[Ant Control] HELP! Why Doesn't This Chemical Always Work?

Ants are everywhere, even in our descriptions of infestations. Ants remain the top growth area for most pest management companies. Ants also top the list of reasons for callbacks in management of these pests because of differences in efficacy experienced with chemicals and formulations. Researchers also find inconsistencies in laboratory trials with ants and various chemicals and formulations. This is noted for a number of ants, but particularly for colonies founded by a single queen and colonies that have long life expectancies such as carpenter ants. Why have ants been difficult to manage? Ants are often viewed as individuals rather than part of a larger single organism. W. M. Wheeler in 1910 first proposed the concept of an ant colony existing as a single “organism.” The 2008 book, The Superorganism: The Beauty, Elegance and Strangeness of Insect Societies by B. Hölldobler and E.O. Wilson, emphasized this unique quality with the central theme that a colony is a single animal raised to a level of organization intermediate between single organisms and the ecosystem. The authors define a “superorganism” as a colony that is tightly integrated by its communication system and caste-based division of labor. Does this explain the differences observed in field applications and in laboratory trials of various chemicals and formulations?

Everyone recognizes that not all humans respond the same to different foods, medications and chemicals. Age, sex, health and genetics all are important factors. Since all humans are not the same, perhaps one needs to consider differences that occur between different ant colonies, particularly in colonies that live for many years. This means considering the colony as a single organism rather than a colony of individuals.

LABORATORY DIFFERENCES. A researcher might assume that in carpenter ant baiting trials in the laboratory with five or more replicates of 50 to 200 ants that are from the same colony that the results would be within a similar range. This often is not the case and there often exists a wide range of results in the replicates. Adult ants have a range of activities that often are dictated by age. Two common activities are recognized: young ants are involved in nurse duties to care for the developing brood whereas older ants forage for food outside the colony. In selection of ants for trials, the age and therefore the activity of individual ants are unknown. Selection of ants by age is not practical in laboratory trials. A range of results occurs when testing for chemical efficacy. This is, of course, why we have averages and statistics, but deviations among the replicates occur and suggest why inconsistencies also occur in field applications. Differences in laboratory trials also occur when the same bait is tested with replicates from two different colonies. In trials with carpenter ants when both colonies were collected in the same locality at the same time of year, responses of these two colonies to the same baits were very different. With one colony, there was 98 percent mortality after feeding on an experimental bait for three days; in the second colony, feeding on the same bait, there was 19 percent mortality in three days and only 85 percent mortality after 12 days. Was this a factor related to colony age, genetics or health? Laboratory trials also demonstrate differences in mortality between colonies with the transfer of a toxicant when exposing ants to nest mates killed by a toxicant or to nest mates exposed to a toxicant.

In field applications with chemicals in various formulations, a pest management professional finds the same product may not have the same results with the same ant species at different locations or at different times of the year. Factors that need to be considered are the age of the colony, season of activity, health of the colony and the size of the colony.

The size and age of a colony are important factors in management. Small carpenter ant colonies including the queen, workers, larvae, pupae and after several years winged males and females are all located at a single location. In management, if a chemical is taken into the main colony through transfer with a non-repellent material or a bait, the entire nest may be eliminated.

With a repellent chemical the presence of dead workers where chemical has been applied often is satisfying to the homeowner and the technician, but callbacks will occur because the parent colony was not destroyed. Generally no more than 10 percent of the workers will leave the colony to forage. If these foragers are eliminated, additional workers may or may not immediately replace these foragers. Factors that determine replacement include the season, health, and size of the colony.

COLONY DETAILS. Large carpenter ant colonies exist as a parent nest containing the queen, workers, and young brood plus satellite nests. The latter may contain mature larvae and pupae, winged males and females, and workers. The size of the satellite nest may vary from a few hundred to several thousand. One satellite nest of carpenter ants was collected with more than 12,000 individual ants with estimates of its existence for several years at the same location.

Number of satellite nests varies with the size, age and location of the colony. In the southern latitudes two or three satellites may occur; in northern areas the number may increase to over a dozen satellites. Laboratory nests without a queen have been maintained for several years. The workers without a queen will forage for food and water during this time and produce trophic eggs and winged males. Management of large carpenter ant colonies with multiple satellites is complex. Movement of mature brood from a parent nest to satellite nests occurs about mid summer in northern latitudes. Movement between parent and satellites during the remainder of the year is variable and chemical may not be transferred to all segments of the colony. As with small colonies, chiefly only the workers that are foraging and contact baits or sprays will transfer the chemical to the non-foraging ants and brood.

Small colonies and parent nests require areas with high moisture content. These nests are often located in wood that has had water damage or in areas where there is wood decay. Structures may be the site of parent colonies if there has been a permanent water leak or damage due to long-standing faulty plumbing, gutters, roof leaks or wood in contact with soil. More commonly the parent colony is located outside the structure in buried wood, tree stumps or roots, or landscaping timbers. Satellite nests may be located in very dry areas with sound wood. These nests have been found under insulation in attics or subfloors, wall voids or in hollow areas within structures. Oftentimes, the first indication of an infestation is in the early spring when workers seek water and are found foraging in kitchens, bathrooms or areas where there is moisture. Satellites may also be established in outdoor areas where there is firewood, stacked lumber, hollow areas in trees, landscaping timbers and fencing.

TREATMENT OPTIONS. Treatments during the foraging season are more successful than during times of the year when carpenter ants are overwintering when they have a lower metabolic rate. During the active season, with the proper placement and the use of non-repellent materials, the ants will travel over treated surfaces and transfer the material to non-foraging ants. Gel and liquid baits are attractive to the ants as they begin to forage early in the season. Carbohydrate baits are also attractive to these early foraging ants. The introduction of materials with delayed toxicity is effective because it allows the chemical to be dispersed throughout the colony.

There are a number of excellent materials on the market in a variety of formulations. The gel baits have been formulated so that they remain available for consumption much longer than earlier baits that dried within a few hours. The new non-repellent materials that can be transferred either through dead ants by necrophoresis (carrying dead) or by living ants exposed to the chemical allow the toxicant to be spread through a colony without having to locate the parent and each of the satellite nests. However, spread to all the satellites is highly dependent on the season, particularly with a large number of satellites. Less communication occurs between nests as the season progresses and foraging is curtailed. With the variety of materials available, pest management professionals have a “menu” of chemicals and formulations to offer to ants. These may need to be offered in tandem with enough follow-up to determine efficacy and allow for alterations in strategies. It is important to remember that no two infestations or two colonies are the same. Remember to think of the colony as a single organism that differs in size, genetics, age and health and that ant activity varies with the season.

The author is professor at Spokane Falls Community College.  Contact her at lhansen@giemedia.com.

References
Hansen, L.D. and J.H. Klotz. 2005. Carpenter ants of the United States and Canada. Ithaca: Cornell University Press.
Klotz, J., L. Hansen, M. Rust, R. Pospischil. 2008. Urban ants of North American and Europe. Ithaca: Cornell Univ. Press.
Hölldobler, B. and E.O. Wilson. 2008. The Superorganism: The beauty, elegance and strangeness of insect societies. New York: W.W. Norton.
Hedges, S.A. 1992. Field guide for the management of structure-infesting ants. Cleveland, Ohio: Franzak and Foster.
Wheeler, W.M. 1910. Ants: Their structure, development, and behavior. New York: Columbia.
Wheeler, W.M. 1911. The ant-colony as an organism. J. Morphology 22: 301-325.