Search

Abstract Queens of Atta sexdens Forel (Hymenoptera: Formicidae) face biotic and abiotic environmental factors in the environment while establishing their nests. Biotic factors such as predation, microbial pathogens, successful symbiotic fungus regurgitation, excavation effort and abiotic factors such as radiant sunlight, temperature, density, and soil moisture exert selection pressures on ant queens. Biotic factors such as temperature and solar irradiation affect the survival of the initial colony differently, in different environments in the field. Queens of the leaf-cutting ant A. sexdens, were installed in sunny and shaded conditions to test this hypothesis. Two hundred A. sexdens queens were collected and individualized in two experimental areas (sunny and shaded), each in an experimental area (25 m2) in the center of a square (50 × 50 cm). Temperature, irradiance, nest depth, rainfall and queen mortality were evaluated. Atta sexdens colony development was better in the shaded environment, and the depth and volume of the initial chamber, fungus garden biomass and number of eggs, larvae, pupae and workers were greater. The queen masses were similar in both environments but mortality was higher in the sunny environment. The worse parameter values for A. sexdens nests in the sunny environment are due to the greater solar irradiance, increasing the variation range of the internal temperature of the initial chamber of the nest. On the other hand, the more stable internal temperature of this chamber in the shaded environment, is due to the lower incidence of solar irradiance, which is also more advantageous for queen survival and the formation and development of A. sexdens colonies. Shaded environments are a better micro habitat for nesting A. sexdens than sunny ones.

Abstract Claustral foundation of nests by Atta sexdens Forel (Hymenoptera: Formicidae) involves great effort by its queens, solely responsible for the cultivation of the fungus and care for her offspring at this stage. The minimum workers, after 4 months, open access to the external environment to foraging plants to cultivate the symbiotic fungus, which decomposes the plant fragments and produces gongilidea nodules as food for the individuals in the colony. Colony gas exchange and decomposition of organic matter in underground ant nests generate carbon dioxide (CO2) emitted into the atmosphere. We described the carbon dioxide concentration in colonies in the field. The objective was to evaluate the carbon dioxide concentration in initial A. sexdens colonies, in the field, and their development. The CO2 level was also measured in 4-month-old colonies in the field, using an open respirometric system fitted with an atmospheric air inlet. The CO2 level of the respirometric container was read by introducing a tube into the nest inlet hole and the air sucked by a peristaltic pump into the CO2 meter box. The CO2 concentration in the initial colony was also measured after 4 months of age, when the offspring production (number of eggs, larvae, pupae and adult workers) stabilized. Ten perforations (15 cm deep) was carried out in the adjacent soil, without a nest of ants nearby, to determine the concentration of CO2. The composition of the nests in the field was evaluated after excavating them using a gardening shovel and they were stored in 250 ml pots with 1 cm of moistened plaster at the bottom. The CO2 concentration was higher in field nest than in adjacent soil. The concentration of carbon dioxide in A. sexdens nests in the field is higher than in those in the soil, due to the production of CO2 by the fungus garden and colony.

ABSTRACT Leaf-cutting ants are controlled with toxic baits. For the method’s greater efficiency, the baits must be distributed and processed by workers during fungus cultivation. To test hypotheses whether the mode of action of the active ingredients, which blocks the sodium channels and interrupts the production of ATP in the mitochondria, interferes with fragment distribution in nests; the dispersion of pellets fragments during fungus cultivation by workers was assessed, spatially referencing the fragment onto the fungus garden. Pellets fragments were randomly distributed, but the amount pellets fragments was influenced by the way that the active ingredient acts in the colony.

Cyphoderus innominatus Mills, 1938 (Collembola: Paronellidae) was first observed in early colonies of Atta sexdens leaf-cutting ants (Hymenoptera: Formicidae). The colonies were collected on February 6, 2019, from a transition area between the Atlantic Forest and the Cerrado, located in the municipality of Botucatu, São Paulo, Brazil. A total of four colonies collected had an average population density of 227 ± 212 C. innominatus individuals, and most of the latter were found in peripheral areas inside the fungus-garden-growing chamber of the colony. In addition, we observed a possible defensive behavior on the part of workers when C. innominatus individuals were present in the fungus garden chamber. Thus, this is the first record of C. innominatus living in association with early colonies of A. sexdens.

Leaf-cutting ants are widely distributed in Brazil, particularly species of the genus Atta. We therefore described the occurrence of leaf-cutting and grass-cutting ant species of the genus Atta. Five routes comprising some of the main highways in the South, Southeast, Center-West, and North of Brazil were sampled, in addition to ants received from other 82 municipalities, composing 300 municipalities sampled. This is the first comprehensive study of Atta in Brazil. The following species were found: A. laevigata, A. capiguara, A. sexdens rubropilosa, A. sexdens piriventris, A. sexdens sexdens and A. cephalotes. Atta laevigata and A. capiguara were the species present in the largest number of the Brazilian municipalities sampled. Atta laevigata assumed the position of species of greatest occurrence in the Brazilian territory. Atta sexdens piriventris was only recorded in the southern region of Brazil. Atta bisphaerica presented lower expansion than A. capiguara. Atta cephalotes and Atta opaciceps are species of very restricted occurrence. Southeastern region was characterized by the occurrence of A. capiguara and A. laevigata. Atta laevigata exhibited a generalized pattern of occurrence in the Center-West and North. Our study contributes to a better understanding of the so far unknown occurrence of leaf-cutting and grass-cutting ants within Brazil.

