Range and Pasture XIX-3

Western Harvester Ants

John B. Campbell

Carpenter Ant

Field Biology and Identification

Harvester ants rank next to grasshoppers in terms of rangeland damage. They create mounds that are about a foot high and 30 feet in diameter. The area around the mound is denuded of all vegetation. Not only is the vegetation destroyed but wind erosion increases in the denuded areas. The ants also collect seeds that may affect plant production, particularly with annual plants.

The ants have complete metamorphosis (egg, larva, pupa, and adult) in the life cycle. Colonies are established by mated queens in late summer. After mating, the queen sheds her wings and digs a burrow several inches deep in the soil. The first brood is composed entirely of worker females. This brood forages, enlarges the colony, cares for the young and protects the mound. The workers move the eggs to newly constructed chambers which are stocked with seeds and insect parts as a food source. Colonies survive as long as 15 to 20 years and may consist of as many as 60 chambers. The winged reproductive males and females usually appear in May following a rain. They mate and establish new colonies. Not all emerge in early summer, some emerge in late summer and early fall.

Plant Response and Damage

The area surrounding the mound is totally denuded. Most of the mound expansion is in the spring but there is no regrowth later. Wyoming has reported as few as 15 mounds per acre destroyed ¹¦₇ of the vegetation on that acre. Damage seems to be more intense in short or mid-grass prairie than on the more arid sagebrush shrub range. Researchers have analyzed the effects of grazing pressure on the abundance of ant mounds for long periods of time (10 to 30 years). There appeared to be no difference in light or moderately grazed pasture but mound numbers increased dramatically on overgrazed range.

Management Approaches

Cultural Methods

Research indicates that grazing management is one method that, if not reducing mound numbers will at least slow the increase of new mounds. As with the situation with grasshoppers, overgrazed range suffers the greatest loss from the red harvester ant.

Host Plant Resistance

There are no known resistant plants.

Biological Control

Little can be found in the literature concerning biological control agents that have an impact on harvester ants. The protection furnished by the workers by stinging and biting mound invaders probably prohibits arthropod predators from attacking ants. Birds and lizards are probably the only successful predators. The impact of micro-pathogenic organisms is unknown but probably of little importance.

Chemical Control

There are insecticides registered for mound treatment but their use requires considerable time and effort. It is possible that seeds utilized by the ants could be treated with insecticides. But unless control is approached as it is with grasshopper control, on a cooperative area basis, individual efforts would be short lived.

Categories: Range, Insects, Pasture, Western harvester ants, Ants

Date: 06/11/2002