Argentine ant national survey case study

Tracking the spread of invasive ants

When Argentine ants (Linepithema humile) were first detected in New Zealand a decision was made not to attempt eradication. The decision was made because the ant appeared to already be well established, and because no effective control methods were then available for Argentine ants at that time.

In 2002, twelve years after their initial discovery, a national Argentine ant survey was undertaken to find out how they had spread since 1990.

Survey methods

Argentine ants are strongly associated with human habitation, so it was decided to focus the trapping effort in towns with populations greater than 35,000 people.

Primary survey sites included domestic and international sea ports and international post offices.

Additional surveys were undertaken at towns with <35,000 people and areas outside towns with high probability of detection (i.e. transport hubs).

The survey was conducted between March and May 2002 (autumn).

Sticky traps were used for the survey.

 
site all towns 
> 35,000 people
selected towns
<35,000 people
outside town limits
sea ports yes    yes  
post offices yes   yes  
railway yards yes * yes  
commercial trading centres yes * yes  
hospitals yes * yes  
other high traffic areas   yes  
factories     yes
meat processing plants     yes
dairy processing plants     yes
mills     yes
other transport hubs     yes 
*if not already intercepted by a starburst transect

The traps were made by placing a blob of jam in the centre of one side of a 120 x 15 mm strip of double sided tape, which the ants would stick to. The tape and jam were then put inside a 150mm length of clear plastic tube with an internal diameter of 19mm.

The traps were trialed in an area known to be infested with Argentine ants. The ants discovered 67% of the traps within 4 hours, and all of the traps within 20 hours. The traps also attracted other ant species that are known for their preference for protein lures. This indicated that the traps would remain effective if the Argentine ants’ food preference changed during the time the traps were set.

Where Argentine ants had been recorded previously, traps were laid out in a “starburst” pattern centred on the area of known infestation. Traps were placed at approximately 100, 200, 400 and 800m along each of up to eight transects emanating from the central point. Up to 33 traps were laid per Starburst. The number of traps and number of transects varied if the area made it impractical to use the full method.

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Additional traps were placed in railway yards, commercial trading centres, commercial transfer areas (e.g. warehouses) and hospitals if these sites were not already intercepted by a starburst transect. In selected towns with populations smaller than 35,000, traps were also placed in these and other high traffic areas. In order to increase the probability of traps recording a rare event, traps were also placed in factories, freezing works, dairy processing works, mills and other transport hubs outside town limits.

Traps were removed 24 hours after they were initially placed. A total of 2850 traps were distributed countrywide, of which 2216 were returned in a usable state. Each usable trap was scanned under a binocular microscope to establish which, if any samples might be Argentine ants. The possible Argentine ants were removed from the double sided tape with kerosene and examined in detail to confirm their identity.

Conclusions

Of the 2216 traps examined 84 contained Argentine ants. These 84 traps were collected from 12 locations, 10 of which had previously had Argentine ants recorded, all in the North Island. No Argentine ants were found in the South Island, despite Argentine ants being recorded in one South Island location during a preliminary study.  

The results of the study suggested that Argentine ant dispersal is slow. Dispersal through human transport and establishment in new locations within New Zealand may be inefficient.

The authors concluded that containment or even local eradication was technically feasible due to the slow spread and isolated, discrete infestations. Decisions on whether to take action revolved more around cost and the social and political will to act, rather than feasibility.

Information Sources

Charles, Suckling, Allan, Froud, Dentener, Connolly, Verberne. 2002. The distribution of Argentine ant in New Zealand: can a ten-year old decision not to eradicate be re-visited? In Goldson, Suckling (eds. ) Defending the green oasis: New Zealand biosecurity and Science. Proceedings of a New Zealand Plant Protection Society Symposium pp.153: 109-224

Landcare Research Argentine ants in New Zealand website