Variability in the selection of substrate by the leaf-cutting ants atta cephalotes (L.) and acromyrmex octospinosus (Reich) (Formicidae, Attini)

In laboratory colonies of Atta cephalotes (L.) and Acromyrmex octospinosus (Reich) pick-up responses to paper discs impregnated with arrestant and inhibitory solutions and feeding responses to measured amounts of these solutions were progressively increased by increasing the concentration of arrestants and decreasing the concentration of inhibitor; the inhibitor had a much greater effect on feeding than on pick-up. Both species picked up paper discs impregnated with arrestants and Acromyrmex readily incorporated these into the fungus garden, while Atta did this less readily. Foraging of arrestive paper discs presented regularly for several days was maintained by Acromyrmex but declined in Atta. Both species were less selective during periods of high foraging activity and during a shortage of suitable substrate. Incoming foragers did not seem to influence the preferences of the outgoing ants, but different colonies showed different preferences. Initially little preference was shown for new materials but on subsequent encounters widely fluctuating preferences emerged, which stabilised within two weeks. Over longer time periods further changes in preferences occurred. The ants' pick-up, cutting or feeding response to a particular material is determined by its chemical and physical nature, the amount of chemical information which the ants obtain, and the threshold of their response. This threshold is modified by such factors as the genetic composition of the colony, the previous quantitative and qualitative availability of the substrate, and the general level of foraging activity. Practical implications of these findings for toxic bait production are discussed.