Secret Site Map
Tuesday, June 30, 2015

Laurel Hansen

The author is professor at Spokane Falls Community College. Contact her at


[Ant Control] Taking the Bait


There are a variety of tools available for the management of ants. Here, one of the industry’s leading ant researchers answers key questions about how carpenter ants and odorous house ants interact with baits in the field.

May 28, 2015

Integrated Pest Management (IPM) comfortably fits into an urban pest management strategy for ants. Control is accomplished by physical alterations to ants’ nesting sites and foraging patterns plus chemicals in the form of sprays and baits. These are all important tools for the management of two key ant pests: carpenter ants and odorous house ants.

Both carpenter ants and odorous house ants may occasionally feed on food available inside a structure, particularly in early spring when these ants are becoming more active and food is not available outside. Their natural food during the foraging season involves aphids and other homopteran insects. This feature has led to the development of baits to add to our arsenal of control agents.

Baits have become increasingly important tools and are available as gels, granules and liquids. Gel baits can be placed inside structures in secure locations, such as in crack and crevice areas, where ants are found trailing. These baits also are effective on the exterior and particularly are effective when placed in areas where ants are entering the structure. Baits placed on foraging trails will be encountered and other ants will be recruited to these sites. Granular baits also are effective on the exterior of the structure but again, they should be placed in foraging trails. Liquid food is carried in the crop of foragers and distributed to nest mates and larvae. Solid food that is collected cannot be passed directly into the digestive system of worker ants and larvae have been credited with the digestion of solid food that is then passed back to the workers.

Digestion of Granulars.

An important question arises regarding granular baits in the control of ants: Are larvae required to digest granular baits? Liquid and gel baits can be distributed easily by foragers to adult ants and larvae. To answer the question of whether granular baits must be digested first by the larvae to release the toxicant before being passed back to worker ants, the following laboratory trials were performed. Laboratory trials were established with five different granular baits plus three gel baits and fed to satellite nests of 40 workers. Half of the nests possessed larvae; half of the nests contained no larvae. These trials were established with carpenter ants (Camponotus modoc) and odorous house ants (Tapinoma sessile). Ants were supplied with a constant source of bait plus honey in choice tests. Granular baits selected for these trials were:

  • Advion Insect Granules (0.22% in- doxacarb)
  • Maxforce Granular Insect Bait (1.0% hydramethylnon)
  • InVict Xpress (0.5% imidacloprid)
  • Niban-FG (5% boric acid)
  • Advance Select Granular Ant Bait (0.011% abamectin B1)

In addition, three gel baits were tested for comparison.

After 28 days, there was no difference in the mortality of carpenter ants with and without larvae with the exception of one gel where the mortality in nests without larvae was significantly higher. Mortality with the granular baits ranged from 60 to 100 percent. Similar results occurred in odorous house ant trials where nests containing larvae had a lower mortality than nests without larvae. In these trials, larvae of neither of the ants were required to digest the granular baits to release the toxicant to workers. Similar results also occurred in the trials with gel baits in that nests without larvae had a higher mortality than nests with larvae.

Solid foods traditionally have been described as being brought to the nest by foragers, fed to larvae who digest the food and regurgitate a liquid form that can be ingested by workers. In the examples of baits in these trials, the toxicant from the bait provided a high level of mortality either through penetration of the exoskeleton or through ingestion as the food was transported to the nest. Workers carry solid food in the infrabuccal sac (located behind the mouthparts) where a number of glandular secretions may be involved in digestion of solid materials before nutrients are passed into the pharynx or transferred to larvae. Toxicants in both the granular and gel form are effective agents in the control of worker ants. As the larvae (and pupae) are completely dependent on workers for survival, these stages cannot survive.

HOW LONG are BAITS ACTIVE? A second question: How long do baits remain active in the field to control ants? To answer this question, laboratory trials with carpenter ants were fed baits that were aged indoors for periods up to four weeks. These aged baits were then added to satellite colonies to determine efficacy. Gel and granular baits used in these trials included:

  • Advion Ant Gel (0.05% indoxacarb)
  • Optigard Ant Gel Bait (0.01% thia- methoxam)
  • Maxforce Carpenter Ant Bait Gel (0.001% fipronil)
  • Gourmet Ant Gel Bait (6% disodium octoborate tetrahydrate)
  • Advion Insect Granules (0.22% in- doxacarb)
  • Niban-FG (5% boric acid)

Baits aged three days, one week, two weeks and four weeks were equally effective through the 14 days of feeding with a slight decline in the first five to seven days in baits aged for four weeks. Maxforce gel (fipronil) was the only bait that was not consumed but it had 100 percent mortality, indicating mortality occurred through contact with the bait rather than ingestion.

