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Eradicating the red imported fire ant by numbers

Since first being detected in Brisbane, Queensland, in 2001, red imported fire ants (Solenopsis invicta) have shown themselves to be an extremely damaging invasive pest, affecting agricultural crops, native…

Can mathematics help eradicate fire ants from Brisbane? Storm_XL

Since first being detected in Brisbane, Queensland, in 2001, red imported fire ants (Solenopsis invicta) have shown themselves to be an extremely damaging invasive pest, affecting agricultural crops, native species and human health (in the form of painful stings and potentially fatal allergic reactions).

Attempts to eradicate this biological invader have to date been unsuccessful, despite concerted efforts. In this week’s online version of Proceedings of the National Academy of Sciences (PNAS), we report the results of a new analysis in which we reconstructed the historical trajectory of the Brisbane fire ant invasion to shed light on the effectiveness of the past eradication strategy – with a couple of surprising results.

Meet the enemy

Red imported fire ants are native to South America, and may have been introduced to Australia as early as 1990. They live in colonies consisting of workers, alates and queens:

  • Workers are sterile females that carry out a range of tasks, including foraging. They are small (2-6 mm), reddish-brown in colour, and generally live for only a few months.

  • Alates are winged males and females that leave the colony to mate in flight. Males die after mating, but females shed their wings and become new queens, founding colonies underground.

  • Queens are capable of laying up to 800 eggs a day, and can live for up to seven years.

General worker ant anatomy. Wikimedia Commons

In Australia, the ants were found at two well-separated locations on consecutive days, with samples confirmed to be fire ants by the Queensland Museum and the Queensland Department of Primary Industries.

The two incursions – one centred on Brisbane’s main cargo port at Fisherman Islands, and a larger one centred on the southwestern suburbs of Wacol and Richlands – were later shown to be genetically distinct, indicating two separate introductions.

Fire ant queen, about to take off. Martin LaBar

The Queensland Department of Primary Industries immediately mounted an emergency response to delineate the invasion and if possible eradicate the fire ant, despite scepticism that eradication could be achieved, given no other country has been able to eradicate them.

The National Red Imported Fire Ant Eradication Program, launched in September 2001 by Biosecurity Queensland, initially appeared to be highly successful.

But, although the smaller incursion at Port of Brisbane was declared successfully eradicated, the larger incursion in Brisbane’s southwest continues more than 12 years after the program began.

Indeed, although more than $A275 million has been spent on the eradication program thus far, the known infested region has doubled in area since 2004.

What we did

For our PNAS analysis, we wanted to evaluate whether the eradication program is on track to achieve eradication, and to learn lessons that will inform future management decisions. The results were surprising and frustrating in that an opportunity for eradication seems to have been narrowly missed. They also highlight the crucial importance of mathematical modelling of biological invasions.

The plots below show our estimates of the month-by-month trajectory of the fire ant invasion during 1996-2011, in terms of the number of reproductively mature nests and their geographic range. They illustrate just how close the program came to successful eradication, with the total number of reproductively mature nests dipping below 150 around the end of 2003.

The sequence of maps shows the changing density of ant nests in infested areas, which declined and recovered in a similar manner to the total number of nests.

Estimated number of reproductively mature nests in each month from Jan 1996 to Dec 2011. KO;JKO'JK;

Maps showing the estimated density of fire ant nests in December of each year 2000-2011. Dark red indicates the most densely infested regions, yellow the least densely infested regions.

A comparison of these density maps with the areas searched and treated (using baits toxic to the ant) in each year shows that the eradication program never fully delimited the invasion.

There was always an infested area outside the searched and treated areas. This is not to be critical of the program: delimiting the invasion was recognised as crucial from its inception and extensive research was conducted to develop surveillance tools for that purpose. But inferring the boundary is a difficult problem.

If accurate estimates of the invasion boundary had been available, resources could have been reallocated to fully cover the infested region. This could have been done at essentially no extra cost by reducing the number of follow-up treatments in formerly infested areas, as we discuss below.

