ensure adequate bait coverage. Baiting around the coast line occurred above the mean high water mark to minimise bait entry into the marine environment. A deflector arm was attached to the spreader bucket to restrict the arc of the swathe to 180o and was used when baiting the edge of buffer zones and minimised bait entry into the marine environment when baiting coastal areas including cliffs. The sowing rate, bait direction and swathe width was all controlled within set limits and was adjusted as required for specific requirements for different types of flight lines (inland, coastal or buffer zone). Other aerial dispersal methods included the idling or turning off of the spinning motor on the spreader bucket which results in bait trickling vertically below the helicopter for narrow areas when required. The combination of techniques enabled all terrains on the LHIG to be effectively baited.
Buffer zones for aerial application to individual properties was agreed with the relevant occupiers and in accordance with relevant regulations and considering outliers from the bait swath. The LHIB committed that this would be no closer than 30 m to dwellings, by agreement or if agreement to the contrary was not reached, then the buffer zone was to be 150 m. In these buffer zones bait was applied by hand, or in bait stations. This was covered in a Property Management Plan for each property. 30 m buffer zones were also established around containment areas for the dairy herd.
GPS was used to guide the helicopter along a set of pre-determined flight lines designed to ensure that all areas are adequately baited. Computer-generated plots of the actual path flown were inspected at predetermined times during and at the completion of the flight to confirm that this has been done. Any identified gaps were treated. Flight-path height was set at an altitude that ensured effective and safe baiting. It was determined in discussion with the baiting operator, and taken into account topography, weather conditions, aircraft safety and the need to avoid significant disturbance to roosting birds.
This baiting methodology was similar to (and was based on) established techniques for other island pest eradications undertaken worldwide. In Australia this technique has been used on islands such as Montague (2007) and Broughton (2009) islands in New South Wales and Hermite Island (1996) in Western Australia. It was also used on World Heritage listed Macquarie Island in Tasmania over autumn and winter 2011.
The aerial baiting technique was first trialled on LHI with non-toxic bait and a custom built spreader bucket (DECCa, 2007). The trials showed aerial baiting to be an effective technique that could be utilised in an operation on Lord Howe Island. The trial report is included in Appendix D – LHI Trials Package. The trial provided an opportunity to establish the correct flight configuration: air speed and settings to produce the required flow rate to achieve the on ground density of bait during operations. Methodologies for loading procedures, and determination of bait usage on flight runs were developed for use in future baiting operations.
Further detailed calibration of the equipment with non-toxic baits (i.e. helicopter, spreader bucket, GPS equipment etc.) were undertaken immediately prior to the operation as part of an operational readiness check overseen by an international eradication expert from the New Zealand Department of Conservation’s Island Eradication Advisory Group.
Hand Broadcasting of Bait
Hand broadcasting of bait was conducted concurrently with aerial baiting. It has been undertaken throughout the settlement area where agreed by residents under individual Property Management Plans and in buffer and exclusion zones (i.e. the lagoon foreshore and Ned’s Beach). In the settlement area, either 10mm (2 g each) or 5.5 mm Pestoff baits (0.6 g each) were hand-broadcast at a density of 12 kg/ha (one bait every two square metres for the 10mm pellet or one bait every half square metre for the 5.5 mm pellet on average) for the first application of bait and at 8 kg/ha for the second application.
Trained personnel moved through hand broadcast areas and applied bait at the designated rate. All personnel carried a GPS unit capable of continuously tracking their path. Computer-generated plots of their paths were used to check baiting coverage. The aim was to distribute baits in garden beds and other areas of vegetation around dwellings, rather than broadcast on lawns. These details are contained in the individual property management plans which have been established between property occupiers and the LHIB.
It was essential that all hand-broadcast bait be out in the open so it could be subject to degradation by weathering. No bait was hand-broadcast directly in or under buildings where it will not be subject to weathering.
Commercially available bait stations (see Figure 17) have been used where aerial or hand broadcasting could not be undertaken. Bait stations have also been placed within all areas containing livestock (i.e. dairy herd, horses and goats). The bait stations used in livestock areas were designed specifically to be able to withstand interference and trampling by stock. Where practicable, and with the agreement of householders, small amounts of bait in open containers (‘bait trays’) similar to commercial products currently available, have been placed within buildings including kitchens, pantries, pet food storage areas etc. Where possible, bait trays have also been put in accessible roof spaces and under-floor cavities.
Note: there has been a potential for currently registered Brodifacoum products to be used in accordance with label conditions by residents in some dwellings. This has been considered on a case by case basis assessing higher palatability of pellets vs. higher dosage, quality control and resident acceptability.
All bait trays and bait stations are monitored regularly and bait replenished as until the end of the baiting period (Nov 1st). Bait uptake provides an indication of rodent activity, along with other detection techniques such as detector dogs, chew blocks and tracking tunnels. Bait in protected locations have not been exposed to weathering, and so any remaining bait will be removed at the cessation of the planned baiting period.
When using bait stations or trays it is important that they are set close enough together that individual rats and mice encounter at least one station during their nightly movements. Rats are wide-ranging and can be eradicated using a grid spacing of 25 m -50 m. Mice, however, are not as wide-ranging, and require a grid spacing as close as 10 m.
Figure 17 Bait Station Examples
Property Management Plans
The LHIB has consulted with all property owners and residents on the island to develop individual Property Management Plans (PMPs) as part of the REP. The PMPs include the agreed baiting methods for each lease on the Island, including the settlement area. This can include the desired combination of hand broadcast and bait stations on individual properties and if properties are on the edge of the settlement area, the appropriate buffer distances for aerial distribution.
The PMPs are confidentially discussed and negotiated with the leaseholders / residents individually and consider mitigation of specific risks and areas of concern on individual properties and in accordance with any regulatory approvals or conditions received. The PMPs only need to be signed once all government approvals have been received and the final decision to proceed with the eradication project has been made by the LHIB. The PMPs do not impact on the tenure of the leases.
At the manufacturing plant in New Zealand, the bait was packaged into 25 kg bags and loaded in approximately 1 tonne weatherproof bait pods for transport by ship to mainland Australia. After customs and quarantine clearance in Australia, the bait was barged to LHI. On arrival on LHI, bait continued to be stored in the weatherproof bait pods in a secured premise at the LHI Airport.
A limited amount of contingency bait was purchased with the order in case of physical damage including weathering or bait loss so it is anticipated that there will be bait remaining at the end of the operation.
Unused Pestoff 20R is being retained in case it is needed for follow up or incursion response. Old bait is either reused (if condition is appropriate) or is collected and transported to the mainland for disposal. Emptied Pestoff bags have either been disposed of in a similar manner as discarded bait pellets or incinerated on LHI in accordance with all legal requirements.
Rodent and non-target carcasses have been collected wherever possible by ground staff during and immediately after the operation, particularly in the settlement area, however due to the large size of the island and rugged and inaccessible terrain it has not been possible across most of the island. Collected carcassses have been dissected and disposed of at a licensed facility on-island.
In the event of a spill, the area was to be isolated and all practicable steps taken to manage any harmful effects of the spillage including preventing baits from, as far as practical, entering streams or waterways. Spilled baits would be collected and put into secure containers. Fine material would be swept up and placed into bags for disposal as above.