[Convention Extra] Ants on the Move: Desert Edition

The desert contains several species of ants that can be major or minor pests in the Southwest. The following is a review of three desert ants that pest management professionals in the Southwest could encounter: Imported rover ants, Argentine ants and red harvester ants.

IMPORTED ROVER ANTS. One of the most prominent pest ants is the imported rover ant — Brachymyrmex sp. in the Subfamily: Formicinae. Both rover ant species B. obscurior (exotic) and B. dipilis (endemic or native to the United States) have become ever more invasive in homes and commercial buildings; in particular, in southern California and the metropolitan areas of Arizona. B. obscurior ranges in color from amber (males) to brown (workers and females). In general, they form small monogynous colonies founded by mating flights which occur in the evening, primarily during the summer. Their diet consists of honeydew produced by subterranean homopterans and they prefer nesting in soil and or rotting wood. Alates are strongly attracted to artificial lights and swimming pools.

Imported rover ants are easily distinguished by their 9-segmented antennae with no club. They are small (1.5-2 mm long), with bodies that vary in color from yellow to black depending on the species. The workers are monomorphic.

Control of rover ants in and around structures can be challenging. Using exterior perimeter treatments and indoor spot treatments with typical residual liquid formulations can result in a temporary solution. The problems are associated with successive resurgences of activity occurring, resulting in callbacks.

Outdoors. Rover ant control in the landscape surrounding structures is focused on a strategic perimeter placement of reservoir stations containing sweet ant bait, and/or foundation perimeter sprayed with a labeled residual insecticides.

Indoors, the approach is to eliminate nesting sites within structures by:1) Locating the colonies within buildings, looking for high moisture areas; 2) Once located, injecting the nests with labeled residual  insecticides; and 3)  Following up with a monitoring program inside using sweet liquid baits to  determine the efficacy of the treatment.

Inspections can be focused around homes in potted plants and under objects in gardens.

ARGENTINE ANTS. Another pest species that is found in southern California is the Argentine ant — Linepithema humile (Mayr, 1868) in the subfamily: Dolichoderinae. The colonies are polygynous and contain thousands of workers. Workers are all the same size, generally small, 1/8-inch long, uniformly a dull brown color, and a petiole with one erect node. A musty odor is emitted when crushed. Colonies propagate by budding or fission. As for their diet, it is primarily honeydew produced foods such as sweets, fresh fruits, and buds of some plants. In addition, they tend honeydew-producing species and forage for oils and sweets in homes. They can be located traveling rapidly in distinctive trails along sidewalks, up sides of buildings, along branches of shrubs and trees, and along edges of carpets and baseboards. Colonies may split in spring and summer when the queen and the workers move to a new site and they are not antagonistic toward each other. Their nests can be located in exposed or covered soil. Colonies can be found under stones, logs, debris, firewood, potted plants, and in mulch in landscaping beds making control difficult at times. Sanitation is important. 

The elimination of moisture sources (e.g., leaky faucets, plumbing, and free-standing water) is critical because these ants are scavengers. You can remove the food source if ants are trailing to food with a mild detergent. Wipe ant trails after food is removed to eliminate the trail pheromone. Check potted plants for ants before bringing them indoors. One way to check for ants is watering the soil thoroughly to force ants out of the soil. Look for cracks and crevices in the structure and seal them. Keep branches from coming in contact with the house (ants will walk on them into the house). A targeted application of residual insecticides is important. Spot treatments at points of entry into the house such as around window sills and door thresholds, can be effective. Using a broadcast application for these ants is not a good solution. A liquid insecticide can make the area repellent to ants, and thus the ants will not feed on bait that is placed in the vicinity of a repellent liquid insecticide. It is important to control them early on when the colonies are small.

RED HARVESTER ANTS. The third pest species is the red harvester ant, Pogonomyrmex barbatus (Smith, 1858) in the subfamily: Myrmicinae. The brood and reproductives are found in nests in mid summer. They can be highly aggressive, with monogynous colonies, founded by a single queen that may survive for a long time. The antennae are 12-segmented without a club, with a row of hairs (psammaphore) on the underside of the head, which enables them to carry large loads.

Harvester ants are more commonly found in cultivated land rather than non-cultivated because of the available food. Nesting areas are usually located in barren circular patterns of varying size with a single entry. Nesting area is normally about 12 feet in diameter but may range from 3 feet to 35 feet. Their dietary preference is small seeds such as Bermuda grass, mesquite, burr clover, Johnson grass, wheat, and wild sunflower, along with beans and others.

Because they are harvester ants they can strip vegetation in very short periods of time. Before resorting to pesticides, pest management professionals should seek out and destroy their nests. Additionally, they should instruct homeowners to modify the ants’ habitat through regular turning of the soil and mowing.

The author is urban extension entomologist at the University of Arizona.