Selectivity of Workers of the Grass-Cutting Ants Atta bisphaerica and Atta capiguara (Hymenoptera: Formicidae) to Vegetable Oils

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Dynamics of the restoration of physical trails in the grass-cutting ant Atta capiguara (Hymenoptera, Formicidae)

ABSTRACT Dynamics of the restoration of physical trails in the grass-cutting ant Atta capiguara. Leaf-cutting ants of the genus Atta build long physical trails by cutting the vegetation growing on the soil surface and removing the small objects they find across their path. Little is known on the dynamics of trail construction in these ants. How much time do they need to build a trail? To answer this question we selected six trails belonging to two different nests of A. capiguara and removed on each trail a block of soil of 20 cm × 15 cm that included a portion of the physical trail. This block was then replaced by a new block of the same size that was removed in the pasture near the trail and that was uniformly covered by the same type of vegetation as that found on the block of soil that was removed. The time required to restore the trail was then evaluated by the length of the grass blades found along the former location of the trail. The results show that ants rapidly restore the portion of the physical trail that was interrupted, which suggests that they could also do the same after their trails have been recolonized by the vegetation.

Microscopic evidence supports the hypothesis of high cellulose degradation capacity by the symbiotic fungus of leaf-cutting ants

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Trajectory of Water- and Fat-Soluble Dyes in the Grass-Cutting ant Atta capiguara (Hymenoptera, Formicidae): Evaluation of Infrabuccal Cavity, Post-Pharyngeal Glands and Gaster

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Efficiency and soil contamination during underground application of insecticides: control of leaf-cutting ants with thermal foggers

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Energy substrate used by workers of leaf-cutting ants during nest excavation

Energy substrate used by workers of leaf-cutting ants during nest excavation. In this study we aimed to ascertain whether leaf-cutting ant workers lose body reserves (fat or sugars) as a function of nest excavation. For each treatment, we isolated 10 workers of Atta sexdens into two experimental groups, Control (C- without excavation) and Soil (S- with excavation), which were kept for different time intervals (0, 24, 48 or 72 hours), totaling 700 tested workers. We then determined the concentration of soluble carbohydrates and total lipid content in them. The total carbohydrates were determined colorimetrically, based on the reaction between carbohydrates and sulfuric acid-phenol. For determination of lipids, the insects were immersed in organic solvent until they reached a constant weight. Our results showed that carbohydrates are consumed during nest excavation activities. In the experimental groups S24, S48 and S72, there was an average reduction of 5.82 (20.42%), 14.31 (44.96%) and 13.27 (43.96%) µ.mg-1 in soluble sugar when compared with the experimental groups that did not excavate. Furthermore, the lipids were not used during this activity. With respect to dry mass of the workers, their values were C0 = 8%, C24 = 10.4%, C48 = 9.2%, C72 = 10%, S24 = 9.2%, S48 = 8.7% and S72 = 8.5%. Our results show experimentally that the source of energy for nest excavation is carbohydrates, whereas lipids are conserved for other activities.

Preparation of mitotic chromosomes of leaf-cutting ants from the genera Atta and Acromyrmex

Some modifications were made to the methodology of Imai et al. (Jpn. J. Genet. 63: 159-185, 1988) for cytogenetic analysis of the leaf-cutting ants Atta sexdens piriventris and Acromyrmex heyeri (Hymenoptera, Formicidae), shortening preparation time and improving chromosomal preparations. The brain ganglia of prepupae were dissected in a 0.0025% hypotonic solution of colchicine, placed on a glass slide on a cold plate (4 ± 1oC) for 20 min. The material was fixed directly on the cold slide (with cold fixative I), macerated with a histological needle and fixed again with fixative I, followed by fixatives II and III, all of them cold. The slide was flame-dried right after the use of fixative III, and it was allowed to air-dry at room temperature for 2 h. The resulting metaphases presented less contracted chromosomes, with separated and well defined sister chromatids at a high frequency, when the material was processed in the manner described and stained with 3% Giemsa in phosphate buffer (pH 6.8) for 15 min.

