[Ant Management] Off the Beaten Path

Identification of carpenter ant trails is a key factor in determining infestation sites and will assist in the determination of what should be included in a treatment program. What follows are answers to some of the common questions asked by PCOs regarding carpenter ant foraging trails.

When are carpenter ants moving on trails? Carpenter ants move on trails for two reasons. In late summer (July in the Pacific Northwest), carpenter ants use trails to move mature brood into satellite nests. Parent nests are most often established outside a structure in stumps, buried wood, landscaping timbers, standing dead trees, the heartwood of live trees, or any wood with a high moisture level. Satellite nests are established in attics, voids, under subfloor insulation, in crawl spaces, and around the exterior of hot tubs. These areas are generally warmer and drier than the parent colony. The brood is transported into satellite colonies and includes mature larvae and pupae of workers, winged males and winged females. Transport along these trails between the parent and satellite nests usually occurs over a two- to three-day period. When brood is not being transported, workers maintain contact between the parent and satellite nests at irregular intervals during the spring, summer and early autumn.
The second type of trails, and the ones most commonly observed are used for foraging. Carpenter ants generally forage on food sources outside structures. Ants travel between the foraging arena and either the parent or satellite nests. Ants prey on other insects, nectar and homopteran honeydew. Carpenter ants observed in the field prey upon other ants, bees, wasps, termites, caterpillars, crane flies, misc flies, aphids and other homoptera, beetles, moths and butterflies, and cockroaches. Ants also forage on nectar and sugar-based materials such as fruits, sodas, hummingbird feeder solutions and sweet food stuffs.

Where are trails located? Carpenter ants form trails in straight lines in open areas but also follow structural guidelines. An irrigation hose connected to a water connection on the exterior of a structure and extending into the yard near trees is a ready-made trail. Ants will follow the hose to the structure rather than a straight line to the house, especially if the trail requires clearing a pathway through the lawn. Other guidelines include the edges of sidewalks, driveways, landscaping timbers, fences, grooves in siding, and under the edge of siding. Wiring and cables that extend from poles through trees to the roof or into attic areas are also readily accepted as trails by ants.

When are ants foraging on trails? Carpenter ants are seasonal with peak activity occurring in summer. In winter months, the ants have a period of diapause where they do not forage and therefore trails do not exist. When ants break diapause, in early spring, they are particularly attracted to sweet materials and water, and are often observed around water sources in bathrooms and kitchens when nests occur within structures. Foraging commences and trails are established when prey becomes available outside the structure. In the Pacific Northwest, trails are difficult to locate until early May when aphids become active. The trails are obvious until the end of September, with peak activity in June and July.
The heaviest foraging activity is between sunset and sunrise. Ants will forage throughout the day, but higher numbers will be observed on the trails after sunset and the lowest numbers of ants on trails occur in the early morning hours (between sunrise and noon). In ant counts on trails in the Pacific Northwest, the average number of ants on trails varied from 10 ants per minute in the early morning to 50 ants per minute after sunset during the peak of the foraging season in July. Carpenter ants are secretive, moving along dark or shaded surfaces and do not actively forage in the rain. However, trails can be observed with careful inspection at any time during the foraging season.

How many trails will occur from a single colony? The number of trails is determined by the size and age of the colony and the size of the foraging arena. Several years are required for a colony founded by a single queen to reach the population size where it is capable of causing structural damage. These large mature colonies may have more than one foraging trail, especially if more than one foraging arena is involved. Inspections should include all possible foraging sites near the structure. Trees with aphids and other insects are common foraging sites. If small numbers of insect prey are available, several trees may be involved with trails to these foraging arenas.

What information does a trail reveal? The numbers and sizes of carpenter ants on a trail may indicate the health of the colony. A healthy colony will have the largest number of small workers and the fewest number of large or major workers. Foragers carry liquid food to the nest in their crop located in the gaster. As the crop expands with food, the abdominal segments telescope outward giving the gaster a striped appearance because the expanded areas lack the pubescence found on the exposed areas. In extreme cases, the foragers may carry enough food so that the intersegmental membranes are exposed. Ants with expanded gasters will be traveling toward the nest; ants without expanded gasters are leaving the nest to enter the foraging arena. Careful observation of the gasters will indicate in what direction the nest is located.
Ants also carry whole prey in their mandibles to the nest. When the location of foraging arenas is proving troublesome, infestation sites of aphids or other insects can be located to determine if ants are foraging from this area. By following the trail from the foraging arena, entry into the structure may be revealed.

Why are trails important in a management program? Management strategies vary with the site and with client requests. If baiting is an option, location of trails is imperative. Baits placed on or near foraging trails and entrances into structures are more effective than baits placed randomly around the structure. A perimeter spray application generally includes spraying the foraging trails. These trails as indicated previously may follow a guideline away from the structure. Trails may also be constructed through lawns. Chemical application to these trails with a non-repellent material will increase the exposure to foraging ants. Slow-acting chemicals will be transported to the colony. Application of repellent materials to trails may cause ants to alter their trails. Chemicals with a rapid “knock down” will eliminate the ants on the trail but may have no effect on the colony. Generally less than 10 percent of the ants in a colony are foraging at one time.
Knowledge about trails is an important aspect of carpenter ant management. Coupled with knowledge of life cycles, life history, biology, ecology and the correct identification of the species, our overall understanding of this important structural pest will increase the success of carpenter ant management.

Dr. Laurel Hansen, one of the country’s leading carpenter ant experts, is an instructor in the Biology Department at Spokane Falls Community College, Spokane, Wash. She can be reached at lhansen@giemedia.com.