Where to Look
To reduce the threat of invasive ants two major points of action need to be considered. First, measures to prevent their entry, and second, responses to eradicate or control incursions and limit their spread. Both of these points rely heavily on surveillance strategies to detect invasive ants.
However, as it is not practical (and often impossible) to inspect all potential sites of invasive ant establishment, it is crucial that resources are allocated to maximise surveillance effort and detection. Murphy (2004) outlined the concept of high-risk-site surveillance (HRSS). This is the deliberate targeting of high-risk sites associated with risk goods and pathways, that are thus more likely to be sites of establishment that spread invasive species (Murphy 2004). Critical to this concept is the ability to rank sites and identify their location. Efficient surveillance methods, flexible design, and the ability to measure surveillance effort are also important (Murphy 2004).
The ability to rank risk sites is often limited by the quality of information on risk goods (Murphy 2004). However, for invasive ants, several publications give us some ability to characterise risk goods and pathways (Lester 2005; Harris et al. 2005; Suarez et al. 2005; Ward et al. 2006), and thus link these to specific sites.
There are some critical questions that should be considered before any surveillance operation. These questions help define the scope of the operation, the resources needed, and the limitations of the work. Ineffective surveillance is a waste of resources but also can have serious consequences for pest management, such as non-detection of a pest, which could limit management options in the future.
There are two main questions that need clarification. First, the geographical scale of the surveillance. Prevention into a region?, or within regions? (e.g. catchments, landscapes, ecological units, offshore islands, 'valued sites'). These are exclusion categories. For example, despite being present in a region, a management goal might be to prevent their establishment in X and Y catchments, and on all offshore islands. The second question is clarification of either prevention of any populations being established [exclusion], or no NEW populations being established [containment].
Designing a regional surveillance programme: an example
An Envirolink report for Northland Regional Council in 2006 provided a framework for designing a surveillance programme for invasive ant species, and to prevent their subsequent spread (Ward 2006). A modified version of this framework is outlined below, with a focus on Argentine ants.
The approach is site-led surveillance, where sites are prioritised into three categories:
- High-risk sites. High-risk sites have a strong association with, and movement of, risk goods. As a consequence, these sites have a higher probability of acting as a source for the spread of invasive ants, and as sites of early establishment within a region.
- Valued sites. These are the sites where Argentine ants most need to be kept out of. In the Northland example, valued sites were of ecological value, but valued sites may also be of cultural or economic (e.g. horticultural areas) value, and are region-specific. 'Valued' is a subjective term, purposely intended to be flexible in its use.
- Other sites. These are sites not falling into the above, and are not considered further.
It should be said that the above categorisation and statements on probability and risk are not based on any scientific data. Risk goods and sites were determined based on expert experience of invasive ant species. This is a serious data gap. A better understanding of how Argentine ants move around and the sources of new populations is critical to slowing their spread.
The Telecom yellow pages (for Northland 2006/07) were searched for business categories that could be associated with risk goods. These categories were given high–medium–low ranking, and businesses (i.e. sites) from the high–medium categories were compiled into an Excel spreadsheet. Highest risk sites are based around businesses that are most likely to supply and frequently move bark, garden material, and fresh produce (e.g. fruit, flowers, vegetables). Sites of medium risk are based around plant material (lawn turf, trees), rubbish disposal (rubbish bins, scrap metal), landscape suppliers (including orchard suppliers, garden centres), timber and wood products, and camping grounds. Results for Northland were 17 sites with a high-risk ranking, and 163 sites with a medium-risk ranking.
A list of natural sites of significant ecological value in the Northland Conservancy was obtained from Department of Conservation (DOC) staff. To be included on this list, a site needed to meet several criteria based on level one guidelines from the Protected Natural Area (PNA) criteria (see Ward (2006) for more details).
These sites were further divided into a high–medium–low priority ranking on additional criteria, including: (i) their representativeness of an ecological unit in Northland; (ii) broad habitat classification (current knowledge of Argentine ants suggests they prefer habitats that are relatively disturbed and/or have open canopy (Harris et al. 2002, Ward & Harris 2005); (iii) relative number of people that might arrive in the site (a higher number of people generally equates to a greater risk of ants being transported to the site) (see Ward (2006) for more details).
As a result 45 'value' mainland sites were ranked, and 12 were given a high priority ranking. Within the priority framework, sites were also graded on two factors. First, on the basis of habitat, where sand/gumlands and wetland had priority over forest. Second on the basis of visitor numbers, where sites with higher visitor numbers were given priority.
Rankings of sites/categories have been on the basis of expert experience and what is known about invasive ant species. Unfortunately, knowledge of invasive ant species is often limited, especially concerning their movement/dispersal and association with humans.
Therefore, this prioritisation process must be put in context – all sites have the potential to have invasive ants but the probability of this occurring depends on the site being associated with the movement of risk goods and the volume/frequency of that association. But also note that risk classifications are not static. New high-risk sites can be built, and changes at a site may result in a change to its risk status. Furthermore, there is no correct definition of a ‘risk good’; this may vary between species, and invasive ant species are generalists and can thus tolerate a wide range of transport mediums and conditions.
As a consequence of this uncertainty, individual expert opinions may differ in the rankings of different sites and categories. However, such differences in opinion should be satisfied in a flexible surveillance strategy that is based on a strong long-term goal, and through advocacy of businesses and residents. To further assist in developing a longer-term surveillance strategy and optimising the process of site prioritisation, the results of any surveillance programme should be briefly evaluated after the first year.
Potted Clematis marmoraria. Image – Peter Williams
Argentine ants are generalists and opportunists, hence they can be found in many microhabitats. However, the best places to look for Argentine ants include:
- Along pavement cracks, retaining walls, fences
- On curbing and the edges of paths, garden edging (pulling back vegetation)
- On plants, and up trees
- Under stones/metal, leaf litter, twigs
- At the base of plant roots
- In and around timber and firewood
- Around landscaping material, including bark chips, potted plants
- In and around rubbish and compost