The odorous house ant, Tapinoma sessile (Say), is found throughout much of the United States and is often the topic of discussion among pest management professionals from Tennessee, Kentucky, Ohio, New Jersey, Virginia, North Carolina, northern California, Washington state and elsewhere. In fact, the odorous house ant is the most common ant invading structures in the Midsouth region and is ranked as the No. 2 "bad girl" in the United States.
This ant is easily identified by the following characteristics:
1. When crushed between fingers and held under the nose, an odor of rotten coconut (not as fruity as a rotten banana) and pine is detected;
2. 1/8-inch long, dark brown/black;
3. Node (bump) on one-segmented waist is barely visible and is hidden below the gaster;
4. A slit-like opening on the underside of the gaster; and
5. Workers are one size.
WHY A PEST? Both foragers and alates cause a nuisance, especially when they are found indoors. In fact, in Tennessee, odorous house ants (OHAs) can usually be seen actively foraging outdoors from March through November, although foraging has occurred on warm days in the winter months. Male OHAs also have been found attracted to lights in homes from late May through the end of June.
However, the reputation of the odorous house ant is a bit of a paradox. It is the most common structure-invading ant in the Mid-south, yet it has been termed submissive and subordinate. OHAs have been described as submissive because it defends only its nest and not its food source or foraging territory. It has been called subordinate because other ants displace it from a food source even if it arrives first (which rarely happens). However, it is more dominant when it outnumbers other ants. We believe most of our clients contact us when OHAs are very numerous and thus more dominant.
BIOLOGY AND BEHAVIOR. These ants are opportunistic nesters and occupy any object that will provide shelter. Because OHAs are a multiple queen species and some nests are formed by budding, many nests may be located throughout the structure and landscape. For example, we have found OHA nests in or under landscape timbers and rocks, logs, debris, siding, leaves, pine straw, mulch, loose bark and bee hives. Indoors, they are usually associated with moisture, such as wall voids near pipes and heaters, bathtraps, termite-damaged wood and beneath toilets. We’ve even found them clustered in the back corner of a kitchen cabinet that held the garbage can and sink pipes. They did not even bother to hide under anything.
Odorous house ants forage day and night, using guidelines, such as vines, limbs and trunks of trees and shrubs or edges of buildings, baseboards and counters, to move from place to place. Foraging occurs at temperatures from about 43° to 95°F, although some researchers report higher foraging temperatures. In some of our studies, foraging decreased as temperatures approached 90°F because Forelius ants out-competed OHA when temperatures exceeded 90°F.
Outdoors, odorous house ants feed on dead animals (skunks, birds, moles, voles, etc.); living and dead insects; excrement (honeydew) from aphids, scales and mealybugs; and nectar from plants such as rhododendron. Indoors, they can be found feeding on sweets and other household foods and are often found trailing to water. In general, oils are not preferred.
OHA often occur in large numbers along the outdoor base of the structure; however, client calls are related to ants seen indoors. To determine if ants indoors were, at the very least, sharing food with ants outdoors, we placed sugar-water containing dye near indoor OHA activity. At hourly intervals, white paper was placed over outdoor foraging trails and ants crushed on the paper. Dye was noted on the white paper two hours after placing the dyed solution indoors.
In 2002, before including a structure in our research study, we conducted the above procedure and determined that OHAs found inside 17 of 18 houses foraged outdoors or exchanged food with outdoor ants. In the one house where we failed to establish an association between indoor and outdoor ants, only a few foragers were seen inside and recruitment to the sugar-water failed. After establishing food flow between the indoor and outdoor ants, we knew we could treat outdoors and affect the indoor ant populations without disturbing the dweller.
PREVENTIVE MANAGEMENT OPTIONS. Ants occur in structures because food or water and/or nesting sites are present. Management efforts, especially bait treatments, are more effective if other food sources for ants are removed. Inform clients that reducing these resources, such as water from leaky faucets, dropped food and dead insects on window sills, can reduce indoor foraging. For example, ants were found in one home from the spring through the first week in December. The only day ants were not seen was the day the family returned from vacation in July. We believe this occurred because prior to leaving on vacation, the family had emptied the indoor garbage that had served as a food source. Clients need to remove garbage from the structure regularly and place outside garbage cans away from the structure.