Atta bisphaerica is a species of grass-cutting ants commonly found in the Cerrado biome. The Brazilian Cerrado (savanna) biome covers 2 million km representing 23% of the area of the country. It is an ancient biome with rich biodiversity, estimated at 160,000 species of plants, fungi and animals. However, little is known about their nest architecture development. This study investigated the architecture of fourteen A. bisphaerica nests from Botucatu, São Paulo, Brazil. Molds were made of the nests by filling them with cement to allow better visualization of internal structures such as chambers and tunnels. After excavation, the depth and dimensions (length, width, and height) of the chambers were measured. As expected, there was a lateral development in the nests and increase in the number of chambers over time. Results showed that in nests with an estimated age of 14 months, the average depth was 1.6 ± 0.4 m; for those with 18 months it was 2.2 ± 0.7 m and at 28 months it was 2.5 ± 0.7 m. The number of chambers varied from 4 to 7 in 28-month nests, 2 to 4 in 18-month nests, and from 2 to 3 in 14-month nests. With respect to the dimensions of the internal tunnels, there were variations in their average width, increasing with time. The fungus chambers were located beneath the largest mound of loose soil. This study contributes to a better understanding of the so far unknown nest architecture development of A. bisphaerica grass-cutting ants. Keywords: Grass-cutting ants, Nest, Social insects

ABSTRACT Leaf-cutting ants live symbiotically with a fungus that they cultivate on the plant leaves that they cut. The innumerous studies on the plant selection mechanism used by leaf-cutting ants show the researchers’ interest in this issue. Many classical studies propose that plants are selected according to the fungus garden nutritional needs and the absence of potentially harmful substances. This hypothesis is corroborated by behavioral experiments using cycloheximide (fungicide) with citric pulp or forage plants greatly accepted by leaf-cutting ants. According to this hypothesis, under the action of a fungicide, the fungus emits an allomone that informs worker ants that some food is inadequate to its growth. Although some authors state that the cycloheximide “fungicide” used is specific and non toxic to ants, our findings are distinct. In our study, various concentrations of cycloheximide were administered orally to leaf-cutting worker ants in a citric pulp paste diet. After the ingestion period, the ants were isolated and offered the symbiotic fungus for 21 days and the mortality rate was evaluated. As expected, the treatment with 0.01% cycloheximide showed a low mortality rate (8.86%). At 0.1%, the mortality rate was mild (27.85%), and treatment with 1% cycloheximide resulted in moderate mortality (45.57%). In contrast, the positive control with 0.1% sulfluramid showed a high mortality rate (91.14%). Therefore, we concluded that the ingestion of high concentrations of cycloheximide results in a moderate mortality rate in leaf-cutting worker ants.

Leaf-cutting ants live symbiotically with a fungus that they cultivate on the plant leaves that they cut. The innumerous studies on the plant selection mechanism used by leaf-cutting ants show the researchers’ interest in this issue. Many classical studies propose that plants are selected according to the fungus garden nutritional needs and the absence of potentially harmful substances. This hypothesis is corroborated by behavioral experiments using cycloheximide (fungicide) with citric pulp or forage plants greatly accepted by leaf-cutting ants. According to this hypothesis, under the action of a fungicide, the fungus emits an allomone that informs worker ants that some food is inadequate to its growth. Although some authors state that the cycloheximide “fungicide” used is specific and non toxic to ants, our findings are distinct. In our study, various concentrations of cycloheximide were administered orally to leaf-cutting worker ants in a citric pulp paste diet. After the ingestion period, the ants were isolated and offered the symbiotic fungus for 21 days and the mortality rate was evaluated. As expected, the treatment with 0.01% cycloheximide showed a low mortality rate (8.86%). At 0.1%, the mortality rate was mild (27.85%), and treatment with 1% cycloheximide resulted in moderate mortality (45.57%). In contrast, the positive control with 0.1% sulfluramid showed a high mortality rate (91.14%). Therefore, we concluded that the ingestion of high concentrations of cycloheximide results in a moderate mortality rate in leaf-cutting worker ants. Keywords: Leaf-cutting ants, Cycloheximide, Mortality, Formicide screening

The success of incipient colonies of leaf-cutter ants depends on multiple factors such as temperature, humidity, soil type, the queen’s fertility and vigor, frequency of harmful microfungi, among others. Based on this observation, three factors related to Atta sexdens were characterized: (1) initial nest morphology (depth and volume); (2) queen oviposition rate; and (3) prevalence of Escovopsis (parasitic fungus) and other fungi in incipient colonies. Our observation showed a variation in chamber dimensions and volume, indicating differences in colony development rates. The oviposition rate of queens from nests of the same age showed a difference among queens. About microfungi prevalence, it was found that many are potentially deleterious. In particular, the parasite Escovopsis was present in 16.7% of the colonies. Our study shows positive correlations between queen condition and nest size/fungus garden size. These correlations collectively suggest that ecological factors such as plant availability can generate substantial variation between nests and thus shape the success of incipient colonies.

