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The optimal foraging theory predicts that colonies of social insects must be able to adjust the intensity of their foraging behaviour as a function of the quality of the food discovered. Here, the mechanisms allowing the regulation of recruitment as a function of food concentration in the ant Myrmica sabuleti were analyzed. Although the total number of foragers engaged in food collection during recruitments increased with increasing concentration of sucrose solutions (0.1 vs. 1 M), neither the proportion of recruiting scouts nor the invitation behaviour performed by the scouts in the nest can explain this relationship. Foragers trail more when coming back from a 1 M than from a 0.1 M sucrose solution. However, this alone cannot explain the collective patterns observed since the mean numbers of workers leaving the nest after the entry of a scout coming back from either 0.1 or 1 M sources were not significantly different. We suggest that a spatial distribution of the foragers in the nest as a function of their motivational state could be part of the regulation process. The ants located near the nest entrance would respond to both low and high trail pheromone signals, but those located deeper in the nest would respond only to high level signals, resulting in higher recruitment rate towards richer sources.

Field investigations showed that the ant Lasius fuliginosus combines stable foraging behaviour based on long-term memory and spatial specialization with more flexible actions, based on food recruitment. The use of food recruitment varies according to the season and the type of food. Baits of meat or large prey elicited recruitment during the major part of the year, except in the autumn, while baits of sugar elicited recruitment only in early spring, before the aphid colonies developed. A permanent trunk trail was established after a recruitment towards a newly discovered and stable source of carbohydrate food. This happened in early spring when the food sources were scarce or when spring migrating aphids were produced.