There are other ways to remove access to these needed resources. Pet food serves as a reliable food source for these ants, so it is important to train pets to eat food within 10 minutes of placement. Trail pheromones to alternate food sources, not bait stations, can be removed by wiping the surface with a mild detergent.
While indoor food sources are more easily removed, it is almost impossible to remove all outdoor food sources. In addition, removing outdoor food sources may also cause ants to enter indoors in search of food. Some feel spring rains wash the sucking insects from plants and thus cause the ants to enter indoors in search of food.
Homes also provide nest sites for ants. Locate the entry point into the structure, such as a hole in the foundation around conduit, plumbing or poorly sealed windows and doors and seal these areas to prevent future invasions. Remind clients to check potted plants and firewood for ants before bringing them indoors. Keep branches, vines and other vegetation from coming in contact with the structure because ants follow these guidelines to gain access. Vines can also serve as nest sites. On one occasion, OHA workers, queens and immatures were found nesting between the stem of English ivy and a brick wall to which the ivy was attached. Pull mulch, pine needles, leaves and other cover away from the foundation of the structure.
Many of the previous recommendations may be more effective if performed prior to OHA infestations, that is, while the ants are next door just eying your client’s property, but haven’t made their move yet.
OTHER MANAGEMENT OPTIONS. Locating and treating nests is most effective when colonies are small and less numerous. In our research studies, when OHAs are present in low numbers and other ant species are present, we have seen other ants displace it from food. In fact, we have set our monitoring times to about 45 minutes because after this amount of time other ants displaced OHAs from our honey-smeared sampling cards. If we left them out for less time, OHAs didn’t find the cards. Thus, when OHA colonies are small, competition with other ants could be a reason for bait failure. If OHAs are just starting to move into the landscape, locating and treating nests may be effective; however, if the colony is well established and many nests are present around the structure, this type of treatment would be less effective. Large volumes of diluted insecticide sprayed through mulch as it is raked to expose nests will probably lower populations for a while, but it is unlikely to provide long-term control.
Baits, on the other hand, are collected by foraging ants and brought back to the nest where the pesticide is potentially circulated to all members of the colony that feed (workers, queen[s] and immatures or brood). Baits exploit the forager caste, causing them to introduce the toxicant into a previously inaccessible nest or nest of unknown location. Still, OHAs have been reported to move their nests about once every three weeks, often in response to rain. Prior to a rain, workers can be seen carrying brood to another location and when involved with this moving task, they are not interested in feeding on baits.
GELS, GRANULES or LIQUIDS? Gel, granular and liquid baits have different advantages. Gel baits are popular with pest management professionals because they are easy to use — gels are easily transported (the syringes can be carried in a holster) and gel baits can be placed into any crack or crevice. However, ease of use does not necessarily relate to efficacy. Bait attractiveness, as indicated by large numbers of ants feeding on a gel bait, may certainly impress a client, but will not guarantee distribution through a colony. A study by Silverman and Roulston indicated that Argentine ants fed eight times longer on a gel, but removed five times less sugar than ants feeding on a liquid. Also, when fipronil was added to these baits, more of the colony died due to the liquid formulation than the gel. They concluded that the liquid bait was more effective than the gel.
At the University of Tennessee we have evaluated several baits for effectiveness against small laboratory OHA colonies, but we haven’t evaluated all baits on the market, so failure to mention a bait in this article may not mean they are ineffective or effective, it just may mean they were not evaluated yet. In addition, one should be cautioned about making the assumption that the results of a small laboratory colony will reflect the results of a bait applied to a structural infestation or even to a large laboratory colony. Test colonies are usually started with about four queens, 400 workers and 1 cm2 brood. Food is removed for two to three days, then about two grams of bait are introduced. Three days later, the regular feeding schedule starts. Baits typically remain in the colony for the duration of the test. In a field situation, however, there will always be competing food sources. Second, in this protocol, each individual worker could have fed on the bait. When a large colony, such as that found around structures, is baited, only a small percentage of the colony actually forages. We don’t know how well any of these baits would be transferred around a large odorous house ant colony.
