Ants have recently scaled the ranks to become among the most important of household pests. Species native to North America, such as the odorous house ant, are gaining great notoriety as being persistent and difficult to control. Adding to the problem, numerous ants have in the past century been introduced from abroad and have subsequently invaded all variety of urban, suburban and rural habitats.
Just a few short years ago, residual insecticides were by far the method of choice for ant control. However, pest management professionals are now increasingly turning to baits. Nevertheless, the success of baiting is not always assured but can be promoted by pest management professionals taking into account ant biology. Each type of ant has its own set of unique traits, which allow it to profitably exploit an environment. By knowing these, a pest management professional can increase the likelihood of successfully mitigating an ant infestation.
Three aspects of ant behavior — related to nesting, trailing and feeding — must be considered when controlling ants. All ants differ in behavior to a certain degree and even the same ant’s behavior can vary greatly over time.
With respect to nesting, PCOs should consider where an ant nests and whether it does so permanently in a single location. But other questions should be asked as well. Does the ant cluster in a single spot or branch out, forming satellite colonies? Is the ant likely to move when disturbed?
A thorough pest management professional should also be knowledgeable of the trailing habits of an ant. Foremost, he or she should address the issues of definition and persistence. Does the ant form well-defined trails or forage in a scattershot pattern? In addition, does the ant remain dedicated to the same path for extended periods of time, or is it transient in its foraging?
The final aspect of behavior meriting consideration relates to the items an ant prefers to eat. Does the ant select sweets over protein, that is, nectar over prey items? This issue is, of course, directly relevant to baiting and in this regard, it should be mentioned that an ant’s dietary preference can change over the course of a season as colony demands shift. An individual may forage for protein in spring and early summer as the colony is growing, but reap more carbohydrates with the approach of fall in preparation for winter.
Here we will consider three ants that illustrate well how behavioral traits can vary across different species.
BLACK CARPENTER ANT. The black carpenter ant, Camponotus pennsylvanicus, has specific nesting requirements, preferring to dwell where humidity is high and temperature constant. In the wild, carpenter ants live chiefly in tree cavities and logs and in the human realm they occupy similarly moist and stable habitats. Such favorable conditions nonetheless probably have limited availability in whatever habitats the carpenter ant occupies. Therefore, these ants, after establishing, are relatively reluctant to pick up and move.
Nevertheless, carpenter ants are known for creating satellite nests, so PCOs must be diligent in locating all possible sites of infestation. As for foraging, carpenter ants usually stick to the same path — so much so that they can create visible trails etched into a lawn or forest floor. Often, however, a carpenter ant trail is difficult to locate, especially by day, when few ants typically are using the trail. Carpenter ant dietary practices tend toward the predatory, but they prefer protein in early to mid-season, when the colony is maximally expanding, and carbohydrates toward fall.
ODOROUS HOUSE ANT. A second species worthy of discussion, not least because its behaviors differ so greatly from those of the carpenter ant, is the odorous house ant, Tapinoma sessile. These small black ants are opportunistic nesters that can occupy seemingly almost any enclosed or covered space. Indeed, around homes they are often found beneath stones, roots, and siding and will inhabit cracks in foundations and subslab spaces.
Unlike the carpenter ant, odorous house ants are extremely nomadic and will hide at even the slightest disturbance. Moreover, odorous house ant colonies will frequently fracture into two or more independent nests, all replete with reproductives. This can make their control quite difficult because knocking out the main colony does not mean all queens have been eliminated, as it certainly does with carpenter ants, which are monogynous. Despite these difficulties, pest management professionals are not without hope in attempting to control this species.
Odorous house ant trails are usually persistent and thus highly evident, consisting of up to thousands of individuals tracking back and forth on an hourly basis. Also making trails easy to find is the fact that odorous house ants readily forage by day, unlike carpenter ants. Notably, the dietary preference of the odorous house ant appears strongly shifted toward carbohydrates; the ants constantly forage up shrubs and trees for honeydew and floral nectar.
DOWN EAST FIRE ANT. A third species of interest, the so-called down East fire ant, Myrmica rubra, is not native to North America, perhaps having been introduced from Europe. Thus far it seems to be restricted mostly to New England, which is fortunate because this medium-sized, reddish ant can inflict a painful sting, much like the red imported fire ant.
Different from carpenter and odorous house ants, though, M. rubra is ground dwelling, preferring to nest in earthen mounds in grassy meadows or beneath leaf litter at forest edges. It can form nests at the base of trees and has even been known to occupy abandoned carpenter ant galleries. Like carpenter ants, it is a persistent nester, inhabiting damp areas, but it tends to form clusters of inter-connected nests, most containing multiple queens. Unlike both the carpenter ant and odorous house ant, M. rubra does not appear to form well-defined trails, but forages in a somewhat haphazard manner; it essentially carpets its territory. Similar to the odorous house ant, though, M. rubra has a sweet tooth, feeding on carbohydrates and regularly tending aphids. Frequently, M. rubra can be found rushing up plant stems and tree trunks to tend large clusters of aphids at the base of leaves.
CONCLUSION. The main objective of the foregoing discussion has been to impress that pestiferous ants exhibit considerable behavioral variation in nesting, trailing and feeding. And make no mistake, other differences do exist among the species. It therefore makes little sense for a pest management professional to follow a single approach in ant control, no matter whether it involves a bait or residual insecticide. The pest management professionals must first correctly identify a pest and only then should he or she begin implementing a control strategy — one that has been designed specifically to target the identified species.
Biologically based pest control certainly can be effective. The authors, in conjunction with Western Pest Services, Parsippany, N.J., conducted an investigation in which we examined the efficacy of liquid bait formulations for the control of odorous house ants. Fifteen sites, both residential and commercial, considered “problem accounts,” were included in the study. Each site was thoroughly inspected for all potential nests and foraging trails and bait stations, containing liquid formulations of either 50 ppm imidacloprid or 1.0 percent boric acid, were put in place, heeding the biological principles outlined above. After eight weeks, ant foraging was reduced by up to 92 percent with imidacloprid and more than 80 percent with boric acid. These results show that baiting has great potential for controlling even the most intractable of tramp ants. It is nevertheless unlikely that such excellent levels of control would have been achieved without proper pest identification, followed by careful analysis of sites for nesting and trailing behavior of the ants.
Given that knowledge of an ant’s biology can help one in making proper decisions on control practices, fundamental research into such topics as ant behavior is imperative. Sadly, for many species, research of this type is lacking. The odorous house ant has received scant attention from the research community. Myrmica rubra is another case in point. While it has been extensively investigated in its native range, Eurasia, little is known about its biology in North America. The significance of this lies in the fact that introduced species have in some cases been shown to behave entirely differently in their new homes. We might also add that almost no applied research has been done on methods of controlling M. rubra. Clearly, more study is needed to fill gaps in knowledge on ant biology. In this regard, we at Penn State are investigating all three of the species discussed in this article and will report back intermittently with our results.
David Bell is a doctoral student in the Department of Entomology at Pennsylvania State University (PSU). Win Higgins is a master’s degree candidate at PSU and is also a regional entomologist for Western Pest Services. Dr. Glenn Holbrook is an assistant professor of entomology at PSU.