Low variation in ribosomal DNA and internal transcribed spacers of the symbiotic fungi of leaf-cutting ants (Attini: Formicidae)

Leaf-cutting ants of the genera Atta and Acromyrmex (tribe Attini) are symbiotic with basidiomycete fungi of the genus Leucoagaricus (tribe Leucocoprineae), which they cultivate on vegetable matter inside their nests. We determined the variation of the 28S, 18S, and 5.8S ribosomal DNA (rDNA) gene loci and the rapidly evolving internal transcribed spacers 1 and 2 (ITS1 and ITS2) of 15 sympatric and allopatric fungi associated with colonies of 11 species of leafcutter ants living up to 2,600 km apart in Brazil. We found that the fungal rDNA and ITS sequences from different species of ants were identical (or nearly identical) to each other, whereas 10 GenBank Leucoagaricus species showed higher ITS variation. Our findings suggest that Atta and Acromyrmex leafcutters living in geographic sites that are very distant from each other cultivate a single fungal species made up of closely related lineages of Leucoagaricus gongylophorus. We discuss the strikingly high similarity in the ITS1 and ITS2 regions of the Atta and Acromyrmex symbiotic L. gongylophorus studied by us, in contrast to the lower similarity displayed by their non-symbiotic counterparts. We suggest that the similarity of our L. gongylophorus isolates is an indication of the recent association of the fungus with these ants, and propose that both the intense lateral transmission of fungal material within leafcutter nests and the selection of more adapted fungal strains are involved in the homogenization of the symbiotic fungal stock.

Temporal patterns of nutrient availability around nests of leaf-cutting ants (Atta colombica) in secondary moist tropical forest

Leaf-cutting ants consume up to 10% of canopy leaves in the foraging area of their colony and therefore represent a key perturbation in the nutrient cycle of tropical forests. We used a chronosequence of nest sites on Barro, Colorado Island, Panama, to assess the influence of leaf-cutting ants (Atta colombica) on nutrient availability in a neotropical rainforest. Twelve nest sites were sampled, including active nests, recently abandoned nests (<1 year) and long-abandoned nests (>1 year). Waste material discarded by the ants down-slope from the nests contained large concentrations of nitrogen and phosphorus in both total and soluble forms, but decomposed within one year after the nests were abandoned. Despite this, soil under the waste material contained high concentrations of nitrate and ammonium that persisted after the disappearance of the waste, although soluble phosphate returned to background concentrations within one year of nest abandonment. Fine roots were more abundant in soil under waste than control soils up to one year after nest abandonment, but were not significantly different for older sites. In contrast to the waste dumps, soil above the underground nest chambers consistently contained lower nutrient concentrations than control soils, although this was not statistically significant. We conclude that the 'islands of fertility' created by leaf-cutting ants provide a nutritional benefit to nearby plants for less than one year after nest abandonment in the moist tropical environment of Barro Colorado Island. Published by Elsevier Ltd.

Effects of leaves of Ipomoea batatas (Convolvulaceae) on nest development and on respiratory metabolism of leaf-cutting ants Atta sexdens L. (Hym., Formicidae)

Laboratory colonies of the leaf-cutting ants Atta sexdens feed daily with leaves of Ipomoea batatas showed ant mortality and a significant decrease in the size of the fungal garden after the second week, with complete depletion of nests after 5 weeks of treatment. The mean oxygen consumption rate of these ants was higher than the control (ants collected from nests feed with leaves of Eucalyptus alba), suggesting a physiological action of the leaves of I. batatas on the ants in addition to the effect of inhibiting the growth of the fungal garden.