Thus far, the only gel-type baits tested that have killed all colony members (workers, queens and immatures) by eight weeks are Terro Ant Killer II (5.4 percent borax), Gourmet Ant Bait Gel (6 percent disodium octaborate tetrahydrate), Whitmire Micro-Gen experimental gel bait (now registered as PT388B Advance Ant Gel Bait, 5.4 percent borax), Outsmart (6.25 percent boric acid), AntX (7.5 percent orthoboric acid) and Pro-Joe-S Ant Bait/Gel Formula 4.5 (4.5 percent boric acid).
Researchers' Note |
Results presented here refer to an experimental use of Termidor SC. We applied this product at 6 gallons/1,000 square foot and the 2002 Termidor SC label allows about 1.5 gallons/1,000 square foot to be applied up to two times per season. Label directions allow one- quarter to one-half of the fipronil used in our study. We urge all pest management professionals to follow all label directions. In addition to fines and other consequences from your local regulatory agency, misuse of products could provide support for withdrawal of these products from the marketplace. |
Granular baits can be scattered around the perimeter, placed in small piles, in cracks and crevices or in bait stations. Of the granules that we have tested as described in the previous laboratory protocol, Maxforce Fine Granule Insect Bait (1 percent hydra-methylnon), the New Niban baits (5 percent orthoboric acid) and Bayer experimental granular baits (0.05 percent, 0.1 percent imidacloprid) have killed all colony members by eight weeks. Regardless of the ease of use of gel or granular baits, however, our best strategy for managing odorous house ants includes a combination of a nonrepellent perimeter spray and a liquid bait in the landscape (see field results on page 100).
Liquid baits, although readily fed upon and quickly distributed through colonies, do have some disadvantages. The difficulty encountered with liquid baits is with the delivery system. Several bait stations have been developed to deliver liquids, most with a filter-type material with a plastic housing. Results from other ant researchers indicated that some stations gained weight (moisture), while other stations lost weight due to evaporation.
Also, some pest management professionals are reluctant to use them because some consider baits bulky, baits may be expensive and could be difficult to anchor and they must be refilled. Rust and colleagues warned that the excellent results obtained with liquid baiting systems developed by researchers for sweet-loving ants, such as Argentine ants, may not be duplicated with commercially available liquid baits because of modifications made during the commercialization process. Thus far, Terro Ant Killer II (diluted to 2 percent borax), Whitmire Micro-Gen experimental liquid bait (now registered as PT381B Advance Liquid Ant Bait, 1.3 percent borax) and Bayer experimental liquid bait (0.025 percent imidacloprid) have eliminated all OHA colony members by eight weeks using the previously described laboratory protocol. However, better liquid delivery systems need to be designed so pest management professionals can use liquid baits with less maintenance.
Repellent perimeter sprays can be used to keep ants out of a structure; however, you must be careful to prevent sealing foragers indoors. While baiting has been the main strategy for managing ants and use of sprays has been downplayed, we may need to rethink this logic as nonrepellent sprays make their way onto the market. Nonrepellent sprays will not deter ants from entering a structure, but, like baits, may allow ants to spread the insecticide to other colony members.
FIELD RESULTS. Because many clientele are not home during the day, perimeter treatments are used by pest management professionals because they do not require access to the structure’s interior and, thus, are easily and quickly applied. Outdoor perimeter applications also reduce the risk of pesticide exposure to indoor occupants.
 Mean percentage reduction in OHA outdoor populations at 2 ft. up and at the base of structures.