What we found

Perhaps the most surprising finding was that the southern and western boundaries of the invasion advanced at a steady rate, seemingly unaffected by the sharp drop in the number of reproductively mature nests, as mentioned above.

DEPT. OF PRIMARY INDUSTRIES/AAP

So the eradication program appeared highly successful in that fewer nests were being reported, but this masked an expansion of geographic range. Such discrepancies may be a general feature of eradication programs, and policy makers need to expect an infestation to continue expanding its range despite declining numbers.

The rate of movement of invasion boundaries should be estimated, and the eradication strategy should be designed to keep pace with the expansion in a preemptive, rather than responsive manner. This demands sophisticated mathematical modelling.

Each month, models should infer areas likely to be infested, even if no nests have yet been found there. These areas can then be treated with baits. We believe our new model provides unprecedented spatial and temporal resolution for this purpose.

Wikimedia Commons

Another unexpected finding was that immature nests (those not yet capable of founding new nests) outnumbered mature nests at almost every stage of the invasion. Destroying immature nests is thus at least as important as destroying mature nests. But many of these immature nests are too small to detect and their presence can only be inferred.

One consequence for policy makers is that monitoring and treatment of recently infested areas should continue for at least eight months (the approximate time between founding of a nest by a new queen and the first mating flight by alates) after nests have apparently been eradicated from those areas.

(Until 2006 the practice was to perform three or four repeat bait treatments per year for three years, which may be overkill, given that each treatment is estimated to kill at least 80% of nests. The program now considers five treatments administered over 18 months to be sufficient.)

Newly infested areas contain only immature nests, so the total area infested is larger than the area containing mature nests. So, again, sophisticated models are needed to infer how far immature nests have spread, and the eradication strategy should include a plan for dealing with those invisible advance invaders.

What comes next?

Is the eradication program on track to success? We found that numbers of red imported fire ants were again in decline throughout 2011. But by December of that year – the end of the period we examined – the infested region was larger than ever, with the invasion boundaries still advancing to the south towards the Gold Coast and Scenic Rim and west into Lockyer Valley.

To put it bluntly, at the current rate of expansion only a scant few years remain before some of Queensland’s most popular tourist destinations and most fertile farmlands become infested.

Unless, of course, we can avert this.

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30 Comments sorted by

  1. Whyn Carnie

    Retired Engineer

    Ah, Yes, Queensland, the Australian home of Prickly Pear and Cane Toads.
    When I reported what I thought were Fire Ants to the Fire Ant control office I was told to go and collect samples and mail them in. I live only 45km from Brisbane CBD. I did not feel like trying that but was told that was all they could offer. So no wonder the blighters are spreading. Another cushy job for specialist bureaucrats hoping to establish a new profession.

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    1. ian nicolson

      writer

      In reply to Whyn Carnie

      There must be more than a million households living within 45km of the Brisbane CBD. Every single one of them has tens of thousands of many sorts of ants all over it. If you had described anything at all closely resembling fire ants, or located in a dangerous place (close to a school or bus stop or that had led to stings to people or pets for example) to the Control Centre or Call Centre staff, you would have been offered a different service and immediate destruction of the nest as soon as the diagnosis…

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    2. Whyn Carnie

      Retired Engineer

      In reply to ian nicolson

      I'd been to a public session on fire ants. We were advised to report any potential sightings. The ones I saw were in little mounds and of various sizes as described. Not trying to give cause for you to sneakily suspect anything. I did not go back and collect samples although they were in our local park. The pics shown of stings put me off that exercise as I did explain to the fellow on the phone. He was adamant that his office was too busy to attend. I did pass the matter on to our council where it was lost among more important calls I suspect.
      Perhaps the administration is doing some canning of less productive units? It would only take a couple of senior bureaucrats to be stung or their kids or dogs for some motivational effort at improved productivity

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    3. ian nicolson

      writer

      In reply to Whyn Carnie

      Thanks for that, Whyn. I am sorry to say that detecting RIFA visually is not always straightforward and there are a couple of native and non-native (but not declared pest) species in SEQ that can present as similar. I am confident that whoever you spoke to made a judgement based on a decision tree that suggested your report was low probability. It is also the case that the Program's current resources have to be managed actively because the total area in which RIFA have been detected is now quite…

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    4. M wilson

      logged in via email @optusnet.com.au

      In reply to Whyn Carnie

      Just a cup of MINERAL TURPENTINE poured down the ant nests and/or sprayed around ant infested areas effectively gets rid of the problem in LESS THAN 10 MINUTES, and costs only a fraction of chemicals such as ADRO which costs over $100 a kilo and need several applications over a long time to work.

