Cane toads

Cane toad trap trials – preliminary observations

Graeme Sawyer

Frog Watch ( Northern Territory )

GPO Box 4508 , Darwin NT 0801

Ph:0411881378 Fax: (08) 8941 1167 Website http://www.frogwatch.org

Abstract

Trials by FrogWatch have shown that traps can catch all the cane toads in an area around a house or block in a few weeks, and that once the toads are gone any new toads moving into the area seem to be quickly caught in the traps. Test sites have been kept relatively toad free by a single cage trap. Landowners have commented that they never see toads around the house anymore, except for those in the trap.

The FrogWatch ‘SuperTrap’, a trap designed for permanent set up in the bush, has been tested and has caught 543 toads in six weeks. The traps are proving to be effective during the wet season as well as the dry season, which is a huge boost to our confidence that the traps will help to reduce cane toad numbers.

Keeping toad numbers suppressed in areas where the blocks of land are small and there is a reasonable density of people would seem to be achievable. But what of a broader control strategy or techniques to control toads on a broader scale?

The cage traps would appear to be able to play a significant role in capturing toads, and in large numbers. The Bonrook trial trap captured 224 in the first week and 543 in total.

The captures and the observations of the station managers indicate a very significant and rapid decline in toad numbers. The results of the “supertrap” at Ringwood station, in the Northern Territory , also indicate some similar trends. So what else do we need to know about traps?What we can do with traps as a part of a broader control strategy will be the focus of the presentation.

Introduction

Frogwatch

FrogWatch is a not for profit organization focused of raising environmental awareness, especially issues relating to frogs. FrogWatch has been active in the Northern Territory (NT) for over 10 years and has developed a very strong profile across the community.

FrogWatch is dependent upon the voluntary efforts of members at all levels of our community. In 2004 we recognised the need to get some paid resources working for FrogWatch in order to organise action against cane toads in the NT. We are actively seeking the support of governments, businesses and individuals to provide us with the resources needed to galvanise public action against cane toads.

Whilst our primary focus is on native frogs, we have played a very significant role in raising community awareness about cane toads and their impact on native ecosystems. FrogWatch has trialled traps in a number of settings, including remote bush locations and areas around dwellings, as a part of their research into ways to minimise the impact of the cane toad invasion of the top end of the NT.

House block urban trials

The trials have shown that, during the dry season, traps can catch all the toads in an area around a house or block in a few weeks and that once the toads have been removed any new toads moving into the area seem to be quickly caught in the traps. Test sites have been kept relatively toad free by a single cage trap. Landowners have commented that they never see toads around the house anymore, except for those in the trap.

Bush trials

The cage traps would appear to be able to play a significant role in capturing toads on a broader scale, and in large numbers. The Bonrook trial trap captured 224 in the first week 543 in total over 6 weeks (Table 1). This was a single ‘Supertrap’ placed at the homestead area of a cattle station near Pine Creek in the Northern Territory .

Table 1. Bonrook trial cage trap for cane toads

The captures and observations of the station managers indicate a very significant and rapid decline in toad numbers around the area. This gave us hope that broader scale control was feasible. To test this we set up trap trials in a bush location.

The FrogWatch ‘Supertrap’ trial at Ringwood station, 130kms south of Darwin , has shown that a large capacity, solar powered and automated trap system can continually capture toads around a wetland and reduce the toad numbers in the area. The traps are capturing toads during the wet season as well as the dry season, which is a boost to our confidence that the traps will help to reduce cane toad numbers.

Bush Trial Results

Preliminary results from the placement of the ‘Supertrap’ (Figure 1) on the shoreline of the dam on Ringwood station indicate that traps can limit the build up of toads. Compared to the nearby control site, the numbers of cane toads at the trapping location were reduced (Figure 2). Results for the start of the dry season (May 2005) indicated a build up in numbers as the surface water decreased at the end of the wet season (late April 2005). The trials have also shown an increase in the effectiveness of the traps in terms of the impact on the population at the trapping site compared to the control site (Figure 2).

