Reduction in the foraging activity of the leaf-cutting ant Atta sexdens caused by the phorid Neodohrniphora sp.

Females of the parasitic phorid Neodohrniphora sp. were collected in the he ld and released singly inside an observation chamber placed between a labor atory colony of Atta sexdens (L.) and its foraging arena. The number and sp eed of loaded and unloaded ants returning to the nest, the weight of forage rs and their loads, the number of leaf fragments abandoned by ants, and the number of small workers 'hitchhiking' on leaf fragments were measured befo re phorids were released, while they were in the observation chamber, and a fter they were removed. Relatively few ants were attacked by Neodohrniphora sp., but the presence of flies prompted outbound ants to return to the nes t and caused a significant reduction on the number and mass of foragers. Ad ditionally, the weight of leaf fragments transported by ants was reduced an d the number of abandoned fragments increased in response to Neodohrniphora sp. Presence of the parasitoid caused no significant changes in the number of hitchhiking ante. The regular ants' traffic was resumed after phorids w ere removed, but foraging activity remained below normal for up to three ho urs. In the field A. sexdens forages mostly at night, but colonies undergo periods of diurnal foraging during which ants are subject to parasitism fro m several species of phorid flies. Considering that daytime foraging may be necessary for nutritional or metabolical needs, phorids may have a signifi cant impact on their hosts by altering their foraging behavior regardless o f the numerical values of parasitism.