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    5. ian nicolson

      writer

      In reply to M wilson

      If you will pardon my bluntness, M. Wilson, that's complete rubbish. Any foul (to ants) smelling chemical or liquid (like detergent or petrol for example), will cause the red imported fire ant to up sticks and move home. They are probably starting that process as soon as you take the cap off. And since you have to kill the queen (and any incipient queens) to kill the colony (and she can be metres deep in the ground), your chances of getting to her are zero. Flooding the nest with an approved insecticide…

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  2. Gordon Angus Mackinlay

    Clinical Psychologist

    It is good to see this article, of far greater importance to Australia and its people's than the majority of faecal matter that now litters the pages of the Conversation.

    Whilst fortunately the primary industry properties that our family trust owns in QLD are not infected (YET) our brother in law who manages the trust, has had dealings with people who are.

    It costs serious money to eradicate the problem, but, since it is only done piece meal, with surrounding properties still infected, back…

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    1. Terry Reynolds

      Financial and political strategist

      In reply to Gordon Angus Mackinlay

      Gordon, down here in Melbourne we were of the impression that the outbreak was limited to the Brisbane wharves and immediate action was taken to eradicate it. It seems that the Queensland authorities have let it get out of control given the massive area it seems to have now spread to.

      I am sorry for you people on the land in Queensland if you cannot get the State Government to act. The National Party's. Barnaby Joyce gets plenty of TV coverage with his wild claims about the Federal Government - what is he doing to assist farmers with this menace?

      Australia is a beautiful country and we have to protect it from such invasions whatever the cost. It seems we are doing a poor job.

      .

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    2. James Hill

      Industrial Designer

      In reply to Gordon Angus Mackinlay

      Dramatic dumbing down is right, especially the bureaucratic ignorance and incompetence mentioned.
      Obviously farmers need more feet on the ground than they can possibly martial.
      Yet there are millions of suburbanites confined to quarters, as it were, and unable to help.
      Since the bureaucracy, especially at the local government level, have been corrupted by the real estate rorters, then perhaps weekend camps need to be set up by a private agreement between farmers and interested suburbanites.
      Their…

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    3. ian nicolson

      writer

      In reply to Gordon Angus Mackinlay

      Hi, Gordon. Can I say some time checking the Program's methods on the DPI website might reduce your shortfall of accurate information. However you are quite right about the magnitude of the threat to life and lifestyle they pose. Witness the woman who died in Florida from RIFA stings while at her condo pool last week.

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    4. Terry Reynolds

      Financial and political strategist

      In reply to James Hill

      James, your concept of volunteers works.

      I have just spent 2500 hours removing a massive weed infestations of box thorn, hedge mustard, fleabain, kikuyu etc from the foreshore at Hampton, on one of the most expensive suburbs on Melbourne's, Port Phillip Bay. It took me three years and about 20 hours a week in the early mornings, but today it looks terrific and the people have come back to enjoy it again. There is still more to be done.

      The 27 acres of woodland suffered from 50 years of neglect…

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    5. James Hill

      Industrial Designer

      In reply to Terry Reynolds

      Thanks Terry, good points.
      Plenty of grey nomads getting ready to move out for the adventure of their lives.
      Look out when they start to get organised.
      Better than hanging about getting parasitised by the so called service industries infesting suburbia.
      Self-reliance looks much more attractive.

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  3. Pam Swepson

    Psychologist

    The density measure of fire ant nests is a function of how many are found - not how many there actually are. The graphs of fire ant density presented here perfectly match the number of staff who were looking for them; around 620 in years 02-03 and around 140 after 05.