To date, the toad population would appear to have been halved by the trapping. The hope is that by the late dry season, toads will be eradicated from the trapping site. We have used the traps alone with no other mechanisms and suspect that we could accelerate the process by using more traps and some manual control.

The low capture rate in March is probably due to a combination of long grass making the lights harder to see and the fact that insects were so prolific that the insect ball at the trap was larger than the trap. This meant that toads could get a feed of insects without going into the traps.

Figure 1 . ‘Supertrap’ at Ringwood station trial site

Figure 2 . Results of capture trials. Codes: █ total monthly captures; █ average monthly count at trapping site (Dam 1); █ average count at control site (Dam 2). Note: counts at the control were limited in February - March due to access during the wet season.

The jump in toad numbers in April was probably due to the onset of the dry season and much of the ephemeral surface water vanishing causing the toads to move in on the more permanent water sources.

The capture of toads during April - May is decreasing the toad population significantly at Dam 1 (red Bars) compared to the control site (Yellow bars).

Bycatch

Somewhat surprisingly there has been no by-catch in the cage trap trials. In over 250 nights of trapping, nothing but cane toads have been caught. It appears that we have an effective, manageable and species-specific cane toad trap.

Preliminary findings

These preliminary results invoke the possibility that the traps can be used as a broad scale control mechanism, and play a key role in cane toad management and threat abatement plans.

This would especially be the case if the toad behaviour in the wet dry tropics makes them susceptible to control, especially in the dry season. Their need for water and ability to move indicates they will congregate on remnant water. Preliminary field observations and research support this.

Keeping toad numbers suppressed in areas where the blocks of land are small and there is a reasonable density of people would seem to be achievable.

Broader scale control or minimisation strategies also seem to be possible leading to regional control strategies or larger scale eradication programs or threat abatement strategies. There is still further testing needed to verify the extent to which toads in an area will congregate on remnant water in the late dry season and as they become more hungry, their susceptibility to traps.

Methods

Frog Watch has set up traps on a man made dam on Mt Ringwood station. The initial trap was set up on January 1 st 2005 (Figure 1). The location is in hilly savannah woodland (Figure 3). A second trap was added on the eastern side of the dam on April 19 2005 (Figure 4). The plan is to just use traps to see what impact they can have on the cane toad population at the site.

Figure 3. Dam 1 trapping site

The population of cane toads at a second dam about 2.5 kms from the trapping site is being used as a comparison. Spotlight surveys of the entire perimeter of each dam, taken on the same night, are used as comparisons samples of the population at each location

It is our expectation that the trap site will have significantly less toads by the end of the Dry season than at the control site, and the subsequent build up of toads will be slower on the trapping site than at the control site.

Figure 4. Dam 2 control site

Observations

Whilst it is too early to be conclusive, we have a number of observations and findings which we are hoping to follow through on with additional research and trials at other sites. During the late dry season in the wet dry tropics, cane toads are very susceptible to trapping with light based traps.

Male captures are much more prevalent than female captures close to water (less than 10m from edge) during the wet season.

There were differences in the counts at the trapping site indicating that the trap was reducing the number of toads on that side of the dam. Consistently the count was lower on the trap side. In the week when the light failed and no toads were caught the numbers were more even.

A second trap was added, as it was apparent the toads on the eastern side, the right of the image above, were not moving to the trap during the wet season when insects were plentiful. It is unclear from what distance the traps will attract toads. We have a trap on each side, making a trap approximately every 500 metres of shoreline.. This second trap was set 30 metres from the edge of the water to see if more females would be caught. Preliminary indications are that females toads are keeping back from the edge of the water but appear to be moving in to the edge more as the dry season sets in.

The population mix of males, females and sub adults appears different on the control non-trapping site compared to the trapping site. This may indicate an impact on breeding.

Quite small toads are being retrieved from the traps. Toads small enough to pass through the wire mesh are refuging in the traps and being collected when the traps are cleared.

Questions

Would other controls, such as increasing the number of traps and manual control supplements, increase the rate of toad removal?

What combination of lights needs to be used to maximise capture at different times of the year?

What other attractants can be used to supplement the effectiveness of the lights as lures?