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The objective of this study was to determine which treatment, a perimeter bait, spray or a combination of bait and spray, would reduce outdoor and indoor OHA populations. To evaluate the effects of these applications, outdoor OHA populations were sampled twice before treatments were applied and at 1, 2, 4, 6, 8, 10, 12, 14 and 17 weeks after treatment. Residents were phoned at each sampling period to determine indoor ant presence. The perimeter spray of 0.06 percent fipronil (Termidor SC) was applied at 244 ml/m2 (6 gallons per 1,000 sq. ft.) to 1 foot up and out from the foundation base. The bait treatment applied consisted of a 1.3 percent borax experimental liquid bait (PT381B Advance Liquid Ant Bait, Whitmire Micro-Gen) placed in an Advance A.C.E. station and located on the ground against the structure. Baits were placed where more than 10 ants were found on a sampling card. A combination of the 0.06 percent fipronil nonrepellent perimeter spray and the 1.3 percent borax experimental liquid bait was applied as described above, except the bait was placed outside the spray zone to intercept ants coming from the landscape. Control structures received no treatment.
By week one, cards spaced 15 to 20 feet apart and placed two feet up and at the base of the structures indicated that the bait and spray combination treatment reduced populations by greater than 94 percent. This level of control remained throughout the 17-week monitoring period. The spray alone reduced populations by 93 percent or greater at four weeks after treatment and throughout the remainder of the study. The bait and spray combination reduction percentage was never significantly different from the spray alone on outdoor populations. The liquid bait reduced outdoor populations by 82 percent by week four and reductions were greater than 94 percent from weeks 10 through 17.
 Mean percentage reduction in OHA indoor populations.
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The liquid bait and spray combination treatment was the most effective in eliminating OHA indoors. Although ants had been seen in all four homes before treatment, no ants were seen inside any of the structures treated with a combination of a perimeter liquid bait and spray any time after treatment. The lack of rain following the initial placement of the bait and the drought-like summer conditions in general may have made the liquid bait more attractive. This system may not provide as quick a reduction in indoor ant populations when moisture is more easily accessible.
CONCLUSION. Our best results for reducing indoor and outdoor odorous house ant populations around homes were obtained with a treatment combination of 0.06 percent fipronil spray applied to the outdoor foundation 1 foot up and out combined with a 1.3 percent borax experimental liquid bait (PT381B Advance Liquid Ant Bait) applied in stations in the landscape near the structure. As efforts continue to reduce potential pesticide exposure to people, property and pets, we feel this perimeter bait and spray system, in which we have reduced the spray zone to one-eighth the area of a traditional barrier treatment, fits this philosophy well.
Author’s note: An example of a non-repellent perimeter spray for odorous house ant management was provided previously. In theory, other nonrepellent pesticides could act in a similar manner and we hope to evaluate some of these products this summer. We’ll keep you posted as new data is generated.
The authors are an associate professor in Entomology and Plant Pathology; Entomology and Plant Pathology Department graduate student and J. M. Waller Associates’ Pest Management Consultant; and English Department graduate student at the University of Tennessee.
References
Drees, B.M. and B. Summerlin. 1997. House-infesting ants and their management. L-2061. Texas Agricultural Extension Service.
Feller, J.H. 1987. Interference and exploitations in a guild of woodland ants. Ecology. 68: 1466-78.
Hedges, S.A. 1998. Field Guide for the Management of Structure-Infesting Ants. 2nd edition. G.I.E. Publishing, Cleveland, Ohio.
Hedges, S. 2002. 2002 Pest wrap-up. Pest Control Technology. December: 56-60, 86.
Oi, F. and D. Oi. 1997. IPM tactics for Argentine ant control. Circular ANR-999. Alabama Cooperative Extension System.
Paulsen, G.S. and R.D. Akre. 1991. Behavioral interactions among the formicid species in the ant mosaic of an organic pear orchard. Pan-Pacific Entomologist 67: 288-297.
Silverman, J. and T. H. Roulston 2001. Acceptance and intake of gel and liquid sucrose compositions by the Argentine ant (Hymenoptera: Formicidae). J. Econ. Entomol. 94: 511-15.
Vail, K.M. 2002. Structure-invading ants. PB1629. The University of Tennessee Agricultural Extension Service.