    A feasibility study on using mathematically modelling was conducted in 2002 by R George. He found that 'it is not possible to satisfactorily fit observed patterns with know mathematical processes du to the confounding influences of variable sampling intensities.'

    While 3 or 4 repeat bait treatment per year for 3 years might seem like overkill to the authors, the fact remains that the only sites that have been declared fire ant free have received many many more than this plus nest injections.

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    1. Jonathan Keith

      Senior Lecturer, School of Mathematical Sciences at Monash University

      In reply to Pam Swepson

      Dear Pam,

      Thank you for your carefully considered comments. We have tried to address the issue of variable sampling intensities by estimating how many nests went undetected and where they would have been situated. The results we presented include these estimates. This was difficult, and I agree it would have been impossible with methods available in 2002.

      With regard to how many follow-up bait treatments are needed and for how long, I concede this is something we haven't studied in detail. I think we've been careful in our article above to avoid making a definitive statement on this matter. Ideally there would be overkill, but with limited resources needed elsewhere, it's also important not to do too many. I think it's an issue worthy of further study.

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  4. M wilson

    logged in via email @optusnet.com.au

    It worked for me, so good ol' queenie must have starved to death since the rest of her red fire brigade deserted her, because there is no sign of them in my 1700sqm place since I "fed" them with a cup of turps.

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    1. ian nicolson

      writer

      In reply to M wilson

      Wait.

      Or talk to your neighbour.

      Did you let anyone at the Fire Ant Program know what you found and where? If not, why not? If you did, what advice did they have? Did they confirm your diagnosis of fire ants? Perhaps you didn't bother submitting a sample? Without active, responsible and educated public co-operation, the best endeavours over a decade of hundreds of staff, citizens and volunteers and the expenditure of millions of public dollars are at risk. If the Program does fail to achieve eradication, many small individual failures or oversights will be involved. As has always been the case. This is hard work, but well worth maximum effort.

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    2. Terry Reynolds

      Financial and political strategist

      In reply to ian nicolson

      Back in Joe Bjelke-Petersens day in the seventies, all you would have to have done is publish in the Courier Mail that fire ants were invaders from a socialist country and were really "red ants". Queenslanders would have gone crazy to get them out of Queensland for ever. They did with ABBA because ABBA came from a socialist country Sweden. No reds under the bed in Queensland albeit ironically they elected the only ever Communist member of parliament in Australia. Perhaps they should have said cane toads were communists invaders.

      I appreciate it is not really a laughing matter but Queensland is going top have to do something serious to eliminate them. Rudd spent $1.4 billion insulating houses as a stimulus, much of it in Queensland, Perhaps he and the State Government could have another stimulus spend to rid the state of fire ants. This time Queensland free enterprise, train all your workers properly first.

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    3. M wilson

      logged in via email @optusnet.com.au

      In reply to ian nicolson

      Actually I did report the problem to my local council - and the response was another example of bureaucracy gone mad - to the effect that it was my baby so "call in the pest control people if you have a problem".

      I may not be an expert on fire ants, Ian, but neither did I need one to tell me what an annaphylactic reaction felt like after being bitten by the red critters in two separate occasions and needing urgent medical treatment to deal with it, and then holding on to ice packs for a week…

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    4. Whyn Carnie

      Retired Engineer

      In reply to M wilson

      Your experience is quite similar to mine. Both the response by authorities and the turps trick. My mother used to knock ants with turps, my neighbours today do termites the same way. I have used both petrol and bulk fly spray on termites.
      As to the responses we've received here and in the other world, it seems the problem really is that the government experts aren't. And don't like being told.

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    5. ian nicolson

      writer

      In reply to Terry Reynolds

      Yup, that'd work better than what we have now. Rehire the 400 experienced staff the Program has sacked at the behest of national funding bodies or the current Qld administration. The missing 7 years of labour will be a problem though . . .

