Non-random search pattern of ants foraging on honeydew of aphids on cashew inflorescences

Abstract

Cashew, Anacardium occidentale L., trees harbour aphids, Toxoptera odinae Van der Goot, in the flower clusters borne on racemose inflorescences. The foraging pattern of the ant, Camponotus compressus Fabricius, on the honeydew secreted by these aphids was studied. The ants foraged in spatially nonrandom patterns. From any given point along the linear axis of the inflorescence, they tended to orient towards that part where they were more likely to encounter aphids. Analysis of the three-dimensional movement pattern of the ants showed they tended to explore the inflorescence systematically by visiting successive branches in their subsequent moves. They attained this by moving sinusoidally around the inflorescence axis. However, the ants concentrated their foraging effort in areas of high aphid density. Although this led to deviations from the systematic exploration of the inflorescence, it increased their foraging efficiency. There was a significant association between the numbers of aphids of any two randomly selected branches of an inflorescence indicating that the resource quality of a randomly chosen branch reflects that of the whole inflorescence. Using this association, ants could assess the quality of an inflorescence by sampling a part of it and accordingly decide whether or not to continue foraging on an inflorescence; they abandoned the inflorescence if the sampled branch was of poor quality and continued to forage if it was of good quality.