Order/Family: Hymenoptera/Formicidae Scientific Name: Iridomyrmex Humilis (Mayr)

Description:  Argentine ant workers are 1/1 6-inch long and light to dark brown, the queens are 1/8- to 1/4-inch long, brown, and covered with fine hair. Males are slightly smaller and a shiny brown-black. These ants lack spines on the thorax which is unevenly rounded when viewed from the side, have a single node on the petiole, and do not have a circle of hairs at the tip of the abdomen. The eyes are located very close to the base of the antennae and appear to be looking forward.

Colonies consist of several hundred to several thousand workers' and several queens. They are located in moist areas near a food source. Developmental time (egg to adult) is 33-1 41 days and averages 74 days.

The Argentine ant probably entered the United States in the latter part of the 19th century.  They were first noticed by Edward Foster in 1891, although undoubtedly they had been present for some years before his first observation.  It was surmised that they "hitched a ride" onboard ships laden with coffee from Brazil.

Range and Habits:  Argentine ants are found throughout the southern United States and California. They typically live in nests outdoors near a food source but become major pests when they forage indoors for food. Overwintering nests are large, may have several queens, and are found deep in the soil and in buildings near' heat sources. In the spring, these nests disperse with smaller colonies developing in moist soil, in trees, and under stones and concrete slabs. The colonies are mobile, relocating to more acceptable nesting locations whenever necessary. In the fall, the colonies congregate in communal overwintering sites. Ant numbers decrease somewhat during this period. Argentine ants are very aggressive and eliminate other ant species in the area they colonize. They attack, destroy, and eat other household pests, such as cockroaches. They prefer sweets, often tending aphids or scale insects on plants, and use them as a source of honey dew.

Control: Outdoors look for Argentine ants trailing up next to the foundation, sidewalks and driveways just below the grass line. All cracks and gaps in exterior walls which provide access to voids and interior areas should be sealed.  Cracks in slabs and gaps in expansion joints should be filled. All debris from the exterior of the structure and other items on the ground where these ants nest should be removed.

Damage and Control:   Control is often difficult because the colonies are dispersed. However, the workers are very good at finding sweets in homes and in establishing trails which can be used to find the nests. They are attracted to and feed on sweet and protein-based baits.  The Argentine ant is one of the most common household pests in the wide area where it is distributed. For example, in those sections of southern California where the insect is prevalent, practically every household utilizes some measure to combat it. In homes where its control is neglected, hordes of this persistent creature with its ever present trails may be found in practically any sweet food. It is an extremely persistent pest, especially during the warm summer months. In the winter, immediately after rain, it may invade the home seeking protection from inclement weather.

Baits can be placed where ant trails have been established or in locations where the ants have been sited. Unless using containerized baits indoors, baits should be placed so they are inaccessible to children and pets. Baits should be checked frequently for feeding activity and availability.

The most effective control is accomplished when ant trails are followed to the nests and treated with a residual insecticide. Exterior nests should be drenched with a liquid formulation. Nests in wall voids are more easily treated by aerosol injection or application of a dust formulation. When the nests cannot be located, a barrier spray using a microencapsulated or wettable powder formulation should be applied to foundations, plants, and soil immediately adjacent to the building. This is effective in repelling foraging workers and preventing them from reentering the structure. Controlling aphids on ornamental plants and trees around structures removes a preferred food source and causes them to forage elsewhere for food.

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