The Seasonal Life Cycle of the Argentine Ant, Iridomyrmex humilis (Hymenoptera: Formicidae), in Southern California

Iridomyrmex humilis (Mayr) is seminomadic and often shows striking seasonal movements of entire field populations with unfavorable changes in environmental conditions. The seasonal low in the life cycle occurs from January to February when workers constitute above 90% of the biomass of the colony with the remaining 10% composed of either queens or overwintering worker brood, which completes its development by mid-March. The new seasonal life cycle begins in late February to early March with the production of a large number of eggs. This 1st group of eggs gives rise primarily to sexual forms, which subsequently mature during May. Mating occurs within the nest shortly after the females have emerged from pupae. After mating, the females shed their wings, join in the polygynous colony, and immediately begin to lay fertile eggs. The number of queens in the nest remains fairly constant until January or February of the following year when about 75% of the queens are eliminated by workers in a form of colony regulation. Worker production starts about mid-March with the laying of eggs that will give rise to the 1st worker brood of the season. Worker numbers increase throughout the year with worker brood forming 50% or more of the colony biomass through the summer months of July, August, and September. Production of worker brood drops very sharply in October when the worker population itself is at a seasonal high. With no replacements from brood, the worker population decreases steadily over the winter months, reaching its lowest level the following March or April.