Top 10 Tips for OHAs

Cover Story: Annual Ant Control Issue - Cover Story: Annual Ant Control Issue

Want to have greater control over perhaps the most troublesome of ant species? Read on.

Odorous house ant, Tapinoma sessile.
© Eli Sarnat, Antkey, USDA APHIS PPQ,

Odorous house ants (Tapinoma sessile, OHA) consistently rank as the most important ant driving sales and cause the most service calls (2018-2020 State of the Ant Control Market Reports, PCT). OHA are infamous for being hard to control and consistently rank as the #1 “callback” ant. The laboratory of Dr. Grzegorz Buczkowski at Purdue University has been studying OHA since 2004 and the following are “Top 10 Tips” for achieving better success with OHA and ants in general.


Insecticide resistance is commonly observed in cockroaches, bed bugs and other pests. However, insecticide resistance has never been documented in social insects like ants or termites. It is thought the unique reproductive biology of social insects (i.e., the caste system) is the main factor responsible for the lack of resistance. When experiencing control failures, it is important to remember that most are not due to resistance. Before assuming insects surviving an insecticide application are resistant, eliminate other possibilities such as improper application, incorrect dosage, improper timing or simply re- infestation. When baiting ants, understand the needs of different species and don’t assume that a bait that works well will always work and a bait that didn’t work won’t ever work.


OHA supercolonies may reach enormous size and cover large areas. An OHA supercolony on the Purdue University campus has been estimated to contain approximately 5 million workers, 50,000 queens and countless brood. At any given time, only a small fraction of the colony is out foraging. PMPs may underestimate the size of the populations they are treating. A thorough inspection will reveal nesting locations and help approximate colony size. The amount of bait applied should then be matched to the infestation level and re-applied for as long as the ants are feeding. Numerous bait placements should be deployed throughout the colony’s foraging range and should be placed close to foraging trails to maximize the chance of OHA finding the baits.


Research conducted by our lab demonstrates that OHAs have extreme resistance to starvation. Colonies consisting of workers, queens and brood were collected in the field, placed in boxes, provided with water and kept in the lab for five months without food. The colonies declined slightly in brood and workers, but survived five months of total starvation. They likely used fat reserves, brood cannibalism and trophic eggs produced by the queens for nutrition. The implication for PMPs is that simply eliminating food sources is not going to eliminate OHA. However, sanitation and reduced access to honeydew are important for increasing the attractiveness and efficacy of baits deployed for control.


PMPs use multiple ways to manage ants around homes and businesses and often develop a routine and rely on the same products season after season. When experiencing control difficulties, it pays to explore new possibilities. PMPs should continually educate themselves on new methods and products. The Buczkowski Lab works with all major product manufacturers and is involved in testing new products, formulations and active ingredients in laboratory and field studies. New products and technologies are being developed all the time. In 2019-20, several new, highly effective baits and sprays became available for OHA control.


In the United States, 41 species of ants are considered household pests. They show extreme variation in life history traits, which may result in differences in susceptibility to insecticides and consequently substantial differences in the outcome of control efforts. The Buczkowski Lab tested the susceptibility of 12 species of urban pest ants to residues of fipronil, bifenthrin and chlorfenapyr. Results show substantial variation in susceptibility across species. Across all ants, OHA were consistently the least susceptible to all insecticides — they were the hardest ant to kill (see Figure 1). Comparatively low susceptibility may explain why OHA is so persistent and so difficult to control. The results of the study merit adjustments in insecticide treatments depending on the species treated. To achieve satisfactory control, species that have particularly low sensitivity to insecticides might require higher application rates, more frequent application intervals, or both.


PMPs across the country report control failures with OHA, which decreases profits and increases consumer dissatisfaction. Results obtained in the Buczkowski Lab (see Figure 2) show that OHA colonies exhibit significant regional variation in susceptibility to insecticides. Ten OHA colonies were collected around Purdue University and subjected to residual applications of a pyrethroid insecticide commonly used for ant control. Colonies showed significant variation in susceptibility, with time to achieve 100 percent mortality ranging from 25 to 140 minutes under continuous exposure. In field situations, where exposure is intermittent, the differences are likely to be even greater. Furthermore, differences across the country are likely to be even greater. Highly significant variation in insecticide susceptibility (or perhaps yet undocumented insecticide resistance) may be one of the reasons why OHA are so difficult to control.


OHA colonies go through an annual cycle of expansion and contraction. The colonies overwinter in a central protected location, quickly expand in the spring, reach maximum size in the summer and again shrink in the fall. A field study conducted in 2019 compared early-season preventive treatments to mid-season curative treatments. Results show that the best option for controlling OHA and preventing call-backs is an early season application of liquid spray insecticide. To improve chances of eliminating all nests and destroying the colony, PMPs should treat for OHA early in the season when the colony is still small, ideally before the ants become active again.


Gel baits are by far the most popular pesticide formulation for controlling OHA and other ants. However, to be effective, gel baits must be discovered and consumed by the target pest. Research by the Buczkowski Lab evaluated the fate of gel bait placements for controlling carpenter ants, which are nocturnal and feed mainly at night. Results show that baits applied during the day are frequently consumed at night by non-targets such cockroaches, other ants or rodents. In some cases, carpenter ants discovered the baits but were later outcompeted by other species. One way to increase success with baits is to apply them in areas inaccessible to non-targets, protected within bait stations and always in close proximity to nests and foraging trails.


OHA have the ability to develop large supercolonies that can contain millions of workers and thousands of queens, making them very difficult to eliminate. A single colony can easily take over a city block and span multiple houses. Areas adjacent to treated structures then serve as re- infestation sources. While many treatments focus on the interior of the structure and the perimeter around the structure, exterior treatments away from the structure are an important and often neglected step. Treatments along property lines may be especially important in preventing future infestations.


The best way to eliminate an ant colony is to find and destroy the nest. However, OHA supercolonies may contain more than 100 nests and nests are constantly moved depending on weather conditions and needs of the colony. Some may be inaccessible and OHA are known for nesting in unusual locations. The good news is that OHA nesting locations are highly predictable. OHA don’t build nests, but rather utilize existing materials such as leaf piles, mulch and objects on the ground. Pre-baiting (also known as survey baiting) helps pinpoint areas of activity. To pre-bait, offer food baits (e.g., apple jelly) and allow time for the ants to establish a foraging trail. Then remove the jelly and replace with a toxic bait (“bait-and-switch”).

The author is a research associate professor and director of the Industrial Affiliates Program, Center for Urban and Industrial Pest Management, Department of Entomology, Purdue University.


Buczkowski G and Bennett GW. 2008. Seasonal polydomy and unicoloniality in a polygynous population of the odorous house ant, Tapinoma sessile. Ecological Entomology, 33: 780–788.

Buczkowski G. 2010. Extreme life history plasticity and the evolution of invasive characteristics in a native ant. Biological Invasions, 12: 3343–3349.

Buczkowski G and Bennett GW. 2009. The influence of forager number and colony size on food distribution in the odorous house ant, Tapinoma sessile. Insectes Sociaux, 56:185–192.

Buczkowski G and Bennett GW. 2006. Dispersed central-place foraging in the polydomous odorous house ant, Tapinoma sessile as revealed by a protein maker. Insectes Sociaux, 53: 282–290.