Leaf-cutting Ant Workers Research Articles

Examination of the immune responses of males and workers of the leaf-cutting ant Acromyrmex echinatior and the effect of infection

Parasites represent significant challenges to social insects. The high density, interaction rate and relatedness of individuals within colonies are all predicted to make social insect colonies particularly vulnerable to parasites. To cope with this pressure, social insects have evolved a number of defence mechanisms. These include the immune response, which, aside from in bumblebees, has been relatively little studied in social insects. Here we compare the immune responses of males and workers of the leaf-cutting ant Acromyrmex echinatior and examine the effect upon immunocompetence of prior exposure to a virulent parasite. Males have a far lower immune response than workers, suggesting either haploid susceptibility or reduced investment in immunity by the short-lived males. There was also significantly less variation in the immune response of males than of workers, which may be due to leaf-cutting ant workers being more variable in age or more genetically diverse within colonies. When exposed to the entomopathogenic fungus Metarhizium, workers expressed a substantially reduced immune response 96 h after infection, suggesting that the immune system was either depleted by having to respond to the Metarhizium infection or was depressed by the parasite. The results suggest that the immune response is a costly and limited process, but further experiments are needed to distinguish between the alternative explanations for the effects observed.

A Macroglomerulus in the Antennal Lobe of Leaf-cutting Ant Workers and its Possible Functional Significance

Ants have a well-developed olfactory system, and pheromone communication is essential for regulating social life within their colonies. We compared the organization of primary olfactory centers (antennal lobes, ALs) in the brain of two closely related species of leaf-cutting ants (Atta vollenweideri, Atta sexdens). Both species express a striking size polymorphism associated with polyethism. We discovered that the ALs of large workers contain a substantially enlarged glomerulus (macroglomerulus, MG) at the entrance of the antennal nerve. This is the first description of an MG in non-sexual individuals of an insect. The location of the MG is laterally reversed in the two species, and workers of different size express a disproportional allometry of glomerular volumes. While ALs of large workers contain an MG, glomeruli in small workers are all similar in size. We further compared electroantennogram (EAG) responses to two common trail pheromone components of leaf-cutting ants: 4-methylpyrrol-2-carboxylate and 2-ethyl-3,6-dimethylpyrazine. At high concentrations the ratio of the EAG signals to 2-ethyl-3,6-dimethylpyrazine versus 4-methylpyrrol-2-carboxylate was significantly smaller in A. vollenweideri compared with the ratio of EAG signals to the same two components in A. sexdens. The differences in EAG signals and the species specific MG location in large workers provide correlative evidence that the MG may be involved in the detection of the trail pheromone.

Use of stridulation in foraging leaf-cutting ants: mechanical support during cutting or short-range recruitment signal?

Foraging leaf-cutting ant workers stridulate while cutting a leaf fragment. Two effects of stridulation have recently been identified: (i) attraction of nestmates to the cutting site, employing substrate-borne stridulatory vibrations as short-range recruitment signals, and (ii) mechanical facilitation of the cut via a vibratome-effect. We asked whether foragers actually stridulate to support their cutting behavior, or whether the mechanical facilitation is an epiphenomenon correlated with the use of stridulation as recruitment signal. To differentiate between the two alternatives, workers of two different Atta species were presented with tender leaves of invariant physical traits, and their motivation to initiate recruitment was manipulated by varying the palatability of the leaves and the starvation of the colony. The lower the palatability of the harvested leaves, the lower the percentage of workers that stridulated while cutting, irrespective of the leaf’s physical features. After intense feeding, no workers were observed to stridulate while cutting tender leaves, and the percentage of stridulating workers increased with deprivation time. The results support the hypothesis that leaf-cutting ant workers stridulate during cutting in order to recruit nestmates, and that the observed mechanical facilitation of stridulation is an epiphenomenon of recruitment communication.

The role of leaf‐cutting ant workers (Hymenoptera: Formicidae) in fungus garden maintenance

Abstract. We studied the role of leaf‐cutting ant workers (Atta sexdens (L.) in fungus garden maintenance, by temporarily excluding workers from the garden. This increased its subsequent attractiveness, as expressed by an increase in the numbers of workers licking it. The length of free mycelia on areas of the garden from which workers were excluded increased but was reduced again when workers were returned. Workers therefore removed hyphae from the garden surface. The maximum‘isolation effect’was obtained by preventing ant access for 2–3 days, after which the effect declined. Removing staphylae from portions of garden kept ant‐free for 4 and 6 days restored the effect, as the ants were not distracted by harvesting staphylae. Portions of garden kept ant‐free for longer than this were no more attractive than non‐isolated control garden. Workers were highly efficient in detecting and removing contaminants from their fungus garden. Samples of garden could be isolated from workers for up to 12 days before major growth of contaminants occurred, and this contrasted with the maximum of 6 days for the isolation effect on licking. The isolation effect was therefore not a response to contaminant growths on the garden. Workers on the garden surface may remove hyphae for nutritional reasons, or to‘prune’their fungus and stimulate its growth. In either case, the result is a regulation of fungal growth.