Influence of plant derived chemical compounds on the decision for foraging and improvements in the chemical control of Atta sexdens rubropilosa (Hymenoptera: Formicidae)

Abstract

The leaf cutter ants are considered the most derived among the fungus -growing Formicidae: Myrmicinae and as a result of their age-based polymorphism and polyethism they established foraging strategies which guaranteed their great ecological success. They distinguish themselves for the ability to produce their own food, due to the symbiosis with the fungus Leucoagaricus gongylophorus. This permits exploiting a vast variety of vegetal resources of which workers collect material from different sources, however they are selective in choosing the adequate material both for themselves and for the development of the symbiotic fungus. As well they point out economically, principally when they occur in large agricultural or forestry areas generelly consisting of introduced species, which favours the increase of the populational density, therefore they are considered a true pest. Various studies approached the factors that influence the collection of vegetal material during the foraging. The chemical factor is fundamental for the choice of plants, nevertheless there are still controversies over its real role in the selectivity. Another point to be highlighted is the difficulty in the control of leaf-cutting ants due to their social behaviour and the use of active ingredients which are not suited to circumvent this behaviour. The misuse of insecticides can stimulate behavioural reactions of the workers resulting in the recognition of prejudical compounds by workers. In the present toxic baits used in the control of leaf-cutting ants are made of citrus pulp, a subproduct of the industrial processing of oranges. If failure in the abatement of a colony occurs the workers stop collecting the bait and only return collecting it after a period of 90 to 120 days. They exhibit a "collective memory" due to the association of the harmful effects of the bait with its components, not necessarily with the active ingredient. In the attempt to avoid this bahviour of rejection of new active ingredients or compounds present in toxic baits it is advisable to analyse the possibility of rotational use of attractants, with the intend that the baits are continuously carried by the ants, turning the control more efficient. In the present study it is proposed to clarify from a chemical point of view what is more significant for the workers choice of material during the foraging: the attractiveness or the repellency. Associated to this it is intended to investigate new recommendations on control, carrying out tests of attractiveness with alternative plants to be used in the formulation of toxic baits. Therefore initially two plants with distinct charateristics will be analysed: a very attractive, Acalypha wilkesiana (exotic) and a repellent (or little attracticve), Tocoyena formosa (tipical of Cerrado). The plants chemical profiles will be characterized as well as it will be establish what makes up their attractiveness or repellency for Atta sexdens rubropilosa workers. Afterwards it is intended to analyse the workers responses towards the extracts of these plants, containing the attractive or repellent chemical compound. The level of importance for the selectivity of the ants and how the recognition of the information about the quality of vegetal material by worker subcastes takes place will be observed. Subproducts of the industrial processing of cashew nuts (Anacardium occidentale) and soybeans (Glicine max) will be evaluated in the laboratory and in the field concerning their attractiveness and potential use in the fabrication of granular baits. Enlarging the knowledge about the interaction between leaf-cutting ants and plants could contribute to a better understanding of the complex social system and could provide additional information for turning the methods of control more effective. (AU)