The SPREP / SPC Guidelines for Invasive Species Management in the Pacific provides a framework for decision-making regarding invasive species management in the Pacific.
Management of invasive species typically involves choosing from one (or more) of four options:
For ants, if eradication or permanent control are chosen, treatment options are very similar. The key difference is the frequency of treatment.
In all cases management (and follow up monitoring) should include ways to reduce resource availability for invasive ants. This should include management of plant pests such as mealybugs and scale insects.
At the very least hygiene and waste management - which reduces the nesting resources and potential for transport - should be enhanced.
Containment involves restriction of the invasive species' movement beyond a known infested area.
Containment restrictions combine manual inspection of any potential vectors (e.g. vehicles) leaving the infested area, with insecticide buffers and the reduction of potential nesting sites immediately outside the infested area. This is to reduce the ants' chances of spreading. Details of the protocols used for these restrictions are in the movement controls section. Containment protocols should be included in an incursion response plan. The plan should clearly outline which agency is responsible for each of the restriction actions. Containment is also heavily reliant on community engagement and should ideally be accompanied by public awareness raising and legislation to enforce the movement controls.
Containment in one area may complement other management options. For example on an atoll where there are uninhabited islets with high conservation values, and less high-value islets that are inhabited, eradication on the high conservation value islet would need to be complemented by movement controls and containment on other islets.
Permanent control involves an ongoing cycle of pesticide assisted and/or mechanical removal of an invasive species followed by monitoring in the infested area. The aim of this control cycle is to maintain the species at an acceptable level where their impact is minimal. Ongoing monitoring of population distribution and density is required to inform future management activity.
Permanent control is not often considered an ideal option for many invasive species, but for invasive ants it may be the most sensible option at the moment.
Permanent control for ants need not always involve many cycles of pesticide use. Sometimes one round of treatment is enough to get ant numbers to a level below which they are causing effects on people, and sometimes on the environment as well. But even if one cycle is effective, future control events may be required. For example, on Christmas Island in the Indian Ocean, heli-baiting of yellow crazy ants happens every few years.
Eradication is the complete removal of a species from its invaded range. In the Pacific, this may be an island, atoll or islet.
While eradication may be the preferred goal, it is often difficult and costly to achieve and may not be realistic where infestations are large and well established.
In addition to their expense, the use of pesticides carries the risk of non target poisoning or other environmental impacts. This potential should be weighed against the benefits of total eradication versus reducing the number of invasive ants to a level where they no longer pose a significant problem.
To determine if a management programme has succeeded in eradication, treatment must be followed by monitoring. Depending on the outcome of this monitoring further treatment may be required. Typically eradication is only declared once ants have been absent for two or more years.
Monitoring costs can be very expensive, and ants are difficult to detect when they are at low abundance.
Only around half of all ant eradications that have been attempted have been successful, despite well-financed long term projects. Even protocols that are highly effective in one location have been found to be ineffective in other situations.
The likelihood of success should be considered when deciding whether or not to attempt eradication.
Another factor is re-infestation. It makes little sense to eradicate an ant from one motu (islet) in an atoll if the risk of re-invasion is high. Keeping numbers down, and, more importantly containment and improving biosecurity, may be better options.
We recommend that eradication should be considered as a goal for all initial detections of any of the five worst threat species.
Biological control involves the use of an invasive species’ natural enemies from its home range or the natural enemies of a similar species to the pest species. These biological controls may be insects or other invertebrates, fungi, bacteria or viruses with a narrow or specific host range. There are currently no biological controls commercially available for ants.
However, a number of research groups are actively researching biological controls for ants. Foremost among these groups is the USDA Agricultural Research Service. This group has discovered three viruses, a microsporidian and a phorid (parasitic) fly infecting red imported fire ants. Some of these have promise as biological controls. A virus was also recently discovered in Argentine ants by researchers from Victoria University.
As well as targeting the ants directly, biological control can involve the pests that facilitate ant invasion success. For example, researchers from La Trobe University have identified a parasite that could control the yellow lac scale, which helps the yellow crazy ant cause devastation on Christmas Island.
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Green, O'Dowd, Neumann, Wittman. 2013. Research and development of biological control for scale insects: indirect control of the yellow crazy ant on Christmas Island 2009-2013. Final Report to the Director of National Parks
Hoffmann, Luque, Bellard, Holmes, Donlan. 2016. Improving invasive ant eradication as a conservation tool: A review. Biological Conservation 198 (2016) 37–49
Lester, Gruber. 2016. Booms, busts and population collapses in invasive ants. Biological invasions. in press
Oi, Porter, Valles. 2015. A review of the biological control of fire ants (Hymenoptera: Formicidae). Myrmecological News. 21:101-116.
Sébastien, Lester, Hall, Wang, Moore, Gruber. 2015. Invasive ants carry novel viruses in their new range and form reservoirs for a honeybee pathogen. Biology Letters 11:20150610
Valles, Oi, Porter. 2010. Seasonal variation and the co-occurrence of four pathogens and a group of parasites among monogyne and polygyne fire ant colonies. Biological control 54:342-348
Valles, Oi, Yu, Tan, Buss. 2012. Metatranscriptomics and pyrosequencing facilitate discovery of potential viral natural enemies of the invasive Caribbean crazy ant, Nylanderia pubens. PLoS ONE 7(2):e31828