Back to the future – using fixed-wing aircraft for aerial application of 1080 baits
The Cresco aircraft (modified by Ravensdown Aero Work) used in this trial being loaded with bait. Image – Grant Morriss
Aerial application of 1080 bait is the only cost-effective option for controlling possums and ship rats over large areas of remote forest in New Zealand. However, even with aerial application, the total area that can be treated is limited as the cost per hectare is high (about $20/ha). To make limited control budgets go further, Landcare Research staff have been investigating different aerial application strategies for reducing the amount of bait sown and the flying time required to sow it (see Precision aerial sowing of baits for possum control). To further decrease costs, Bruce Warburton and Grant Morriss have been working with Ravensdown Aero Work to re-engineer and test the suitability of a Cresco fixed-wing aircraft for sowing bait at low rates compared with helicopters that have dominated the pest control industry over the past decade.
Potential savings result from fixed-wing aircraft sowing bait at twice the speed of helicopters (120 knots compared with 55–60 knots respectively). Although fixed-wing aircraft were used to sow 1080 bait before helicopters, helicopters eventually became the preferred option due to developments in sowing bucket technology and knowledge. Up until recently it was believed that, to obtain high kills of possums and rats, bait had to be distributed uniformly over the landscape. As fixed-wing aircraft could only sow bait in swaths (lines) 25–30 m wide, they had to fly along parallel lines about 30 m apart across the entire control area to achieve complete coverage, resulting in a high number of flying hours. Conversely, some helicopters equipped with underslung buckets with ‘spinners’ can throw the bait horizontally out to 200 m (depending on bucket design), allowing these helicopters to fly along parallel lines 200 m apart (i.e. about 15% of fixed-wing flying patterns).
Research on bait application strategies showed that high kills of possums and rats could be achieved if baits were sown along lines as strips or clusters, and that these strips could be 100 to 150 m apart (Precision aerial sowing of baits for possum control). With this new knowledge, it became apparent that fixed-wing aircraft could again be used because their narrow bait swath was essentially applying bait in a strip. As long as bait density within the strip was sufficient, high kills of pests could be obtained.
The research and success of the re-engineered Cresco aircraft (photo) led to TBfree New Zealand partnering with Muzzle Station (North Canterbury) in August–September 2014 to apply bait in strips to control possums over 10,000 ha in the Clarence catchment. The operational area comprised steep semi-arid hill country from c. 500 m to 2000 m elevation, with a predominant ground cover of matagouri and sweet briar interspersed with scree and rock outcrops. Open areas at lower elevation were vegetated by short tussock oversown with introduced grasses, while at higher elevation tall tussock, spaniards and flax were present.
In one block totalling 6025 ha, the aircraft sowed bait on lines 100 m apart, and in two blocks totalling 1901 ha, bait was sown on lines 150 m apart. The remainder of the area (1980 ha) was sown by helicopter (Fig.) to eliminate any possibility of sowing bait outside the control area. In the blocks with bait lines 100 m apart, non-toxic 2-g prefeed and 12-g toxic RS5 cereal baits were sown at 0.5 kg/ha. In the blocks with lines 150 m apart, the same bait was sown at 0.33 kg/ha. Because the flow rate of bait was the same in both treatment blocks, the bait density was the same on each bait line. The percentage kill of possums was estimated from 37 possums collared with mortality-sensing radio transmitters in the blocks with lines 100 m apart, and from 46 possums spread across the blocks with lines 150 m apart.
The results were excellent with 100% of radio-collared possums killed in the blocks with lines 100 m apart and 98% killed (one survivor) in the blocks with lines 150 m apart.
The mean flying time per 100 ha at a flight line spacing of 100 m for the Cresco (2.7 min) was 45% of the flying time for a squirrel helicopter (5.9 min). The reduced sowing time not only reduced flying costs but also reduced the fuel used and potentially the carbon footprint of aerial control operations.
This new knowledge on possum control and aircraft re-engineering resulting from the team’s research is an excellent example of the rapid implementation of research results making a real difference to operational practice. We are now working with the industry to make additional modifications to other fixed-wing aircraft to reduce costs even further.
This work was funded by the Ministry of Business, Innovation and Employment (C09X1007) and TBfree New Zealand.
