Odorous house ants, or OHA, (scientific name, Tapinoma sessile) are among the most abundant ants in urban and suburban settings, and around human structures in North America. They are considered one of the most ecologically flexible species, adapting to most environments and land elevations, which also makes them capable of invading new areas. They can be active all year long and will even appear indoors during cold winters, looking for food and water. They nest in and under a variety of objects in human environments, but they didn’t start out that way. In fact, odorous house ants are normally simple forest residents with small colonies that live peacefully with their neighbors.
Did you know?
- Odorous house ants were named for the peculiar scent they emit when crushed. Some say it smells like rotten coconut, but others smell blue cheese.
- Odorous house ants can form colonies with many queens and tens of thousands of workers in disturbed natural environments (neighborhoods).
- Like many ants, odorous house ants will care for, protect and “milk” honeydew-producing insects for food, including aphids and scale insects. They are often called “sugar ants”.
In North America, OHA is one of the most widespread species partly because it is not fussy about where it lives. Colonies are easily found at sea level but this ant species can survive elevations up to 4000 feet, which is like living at the top of an Adirondack mountain.
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If you see ants in your house, the easiest way to identify OHA is to crush one in your fingers and give it a whiff. The smell of OHA is distinct, not very pleasant, pungent and like the smell of blue cheese. If crushing ants is not your style, OHA can be identified by sight using some magnification. Workers are all the same size (monomorphic), ranging from 2.4 to 3.2 mm (or about 1/8 inch) in length. They are dark brown to black in color with light golden hairs and a hidden segment between thorax and abdomen. The winged adults (queens and kings) are larger, up to 3.75 to 4.3mm long, with gray wings that have light brown veins. Winged reproductive ants look different than workers and can be hard to identify accurately.
OHA are native to the United States. In natural and undisturbed habitats, such as forests, colonies are small (hundreds of workers) and tend to have one queen and a single nest location. These natural habitat colonies are subordinate to other ants and live among other ant species without much conflict.
In slightly disturbed environments, such as a clearing or a farmland field edge, colonies can become much larger (thousands of workers), but still maintain a single queen and nest location. What makes this species unique is the behavior exhibited in highly disturbed and man-made environments. This ecological flexibility allows OHA to behave like an invasive species in suburban and urban settings. They develop extensive multi-nest colonies with tens of thousands of workers and multiple queens. These “supercolonies” can inhabit large tracts of land, extending across multiple properties. Under these conditions they become serious persistent pests of homes, schools, restaurants and other man-made structures, frequently trailing indoors for food and water. OHA also dominates resources (space and food) in a way that excludes other ant species. They will aggressively fight off other species to defend their territory under these conditions. Oddly, although OHA appear to switch to an invasive mode in urban North America, they have not yet become invasive pests in other parts of the world, except Maui, Hawaii.
Eggs are laid from mid-spring until cold autumn weather sets in, except for nests that remain warm year-round in heated buildings, where egg-laying probably occurs all year. Reproductive swarms emerge at the start of summer. Queens and kings look for mates, and large numbers of unsuccessful alates can end up in pool filters. OHA also mate inside the nest without swarming. Worker ants care for the eggs, larvae, and pupae—newly emerged ants join the colony as workers. Colonies are usually dispersed around an area during the warm months, but come together in one or more protected nest locations or in voids inside heated buildings to survive the winter. This is why OHA may be found foraging year-round in the warmer parts of buildings and homes. Seasonal foraging happens from April through October in natural and outdoor areas. OHA workers are active foragers both day and night.
Lift a flower pot sitting in the yard or on the patio and you might find a clutch of OHA workers and larvae. Lift a stone, a log, a handful of leaf mulch and you may find them. OHA do not typically nest in soil burrows, rather they nest under objects and materials in the environment that trap moisture. You might even find them in between stones or under an outdoor carpet on an elevated deck. Indoors, OHA will nest in spots with moisture and warmth, such as near hot water pipes, wall voids, eaves or attics with moisture damage, bath traps, sink cabinets, under kitchen appliances and even irrigation control boxes. With many small colonies and potentially hundreds of queens, an entire property could be inhabited by a single, but extensive, colony.
Ants enter buildings for specific reasons, but also because they have found a pathway inside. Sometimes a few are just searching, but once food is discovered you could see a flood of ants. Access for ants can be very difficult to locate because they are tiny and use openings we often cannot see. Keep an eye out for where ants are entering and consider ways to seal up the openings. Common points are around window frames and crank handles, around and under doors, gaps around pipes and places where utilities like cable lines enter.
What did you do? You planted and decorated your landscape. Most structures, especially homes, have walkways and patios. Many have landscaping elements that encourage ants, such as flower beds with wood, plastic or stone borders, landscape cloth for preventing weeds, mulch, door mats, stepping stones, planters, wood piles, debris, junk, garbage cans, etc. Additionally, landscape shrubs can attract OHA when sap-feeding insects, like aphids and scale, overtake their limbs. Sap-feeders create honeydew, a favorite ant food. If you or a landscape professional treat trees or shrubs for sap-feeding insects you can cause OHA to seek food elsewhere, namely indoors.
Ants, like OHA and others, forage for various types of nutrients. Their preferences change depending on the time of year and the needs of the colony. The basics are carbohydrates (starch/sugar), fats and protein. Pet food satisfies all of those tastes and is a major attractant to OHA and other ants. Food and drink spills, old soda cans, garbage bins and the sink area with dirty dishes and food debris can become food attractants for OHA. Ant baits with pesticides are formulated to mimic one or more of these food sources.
It is not easy to manage America’s #1 pest, the odorous house ant. They are persistent and usually return, even after professional treatment using residual chemical pesticides. So how do we devise a control plan that works?
A professional’s approach to managing OHA will include several steps, all part of an integrated pest management (IPM) plan. This is the most logical strategy and homeowners should also follow the IPM plan.
Identify the pest
Make sure you know what species you are dealing with. Different ant species have different behaviors and food preferences.
Look for areas of ant activity indoors, find the opening used by ants, locate nests outside, determine the level of infestation (a few ants may not be a big deal). Also take a look at what attracted the ants. Food? Water?
- Sanitation involves the removal of food/water attracting the ants. Read the section above to see what odorous house ants look for.
- Exclusion, although difficult, is worth a try. Look for where ants are trailing and try to close the hole or block the area (under a door, for example) to prevent their access.
- Ants will also climb vegetation to enter buildings at higher points. Consider whether any trees, shrubs, vines or other plants are touching the building.
- If ants are a chronic problem, you may need to make permanent changes such as removing decorative landscape rocks, wood and other solid things around the perimeter of the building. These are objects where ants create smaller nests. This would not include pavers, stone or concrete patios or walkways.
- Consider other ways to reduce ant nesting close to the perimeter of the building or home.
A lower risk, yet highly effective, control tactic for ants is a food-based bait that ants consume. Baits take time to work but can eliminate an entire colony when foraging ants bring toxic bait back to the nest, killing larvae and potentially the queen. Baits are available in hardware stores and from professional pest managers.
Repellent (barrier) and non-repellent sprays with variable human toxicity are often used on the outside perimeter of the structure, up to 3 feet away from the foundation. Repellent barrier sprays keep ants from crossing (they die if they come into contact) and non-repellent sprays are undetectable, so ants cross, come into contact and die from exposure. For many reasons sprays are better if handled and used by a professional.