In a similar study with odorous house ants, baits were aged both on the interior of structures and on the exterior. Not all baits had similar results; however, after 14 days of exposure to aged baits, efficacy of baits aged in exterior areas declined by more than half compared to baits aged on the interior. (Efficacy is defined by the mortality of the worker ants.)

Baits are most effective for both carpenter ants and odorous house ants when foraging trails are located. Baits, either granular or gel, should be placed directly on trails and where ants are entering the structure. Trails may be relocated depending on the food sources of the ants. As trails are moved or relocated, additional baiting may be necessary.

In addition to baits in control of ants, applications of a liquid spray with features of either transfer or quick knockdown are also important management tools. Sprays with a high degree of transfer are effective in eliminating carpenter ant colonies and also are helpful in control of odorous house ants. These chemicals are slow acting and are transferred to other foraging ants and ants remaining in the nest. Chemicals with a quick knockdown will eliminate foragers and may provide residual activity around the perimeter but will not be transferred to ants remaining in the nest. Only about 10 percent of the nest residents forage at one time, leaving large numbers of ants that may later become foragers.

Inspections should include a survey of the exterior of the structure to determine possible foraging patterns and nesting sites. There should be no vegetation in contact with the structure. This includes vines, trees, shrubs and other vegetation. Also there should be clearance around the foundation so that soil does not come into contact with siding, pillars or other structural supports. Fences or landscaping in yards that connects the structure to vegetation, particularly trees, also can provide foraging trails for ants. Any wires that lead away from the house and have contact with tree branches also should be checked. These make excellent avenues for ants to enter attics and to forage in the trees without coming into contact with a perimeter spray application. Wires are often dark in color making ants difficult to see — they use them when wires are shaded.

Location of nesting sites includes inspecting decks, patios, landscaping materials, firewood and stored timbers that have the potential for housing a carpenter ant nest. In contrast, odorous house ants require very little shelter and may nest in stacks of leaves, pine needles, under pieces of wood, playground equipment or other debris present in the yard. Sanitation or elimination of possible sites for these colonies will reduce activity observed within the structure. This is particularly important when sheltering materials are near the foundation of the structure.

Winter months when ants are not actively foraging proves to be the most difficult time to manage ants in structures. These satellite nests are located inside structures, particularly attics or in cathedral ceilings. Heat may cause ants in these nests to become active, often seeking water or moisture in bathrooms and kitchens, during late winter and early spring months. Ants are in a dormant state even though they are seeking moisture. Their metabolism is low and susceptibility to chemical activity is reduced. Drilling into areas to inject dust or aerosol may be necessary to reduce the appearance of ants.

Identification of the pest ant species plus information regarding their lifestyles provides clues for management. Two important pest ants, carpenter ants and odorous house ants, have very different lifestyles. Carpenter ant colonies have a single queen that is located in a parent nest requiring high humidity. These colonies establish satellite nests without a queen and can live in warm, dry areas. Hence, parent nests of carpenter ants are usually found outside a structure where there is wood that contains moisture.

In contrast, odorous house ants have many queens and establish a number of colonies both inside and outside structures. These fragments prove to be more difficult to control because their “nests” are no more than shelters and are commonly moved to accommodate residences in close proximity to their foraging sites. Because there are many queens in many sites during their active season, toxic materials in either baits or sprays may not eliminate all “nests.” These satellite nests differ from carpenter ant nests because each may contain a queen or queens.

There are many other types of ants that are found in urban housing environments. Just as carpenter ants and odorous house ants have varied lifestyles, other ants will differ in nesting sites and foraging. Identification becomes of primary importance in management strategies so that understanding these features for each particular species can be utilized for effective control.

The author is an instructor in the biology department at Spokane Falls Community College, Spokane, Wash. Email her at