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    6. ian nicolson

      writer

      In reply to M wilson

      I am disappointed M. rather than offended, (my apologies if you took that message) but also deeply concerned when people advocate the use of products and chemicals that have been shown to be less than effective. Petrol for example is a popular home remedy in the US. Several people have been killed or maimed using it, and large areas of land and surface and bore water contaminated as a direct result. I can't imagine that turps is a much better product to be spreading in the environment. Or how it…

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    7. ian nicolson

      writer

      In reply to Whyn Carnie

      This pest ant is one of the most studied insects in the world, because of its grievous impacts on wildlife, people and stock. Despite the obvious motivation to resolve the problem cheaply and easily, none of the dozens of non-insecticidal or bait-delivered methods have proved safe and effective on any useful scale. Heat, electricity, pressure, coffee grounds, aspartame, petrol, cinnamon, detergents and so on have all been tried and tested. So has pine oil (turpentine). Some kill ants, but not enough…

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    8. Whyn Carnie

      Retired Engineer

      In reply to ian nicolson

      Perhaps if fipronil were allowed on the market at reasonable cost for amateur use we would see some remarkable progress in red fire ants, termites and paralysis ticks reduction. But what would that do to the lucrative pest control industry? It's time joe public was given a bit more credit for being able to handle simple tasks. If joe decides something is a pest let him take up his own cudgel. Even cane toads react to safe doses of Dettol but its use is down played by frog lovers.
      Fruit bats don't like shotgun treatment but have been given incredible importance as fertilisers of some of our worst woody weeds. And so more horses and humans will die.

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    9. M wilson

      logged in via email @optusnet.com.au

      In reply to Whyn Carnie

      Fipronil is sold to the public under other names, such as TERMIDOR ( http://en.wikipedia.org/wiki/Fipronil ) which is VERY EXPENSIVE ($440 for 2.5 litres on e-bay http://www.ebay.com.au/itm/Termidor-2-5L-Termites-Pest-Control-Fipronil-/111131510054?pt=AU_Pest_Control&hash=item19dff44526&_uhb=1 )

      and is probably not less toxic than turps at under $2 a litre.
      which as Ian says, is "of course also toxic to small animals and humans if ingested", but then again Fipronil is not exactly what you would get any small animals or humans to ingest either!

      There is a difference between use and abuse, so if you do have deep pockets, go for the Fipronil under any other name - Otherwise, save your money and use turps responsibly.

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    10. ian nicolson

      writer

      In reply to Whyn Carnie

      Drifting off the RIFA topic now, Whyn, aren't we? Back on it, I'm confused about why you think that open slather use of a restricted quite toxic and persistent chemical by all and sundry is a better idea than requiring - as I remind you the RIFA legislation still insists - individuals and landholders who suspect they (or know they) harbour fire ants to notify the "carriage" department - in this case the QDPI - of their location. That way an identification can be confirmed and the necessary nest…

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    11. ian nicolson

      writer

      In reply to M wilson

      Oh, dear. Please, please, please do not take this advice with regard to red imported fire ants. Eliminate termites and green ants and jack jumpers and unwelcome paint splashes with it if you must, and responsibly (and at $2 a litre I can see definitions of responsibly wobbling a little), but without the rest of the eradication program armoury (see below) and specifically notifying the Program of suspect nests directly you are guaranteeing we will be hassled or far worse by these highly pernicious ants for the rest of our time. And people will die. Or spend their lives afraid of walking on the grass . . .

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    12. M wilson

      logged in via email @optusnet.com.au

      In reply to ian nicolson

      There is no need to get so melodramatic. It may be that more “people will die” waiting for the authorities to act on an ant’s nests notification than from pouring a cup of turps down the nest hole.

      The concept that just because a litre of turps costs around $2 it may not be used responsible doesn’t make sense either. Heavens! Petrol costs less than that and there is not much that most ordinary people use it for than running their motor vehicles... and getting rid of the odd pest nest ;-)

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    13. ian nicolson

      writer

      In reply to M wilson

      People do die in infested regions. Regularly. Despite Mirex, DDT, Fipronil, Amdro, petrol, turps and you name it. Not melodrama, fact. Let's not go down that "I know better than any expert" route. I note you don't address the core principle. Let the Program know. Pity.

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