Single or multiple capture traps – what should you buy?
Stoat. Image - Grant Morriss.
Funding is the most significant limiting factor on pest control, and will be a major issue for national initiatives like Predator-Free New Zealand, so improvements in cost-effectiveness of current control methods are always welcome. Trapping possums, rodents, and mustelids is expensive because most traps only capture a single animal and require frequent checking to clear and reset. As a consequence, multiple-capture traps have recently become available, but are currently more expensive (NZ$150–$170) than single-capture traps (NZ$9–$30). Deciding what is the most cost-effective option requires an understanding of how many captures a trap might have at a single site over the time between checks.
Multiple-capture traps seem a good idea when pest densities are high, but when there are few animals (as is the case when pests are being maintained at low densities), spending scarce operational funds on traps that can kill 10–20 individuals might not be justified. Bruce Warburton and Andrew Gormley used an individual-based simulation model to determine, for a given density of possums, ship rats, and stoats, what trapping capacity would be needed to maximise the proportion of the population captured using established operational practices. Such information will enable pest controllers to make choices between using multiple-capture traps and multiple single-capture traps, and help developers of multiple-capture traps to optimise the capture capacity of their traps to avoid redundant (and possibly expensive) capacity.
In the model, an area of c. 1000 ha was established with spacing between traplines and between traps along lines based on established operational practices for each target species. The model simulated 30 consecutive nights of trapping, with captured animals ‘removed’ from the population each night. The number of animals captured per trap was recorded. For single-capture traps, traps were ‘removed’ after capture of a single animal, whereas multiple-capture traps remained in service until their capacity was reached (i.e. 6 or 12 captures), or the trapping period ended. The researchers assumed the single- and multiple-capture traps had equal capture efficiency and that a similar lure was used for both.
Possums
At low densities, increasing the number of possums an individual trap could capture (trap capture capacity) had little effect on the proportion of the possum population captured (Fig. 1), with greater than 98% captured when densities were 0.5 possum/ha. Efficacy of single-capture traps decreased with increasing density with only 60% being caught at a density of three possum/ha. When trap capture capacity was at least three possums, greater than a 97% kill was achieved for the entire range of possum densities tested (Fig. 1). Increasing trap capacity to 12 resulted in only a small gain in kill percentage (viz. 98.6%). So, three single-capture traps set at each trap site would potentially be a cheaper option than setting one multiple-capture trap.
Stoats
At low densities, increasing the trap capture capacity had little effect on the proportion of stoats captured, with more than 98% captured when densities were 0.02 stoats/ha (Fig. 2). Efficacy of single-capture traps greatly decreased with increasing density, with only 47% of the population caught in single-capture traps at the highest stoat densities tested. When trap capacity was increased to at least three stoats, a kill of greater than 97% was achieved for the entire range of densities tested (Fig. 2). Increasing trap capacity to 12 resulted in only a small gain in kill percentage (>99% at 0.12 stoat/ha). When the low-cost stoat trap described by Grant Morriss (p19) becomes available, it will support the use of multiple single-capture traps over a single multiple-capture trap.
Ship rats
Assuming no immigration, at the lowest rat densities simulated, increasing the trap capture capacity, slightly increased the proportion of rats captured, from 83% with single-capture traps to 87% with multiple-capture traps (Fig. 3a). Efficacy of single-capture traps decreased markedly with increasing density, with only 32% of the population caught in single-capture traps at the highest densities. When trap capture capacity was at least three rats, a kill of more than 75% was achieved for the range of densities tested (Fig. 3a). Increasing trap capture capacity to 12 resulted in a 16% gain in kill percentage (87% at 11 ship rats/ha). The maximum kill of 87% achieved in these simulations suggests the trap spacing used was too wide, with some rats not encountering traps.
Rat populations recover quickly after control so immigration was modelled with rats from an adjacent uncontrolled area doubling the population on the controlled area over 30 days if no control took place. This resulted in the percentage of rats captured in traps of all capacities being reduced, but traps with a capture capacity of 12 were little better than those with a capture capacity of 6 except at the highest densities (Fig. 3b).
Conclusion
For maintaining possums and stoats at low densities, Bruce and Andrew’s simulations suggest three or four low-cost single-capture traps set at one place provide a more cost-effective option than using the currently expensive single multiple-capture traps. For ship rats, even at relatively high densities (i.e. 10 rats/ha) 6 low-cost single-capture traps could be more cost-effective than one multiple-capture trap. The simulations ran for only 30 days, and if traps and lure remained effective for longer periods then a higher-capture-capacity trap might have more benefit for controlling rats since they occur at higher densities and have higher immigration rates than possums and stoats. For possums and stoats the results suggest that where a control programme aims to maintain both species at low densities, managers should consider establishing sites with multiple traps as a practical alternative to using single multi-capture traps. Future trials will compare the operational costs of using multiple single capture traps with those of multiple-capture traps.
This work was funded by the Ministry of Business, Innovation and Employment.
Bruce Warburton & Andrew Gormley
Fig. 1 Proportion of the simulated possum population captured by traps with capture capacities of 1, 2, 3, 6, and 12. (The line for 6 is obscured by 12).
Fig. 2 Proportion of the simulated stoat populations captured by traps with capture capacities of 1, 2, 3, 6, and 12. (The line for 6 is obscured by 12).
Fig. 3a Proportion of the simulated ship rat populations captured by traps with capture capacities of 1, 2, 3, 6, and 12, assuming no immigration.
Fig. 3b Proportion of the simulated ship rat populations captured by traps with capture capacities of 1, 2, 3, 6, and 12, assuming immigration.
