Image: Argentine ants
Philip Lester
Argentine ants; the large ant in the center is a queen.
updated 11/29/2011 8:36:44 PM ET 2011-11-30T01:36:44

Not all invasive species must be fought back using electrified barriers, natural enemies imported from afar, campaigns to turn them into food, or other, often pricey means.

Instead, it appears that in some unusual cases organisms that have flourished unwanted outside their native range simply retreat on their own.

Researchers in New Zealand have watched this happen to colonies of Argentine ants, Linepithema humile, a non-native species first spotted in the country in 1990. [ Gallery: Invasive Species ]

A natural invader
These ants have traits that appear to set them up for success as an invasive, including an omnivorous diet, a lack of pickiness about nesting sites, a lack of conflict amongst themselves, a high reproductive capacity thanks to multiple queens in a colony, and an affinity for living near humans, according to Meghan Cooling, a study researcher and graduate student at Victoria University in Wellington, New Zealand.

The Argentine ants live at high densities that allow them to push out native ant species, but unlike the invasive fire ants in the United States, they don't sting, according to Cooling.

"What makes them so annoying is the extremely high population densities they can reach, so that they just take over a garden or yard and make sitting outside very unenjoyable. They also invade people's homes and cupboards in search of food, particularly sweet things," Cooling wrote in an email to LiveScience. "They can be a serious problem for agriculture as well, because they tend and protect hemipteran pests [also called true bugs], such as aphids and scales insects, which can lead to outbreaks of these pests."

In 2002, the cost of controlling them was projected to climb to roughly $52 million per year once the ants had established themselves throughout their predicted range.

Invasive species are frequently the targets of elaborate and sometimes expensive efforts to eradicate them, or at least reduce their numbers. For example, an electrified barrier has been installed to keep Asian carp out of the Great Lakes; researchers are turning to European weevils to control invasive garlic mustard in North America; and lionfish in the Caribbean, likely aquarium escapees, are the subject of a cookbook.

Vanishing ants
However, in 2011, Cooling and her colleagues checked on 150 locations where ant populations were spotted between 1990 and 2008. They found that 60 of these had vanished and more than 30 of the remaining sites had only small, low-density populations remaining, according to Cooling.

So why appear to thrive, then disappear? Argentine ant populations have shown low genetic diversity before, so the researchers suggest that inbreeding may have left them vulnerable to disease.

Other invasive populations, like the yellow crazy ant in the Seychelles and the giant African land snail, have declined or collapsed, however, few studies have documented this phenomenon, according to Cooling.

In places where Argentine ants lived in high density, the researchers found few, if any, other ant species. But other species were abundant around small, remnant Argentine ant populations. It appears that other ant communities are recovering after large populations of Argentine ants decline, the researchers write in an article published Nov. 29 in the journal Biology Letters.

An invasive's future
Looking at climate in these regions, they found that the Argentine ants seemed to last longer in regions with higher temperatures. While work overseas has indicated that rainfall can affect the ants' persistence, the relationship in New Zealand remains unclear, according to Cooling.

Climate change appears to give the ants something of a reprieve. From projections created by climate modeling, the researchers found ant populations in part of the country could be expected to stick around a few years longer before disappearing. 

"Given the local presence of this invasive species for short durations of 10–20 years, and the apparent recovery of the resident communities after their collapse, it seems that the long-term ecological or evolutionary effects of Argentine ants in New Zealand may not be as dire as first feared," they write.

You can follow LiveSciencesenior writer Wynne Parry on Twitter @Wynne_Parry. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.

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Explainer: Eight insects with the 'ick' factor

  • Warren Little  /  Getty Images file

    Many insects provide humans with unheralded services such as pollination, sustenance, and pest control, but some of them gross us out — or worse. Take dung beetles such as the one shown in this image, for example. As their name implies, the insects process feces for their livelihoods. The service helps reduce fertilizer costs on grazed agricultural lands and cuts down on the number of flies and parasites the piles of manure would otherwise attract. But a life of dung? Ick.

