competition exists between harvester ant populations, presenting challenges to successful foraging. One example of this is the competition for access to seed resources between the Novomessor cockerelli and Pogonomyrmex barbatus species of harvester ants. Novomessor cockerelli ants engage in interference competition against Pogonomyrmex barbatus colonies by "plugging" entrances to the nest with sand and small rocks. This delays activity of P. barbatus for one to three hours, shifting the onset of foraging until later in the day when the temperature is substantially higher. This reduces productivity in two ways; firstly, the ants are delayed and consequently have less time to forage; secondly, high soil temperatures prematurely drive harvest ants back into their nests, shortening the range of possible foraging. Research indicated that the affected P. barbatus populations did not take efforts to compensate for the interference in their activities, such as by inducing higher rates of foraging. As such, nest plugging by rival species impedes the overall productivity of worker ants in the species Pogonomyrmex barbatus. Division of labor Caste system Red harvester ants are eusocial insects, characterized by a high level of cooperation and an organized division of labor. The worker caste is sterile. As with most ant species, reproduction is performed by a mating caste that consists of winged alates (reproductives) that reside in the nest until weather permits them to fly away and mate. After copulation, the male usually dies, while the now-fertilized queen returns to the ground to search for a suitable nesting site. Once she has chosen a site, she sheds her wings and begins to reproduce, creating a new colony. She produces "worker ants" for 1–20 years until her death. Some queens have been known to live up to 30 years in the wild. In P. barbatus, reproductive and nonreproductive tasks are divided into a caste system, as discussed above. Some individuals are specialized for reproductive function, while other ants are responsible for nonreproductive roles such as defense, caring for the brood, and foraging for food. The system of division of labor naturally arises in conjunction with the formation of the group, rather than as a secondary adaptation; otherwise solitary queen harvest ants placed in forced association had division of labor arise in groups that lack an evolutionary history of such social arrangements. Nonreproductive caste Within nonreproductive P. barbatus worker ants, five major tasks are performed: foraging, maintenance of the nest, defense, midden work,