Starch metabolism in Leucoagaricus gongylophorus, the symbiotic fungus of leaf-cutting ants

Leucoagaricus gongylophorus, the symbiotic fungus of the leaf-cutting ants, degrades starch, this degradation being supposed to occur in the plant material which leafcutters forage to the nests, generating most of the glucose which the ants utilize for food. In the present investigation, we show that laboratory cultures of L. gongylophorus produce extracellular alpha-amylase and maltase which degrade starch to glucose, reinforcing that the ants can obtain glucose from starch through the symbiotic fungus. Glucose was found to repress a-amylase and, more severely, maltase activity, thus repressing starch degradation by L. gongylophorus, so that we hypothesize that: (1) glucose down-regulation of starch degradation also occurs in the Atta sexdens fungus garden; (2) glucose consumption from the fungus garden by A. sexdens stimutates degradation of starch from plant material by L. gongylophorus, which may represent a mechanism by which Leafcutters can control enzyme production by the symbiotic fungus. Since glucose is found in the fungus garden inside the nests, down-regulation of starch degradation by glucose is supposed to occur in the nest and play a part in the control of fungal enzyme production by leafcutters. (c) 2005 Elsevier GmbH. All rights reserved.

Substrate Preparation Behaviors for the Cultivation of the Symbiotic Fungus in Leaf-Cutting Ants of the Genus Atta (Hymenoptera: Formicidae)

Our findings revealed two distinct patterns of substrate preparation: the pattern of leaf-cutting ants foraging on dicotyledons is marked by highly fragmented substrate resulting in a more advanced initial decomposition. The pattern of leaf-cutting ants harvesting grasses is characterized by large pieces of substrate, resulting in little initial decomposition. Ants foraging on both types of plants are apparently intermediary between the two patterns, although more similar to the patterns of those foraging on dicotyledons. Also, the behavior of scraping the substrate was described for the first time, it is very important for the removal of the epicuticular wax layer of the leaves helping the growth of the symbiotic fungus.

Low variation in ribosomal DNA and internal transcribed spacers of the symbiotic fungi of leaf-cutting ants (Attini: Formicidae)

Leaf-cutting ants of the genera Atta and Acromyrmex (tribe Attini) are symbiotic with basidiomycete fungi of the genus Leucoagaricus (tribe Leucocoprineae), which they cultivate on vegetable matter inside their nests. We determined the variation of the 28S, 18S, and 5.8S ribosomal DNA (rDNA) gene loci and the rapidly evolving internal transcribed spacers 1 and 2 (ITS1 and ITS2) of 15 sympatric and allopatric fungi associated with colonies of 11 species of leafcutter ants living up to 2,600 km apart in Brazil. We found that the fungal rDNA and ITS sequences from different species of ants were identical (or nearly identical) to each other, whereas 10 GenBank Leucoagaricus species showed higher ITS variation. Our findings suggest that Atta and Acromyrmex leafcutters living in geographic sites that are very distant from each other cultivate a single fungal species made up of closely related lineages of Leucoagaricus gongylophorus. We discuss the strikingly high similarity in the ITS1 and ITS2 regions of the Atta and Acromyrmex symbiotic L. gongylophorus studied by us, in contrast to the lower similarity displayed by their non-symbiotic counterparts. We suggest that the similarity of our L. gongylophorus isolates is an indication of the recent association of the fungus with these ants, and propose that both the intense lateral transmission of fungal material within leafcutter nests and the selection of more adapted fungal strains are involved in the homogenization of the symbiotic fungal stock.

Evaluation of the adequacy of diflubenzuron and dechlorane in toxic baits for leaf-cutting ants (Hymenoptera : Formicidae) based on formicidal activity

Studies assessing the characteristics of active ingredients (AIs) of toxic baits for leaf-cutting ants are still scarce, although the need for a delayed action of these compounds on adult workers has been well accepted (mortality <= 15% at 24 h and <= 90% at 21 days). Therefore, we determined the insecticidal action of two AIs used in commercial baits, diflubenzuron and dechlorane, over time in workers, and discussed the control of colonies in relation to the existing literature. Dechlorane presented excellent insecticidal activity with a delayed action at all concentrations tested, although its commercial use has been prohibited due to its organochlorine nature. In contrast, diflubenzuron did not cause significant mortality or symptoms of intoxication, indicating that the successful cases reported with the use of baits containing this AI were in fact due to an accidental contamination with dechlorane. We comment about the ineffectiveness of diflubenzuron on alternative targets, i.e., young forms and the mutualistic fungus, supporting the concept that the AI needs to a have a delayed action on adult workers.