    Click the "Next" arrow above to learn about seven more insects with "ick" factors that make us squirm, or much worse.

    — By John Roach, contributor

  • Head lice, the annoying itch

    Sean Gallup  /  Getty Images file

    For moms and dads, the thought of head lice can sow panic at home. School-age kids are prone to pick up the feared infestation of the sesame-seed sized insects in packed classrooms. The critters latch onto hair follicles and feed on tiny drops of blood. At first sight of head lice, many school nurses send infected — and itchy — students straight home. And that's when parents freak out, lathering their kids with shampoos, gels and creams in an effort to kill the lice. However, some lice are proving resistant to the treatments, leaving parents scratching their heads over what to do.

  • Crabs, lice of another kind

    James Castner  /  University of Florida

    Adults, perhaps in the kid-making stage, are also panicked by another kind of lice: crabs. These critters nest in pubic hair and are often transmitted in the course of sexual intercourse. Crab-inspired panic attacks, however, might be on the way out. Researchers at the University of Leeds in Britain noted a decline in crab infestations, first among women and then men, reported at their clinic. The researchers speculated in the journal of Sexually Transmitted Infections that the decline is due to the popularity of the so-called Brazilian bikini wax, which removes most pubic hair, The Associated Press reported.

  • It's getting harder to stop the bedbugs from biting

    Scherzinger Pest Control

    The adage about bedbugs is getting harder to follow, according to entomologists and pest control experts who have noted an uptick in infestations of the blood-sucking insects. Heavy use of insecticides such as DDT all but eradicated bedbugs from the U.S. by the late 1960s, but international travelers appear to have re-opened the door and now, media reports suggest, bedbugs are back with a vengeance. The insects attack warm bodies in the middle of the night and then retreat to dark crevices behind headboards and mattresses. Telltale signs of their presence include pepperlike fecal spots and shed skins.

  • Cockroaches have few fans


    Garbage-loving, foul-smelling and house-infesting cockroaches have few admirers beyond Disney-Pixar's animated robot Wall-E, whose only friend on a post-apocalyptic Earth is, naturally, among the world's most enduring insects. The notoriously difficult-to-kill bugs can spread disease and cause allergies. Researchers are hoping baits that mimic the pheromones females give off when they are ready to mate can at least give humans an edge in the battle for pest-free environments. In this image, a female cockroach at upper right attracts three males with her scent.

  • Ticks can make people bug out

    AP file

    Ticks, although not technically insects (they're arachnids like spiders and mites), make some people bug out. The critters crawl onto hosts such as dogs and people and burrow in their heads to suck blood. Ticks can go undetected for days, ample time to spread sometimes fatal illnesses such as Lyme disease and Rocky Mountain spotted fever. Prevention requires application of insect repellant when outside and regular body checks for potential bites. If a tick is detected, experts advise not to panic, but to expeditiously remove the tick by grasping it with tweezers as close to the skin as possible and pulling upward with steady, even pressure. A brown dog tick is shown here.

  • Fleas no fun for Fido or his best friend

    DesignPics Inc. via Newscom file

    The "how cute" reaction evoked by scenes such as the one shown here can quickly change to "ick" when our dogs start gnawing on their fur to rid themselves of fleas. The wingless, blood-sucking insects can also be more than an itchy nuisance: they are known to spread bubonic plague between rodents and humans, which has killed millions of people. Experts recommend frequent vacuuming, regular washing of pet bedding and treating household pets with topical insecticides.

  • Mosquitoes the icky and deadly

    Rothamsted Research

    For many of us, mosquitoes are more annoying than nasty; though most of us have uttered an ick or two when we successfully swat one on an exposed arm or leg only to create a skid mark of our own blood and bug. But more than ick, the insects are vectors of lethal disease. More than a million people each year die from malaria, a disease caused by parasites in red blood cells that is spread by mosquitoes in some parts of the world such as Africa. In an experiment with a twist, scientists attempting to develop a malaria vaccine recently successfully used mosquitoes to vaccinate humans against the disease.


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