Increase in scout trips due to forager removal in Atta sexdens (Hymenoptera, Formicidae) (Forel, 1908).

Social information exchange through physical contacts and chemical trail deposition forms the basis of food recruitment in leaf-cutting ants. The scout initiates the process and passes the information to nestmates that recruit more foragers, thus amplifying the stimulus and ensuring the success of foraging. An interruption of the contact between workers and a reduction in trail laying can diminish the effectiveness of mass recruitment and alter scouting activity and forager flow. This study verified an increase in scout trips as a consequence of inbound workers (with or without a plant load) removal during Atta sexdens foraging, sustaining the outbound flow of foragers, and consequently foraging activity, either through direct contact or chemical trail deposition. Data indicate as one of the roles of unladen workers along the foraging trail must be to stimulate other workers to go out and so speed up the recruitment process The remarkable ability to organize themselves without central control is a major strength of social insects and the increase in scouting activity observed here is an example of this behavioral flexibility in leaf-cutting ants. Although foraging performance is enhanced through communication between workers, the simple adjustment in scouting activity can maintain the outbound flow of foragers which is an essential activity of the colony.

Effect of the presence of brood and fungus on the nest architecture and digging activity of Acromyrmex subterraneus Forel (Hymenoptera, Formicidae)

ABSTRACT This study investigated the stimuli that trigger digging behavior in Acromyrmex subterraneus during nest building. The hypothesis was that the presence of the fungus garden and/or brood triggers the excavation of tunnels and chambers. For the experiment, the excavation rate of individually marked workers kept in plastic cylinders filled with soil was recorded. Four treatments were applied: (1) 30 medium-sized workers, 5 g fungus garden and 30 brood items (larvae and pupae); (2) 30 medium-sized workers and 5 g fungus garden; (3) 30 medium-sized workers and 30 brood items; (4) 30 medium-sized workers without fungus and brood. After 24 h, morphological parameters of nest structure (length and width of the chambers and tunnels in cm) and the volume of excavated soil were recorded. In contrast to the expected findings, no change in morphological structure, rate of excavation by workers, or volume of excavated soil was observed between treatments, except for tunnel width, which was greater, when no brood or fungus garden was present. Thus, the results do not support the hypothesis that the fungus garden and/or brood are local stimuli for nest excavation or that they mold the internal architecture of the nest. Although this hypothesis was confirmed for Acromyrmex lundii and Atta sexdens rubropilosa, the same does not apply to A. subterraneus. The digging behavior of workers is probably the result of adaptation during nest building in different habitats.

Spatio-Temporal Dynamics of Foraging Networks in the Grass-Cutting Ant Atta bisphaerica Forel, 1908 (Formicidae, Attini).

Foraging networks are a key element for ant colonies because they facilitate the flow of resources from the environment to the nest and they allow the sharing of information among individuals. Here we report the results of an 8-month survey, extending from November 2009 to June 2010, of the foraging networks of four mature colonies of Atta bisphaerica, a species of grass-cutting ant which is considered as a pest in Brazil. We found that the distribution of foraging effort was strongly influenced by the landscape features around the nests, in particular by the permanently wet parts of the pasture in which the nests were located. The foraging networks consisted of underground tunnels which opened on average at 21.5m from the nests and of above-ground physical trails that reached on average 4.70m in length. The use of the foraging networks was highly dynamic, with few sections of the networks used for long periods of time. Three different phases, which could be linked to the seasonal change in the local rainfall regime, could be identified in the construction and use of the foraging networks. The first phase corresponded to the beginning of the rainy season and was characterized by a low foraging activity, as well as a low excavation and physical trail construction effort. The second phase, which began in February and extended up to the end of the humid season at the end of March, was characterized by an intense excavation and trail construction effort, resulting in an expansion of the foraging networks. Finally, in the third phase, which corresponded to the beginning of the dry season, the excavation and trail construction effort leveled off or decreased while foraging activity kept increasing. Our hypothesis is that ants could benefit from the underground tunnels and physical trails built during the humid season to maintain their foraging activity at a high level.

Avaliação dos efeitos tóxicos de extrato hexânico de Azadirachta indica (A. Juss) em colônias de Acromyrmex rugosus (Smith, 1858) (Formicidae, Attini)

O presente estudo teve por finalidade investigar o uso da planta Azadirachta indica como potencial agente natural no controle de formigas cortadeiras bem como verificar alterações comportamentais de operárias de Acromyrmex rugosus expostas ao i.a. azadiractina. Para tanto foi avaliado o carregamento de iscas de A. indica e a potencial atratividade destas para o forrageamento; posteriormente foi feita a avaliação da mortalidade de operárias às quais foram oferecidos extratos hexânicos de A. indica. As iscas de A. indica mostraramsemais atrativas para as operárias de Acromyrmex rugosus em relação a polpa cítrica já comercialmente utilizada porém a altamortalidade das operárias em um curto espaço de tempo descaracteriza a A. indica para utilização como formicida.

Avaliação dos efeitos tóxicos de extrato hexânico de Azadirachta indica (A. Juss) em colônias de Acromyrmex rugosus (Smith, 1858) (Formicidae, Attini)

O presente estudo teve por finalidade investigar o uso da planta Azadirachta indica como potencial agente natural no controle de formigas cortadeiras bem como verificar alterações comportamentais de operárias de Acromyrmex rugosus expostas ao i.a. azadiractina. Para tanto foi avaliado o carregamento de iscas de A. indica e a potencial atratividade destas para o forrageamento; posteriormente foi feita a avaliação da mortalidade de operárias às quais foram oferecidos extratos hexânicos de A. indica. As iscas de A. indica mostraramsemais atrativas para as operárias de Acromyrmex rugosus em relação a polpa cítrica já comercialmente utilizada porém a altamortalidade das operárias em um curto espaço de tempo descaracteriza a A. indica para utilização como formicida.