Lately, Dr. Sean Menke, a postdoctoral researcher for the departments of biology and entomology at North Carolina State University, has been studying the biology and colony structure of the odorous house ant (OHA) throughout the United States. He’s looking closely at factors that influence the patterns of diversity and abundance of this ant in urban systems. His research, thus far, has concluded that the odorous house ant is composed of different species — a fact that pest management professionals must consider when attempting ant control measures.
Updating attendees at October’s NPMA PestWorld, Menke explained that his research team undertook this study because the odorous house ant is a widespread household pest found in every state in the U.S., southern Canada and northern Mexico — and in every possible environment. "In the last 15 years," he said, "it has been reported to be a problem in areas where it historically hadn’t been a problem. Something new appears to be going on.
"And its biology is far less understood than we’d like. In order to effectively treat for the odorous house ant, the pest control industry needs to know more about its colony structure around houses and apartments, as well as its foraging and colony maintenance behaviors."
If this information is acquired, Menke says it may enhance the industry’s ability to predict where this ant will become invasive: in houses, in agricultural settings or in different countries.
"So far, in pursuit of these answers, biologists haven’t been able to find any general answers," he said. "That’s been a huge problem. Most of the work has been focused on one or two other ant species. For example, we’ve learned a lot about the biology and behavior of the invasive fire ant, but extrapolating from that species to the biology and behavior of the odorous house ant is very difficult. That’s partly because most invasive pest species thrive in environments that are different from their native environment — and that makes it difficult to determine what factors make them so successful. That aspect has led us to do this research."
A DEFINING TRAIT. The strong smell of OHAs distinguish them from fire ants and carpenter ants. That’s a significant help identifying them, Menke says. "It smells somewhat like a rotting coconut or blue cheese — incredibly pungent."
COLONY STRUCTURE. Menke says the colonies of odorous house ants in a countryside habitat tend to have a single queen and a few hundred workers. "But in urban areas their colonies tend to have multiple queens — even hundreds of queens and millions of workers in the nest. And ants in other nearby colonies all recognize each other as nest-mates and will sometimes cooperate."
The research study also found that nest sizes could range from one small nest under an acorn beneath an oak tree to a single colony with gigantic nests of almost one city block in length, such as those found around apartment complexes or university buildings.
"Behaviorally, in rural areas, this ant can be considered a sub-dominant species, good at finding food sources but horrible at defending them. But in urban areas, it’s a dominant ant. There, we’ve found it to be successful in defending its food against fire ants and other invasive species. It tends to be more aggressive and far more of a problem pest.
"There tends to be high amounts of aggression between different nests in natural and urban habitats, and in both areas we’ve found widely separated nests that are all parts of the same colony."
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A Systemic Treatment Approach
To eliminate problem ants, Florida-based Collier Pest Control treats systemically for insect pests that produce honeydew, a primary ant food source.
Collier started using this application technique in 2008 when a number of the firm’s clients suffered from serious white-footed and big-headed ant infestations. Technicians faced treatment obstacles because they couldn’t gain access to the ants’ nests since they were located in hard-to-reach areas, including under decks and beneath swimming pools. They applied perimeter treatments and baits, as well as implementing structural treatments. These methods managed the ants initially, but when the pests returned, the company was called back to the location.
To gain control of the problem ants, Collier Pest Control turned to Optigard Flex plant-systemic insecticide, a non-repellent, general pest product from Syngenta that uses the active ingredient thiamethoxam. The technicians applied Optigard Flex as a spot treatment in combination with a granular bait and also as a systemic perimeter treatment for whiteflies and aphids, common honeydew producers (HPDs).
"The results were like night and day," said Phil Hadley, owner, Collier Pest Control. "Right away we had success in knockdown and gained fast control of the nests, without actually treating the nests directly."
To treat systemically, technicians apply the insecticide to landscape ornamentals and shrubs as a soil drench and foliar spray. The active ingredient is then taken in by the plants and controls HDPs from within. "It takes care of two problems at once because by eliminating honeydew producers the ants no longer have a food source, and the colony is either killed off or starves," said Hadley. "Using this method has helped us gain control of problem ants and maximize our resources. And, the improved health of landscape ornamentals was an added benefit for the homeowner."
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MOBILE NESTS. In both habitats, the odorous house ant continually moves its nest, making it very difficult to control, he said. "We’ve found that it moves to different locations every two or three days. So you can find a nest in a house one day, and return the next week to treat it and discover they’ve moved elsewhere — to upwards of 150 feet away. They tend to have incredibly shallow nests in mulch or leaf litter or inside of walls. And their high mobility makes them difficult to track down and eliminate."
Menke cited research by Purdue University researcher Dr. Grzegorz Buczkowski, who noted that in large colonies with multiple interconnected nest entrances, the ants tend to transport their food to a single nest — and not share it with the rest of the colony. "This makes baiting and treating very difficult," he said.
Additionally, Menke said odorous house ants "like sugar, so nests may be near that food source and other sources of carbohydrates."
A MULTIPLE THREAT. Odorous house ants are similar to many other difficult-to-treat ant species, Menke said, in that they are small in size and omnivorous — making them potential agricultural and household problems.
"What makes it difficult is the surprising fact that there’s almost no literature on the subject. They’re so commonly found that apparently no one thought they were worth writing about."
Concerning taxonomy — the classification of this particular ant — Menke’s research delved into several questions:
- Is it really composed of only one species?
- Is the species on the West Coast the same as the species on the East Coast or is it made up of multiple species?
- Is the recent increase of this pest problem caused by a new species?
- Is there a final geographic structure for this ant?
- Can we genetically tell these ants apart from one region to another?
"We collected ants from 54 locations across North America from 22 different states," he said, "and collected related species from four different countries — to find out what exactly we were dealing with. And we tried to compare urban and rural collections as much as possible."
The research determined that there are, indeed, multiple species of odorous house ants, not just one.
"This means that treatments designed to control species in one area of the country may not be working well in another area. We found, for example, that the ants in Oregon are not the same species found in North Carolina, Pennsylvania or anywhere else. And we looked more closely to see if they were associated with urban areas or not. We also found colonies that were collected in rural areas were interspersed with urban colonies within the same geographic area. So there appears to be no significant differences between urban or rural ant populations within a geographic area.
"And we looked at small and large colony structures with single or multiple queens in a specific region and found no significant genetic differences within a species. There doesn’t appear to be a genetic trait causing the size differences in colony structures, either," he said.
"Another fact we learned is that species from rural areas are starting to move into an urban environment and are starting to become a pest problem. We believe there is some environmental factor at play here. That’s something we will continue to study and could influence future treatment possibilities."
The author is a contributing writer to PCT and can be contacted at jfox@giemedia.com.