Iridomyrmex humilis (Mayr)
This species, which is native to Argentina and Brazil, was probably introduced at New Orleans via coffee ships from Brazil before 1891. Argentine ants are found in the southern states and in California, with isolated infestations in Illinois, Maryland, Missouri, Oregon, and Washington.
Workers monomorphic, about 1/16" (2.2-2.6 mm) long; queens about 1/8-1/4" (4-6 mm) in length. Body varies from light to dark brown. Antenna 12-segmented, without a club. Thorax lacks spines, profile unevenly rounded. Pedicel 1-segmented. Gaster with anal opening slitlike, lacking circlet of hairs. Stinger absent, but can bite on provocation. Workers emit a stale greasy or musty odor when crushed.
- Similar Groups
- Odorous house ant (Tapinoma sessile) has pedicel/node nearly hidden by front edge of abdomen/gaster, emit a sweetish rotten coconut odor when crushed.
- Crazy ant (Paratrechina longicornis) with legs very long in relation to body size, antennae scape (lst segment) at least 2 times head length, anal opening circular, surrounded by circlet of hairs.
- Other small dark ants with 2 nodes/segments in pedicel and/or thorax with 1 or more spines/teeth on upper surface.
Colonies are located in moist situations near a food source. They include a few hundred to several thousand workers and many queens; numbers fluctuate seasonally. In the autumn, outside colonies join together to form huge overwintering nests. Development time (egg to adult) is 33-141 days, averaging 74 days. Winged female reproductives are rarely seen because mating takes place inside the nest. The workers are very aggressive and usually eliminate other ant species and some other insects from the area. However, ants from different Argentine ant colonies are friendly and do not fight.
Inside, these ants usually nest near a moisture source such as water pipes, sinks, pofted plants, etc. The workers follow regular trails when foraging, and winged queens can sometimes be found among trailing workers. Workers commonly tend honeydew-producing insects. The preferred foods are sweets such as sugars and syrup but they will feed on almost every kind of food including meats, eggs, oil, fats, etc.
Outside, Argentine ants typically live in shallow nests located in moist situations such as under boards and stones, beneath plants, along sidewalks, etc. This ant prefers sweets such as honeydew, fruit juices, and plant secretions, but also steals seeds, attacks poultry chicks, disrupts bee hives, etc. Their habit of crawling over everything including refuse, sewage, sputum, carrion, etc. affords them the opportunity to transport the causative disease organisms for dysentery, etc.
Argentine ants invade buildings in large numbers when conditions outside are either too wet or too dry for them to live. Also, a decrease in their honeydew supply sends them indoors seeking sweets. They will readily trail along tree and shrub branches, as well as utility lines/wires to gain entrance.