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.

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. Keywords: Leaf-cutting ants, Foraging, Saúva, Social insects

The nest dug by the leaf-cutter ant queen of the genus Atta is a vertical tunnel interconnected to a small chamber to hold its offspring and the symbiotic fungus. The depth of the initial chamber of the Atta sexdens rubropilosa ranges from 10 to 30 cm below the soil surface. This available information, could it be that the ideal initial nest depth is adapted to favorable conditions for the initial colony? We hypothesized this depth can provide a minimum temperature range with almost constant temperature, leading to the development of symbiotic fungus and brood yet to emerge. To test this hypothesis, laboratory experiments were carried out and the soil temperature was measured at different depths. The colony development at different temperatures was studied in the laboratory and the brood production (number of eggs, larvae, pupae and adults) was measured until the first workers emergency. Additionally, lipid content and the survival of queens at different temperatures were determined. Our results show an suitable temperature range (ranging from 24.82±3.14°C to 24.11±1.30°C) at a depth of 5 to 25 cm from the ground, an ideal brood development at temperatures of 24 and 28 °C, and consequently a reduction in lipid content of the queens at high temperatures, without affecting their survival in the trial period. These results indicate that the depth of the initial chamber excavated by the queen is suitable for the success of the incipient colony.

ABSTRACT Atta bisphaerica is a species of grass-cutting ants commonly found in the Cerrado biome. The Brazilian Cerrado (savanna) biome covers 2 million km representing 23% of the area of the country. It is an ancient biome with rich biodiversity, estimated at 160,000 species of plants, fungi and animals. However, little is known about their nest architecture development. This study investigated the architecture of fourteen A. bisphaerica nests from Botucatu, São Paulo, Brazil. Molds were made of the nests by filling them with cement to allow better visualization of internal structures such as chambers and tunnels. After excavation, the depth and dimensions (length, width, and height) of the chambers were measured. As expected, there was a lateral development in the nests and increase in the number of chambers over time. Results showed that in nests with an estimated age of 14 months, the average depth was 1.6 ± 0.4 m; for those with 18 months it was 2.2 ± 0.7 m and at 28 months it was 2.5 ± 0.7 m. The number of chambers varied from 4 to 7 in 28-month nests, 2 to 4 in 18-month nests, and from 2 to 3 in 14-month nests. With respect to the dimensions of the internal tunnels, there were variations in their average width, increasing with time. The fungus chambers were located beneath the largest mound of loose soil. This study contributes to a better understanding of the so far unknown nest architecture development of A. bisphaerica grass-cutting ants.

ABSTRACT Atta capiguara grass-cutting ants are commonly found in the Cerrado biome, in open fields. Although grass-cutting ants build giant nests, little has been elucidated about this building pattern and when chambers and tunnels emerge. The present study describes the nest architecture development of A. capiguara grass-cutting ants from data on 31 cement-molded nests. A. capiguara nests grow with increases in the number of fungus chambers and emergence and increase of waste chambers and foraging tunnels. The structural growth of A. capiguara nests in the first year and a half of age (18 months) is vertical, with the building of the first chambers in the soil profile. After 18 months, the nests grow sideways with the addition of chambers and tunnels, and the first waste chambers appear. Between 18 and 54 months, the number of fungus chambers increases from 1-3 to 21-32, and the chambers are concentrated at the soil surface, although they can be found more than 3 m deep. In addition, the total volume of the waste chambers increases with the increment in the fungus chambers volume. Thus, this study contributes to understanding the nest architecture development of A. capiguara grass-cutting ants and demonstrates that the total volume of waste chambers is proportional to the total volume of fungus chambers suitable for the colony.

The objective of this work was to evaluate the natural attack by Acromyrmex crassispinus in initial Pinus taeda plantations without control measures against ants, as well as the effect of defoliation in seedlings of P. taeda. Evaluations of the attack of leaf-cutting ants on P. taeda plantations were done monthly in the first six months, then 9 and 12 months after planting. The percentages of plants that were naturally attacked by ants were registered. The effect of defoliation was evaluated by artificial defoliation, simulating the natural patterns of attack by A. crassispinus on P. taeda seedlings. The natural attack of A. crassispinus was greater during the first months after planting, being more intense in the first 30 days. Artificial defoliation indicated that there were no significant losses in diameter and height in plants with less than 75% defoliation. However, there were significant losses in diameter and height in plants with 100% defoliation, independently of the cut of the apical meristem, and also plant death. The control of leaf-cutting ants in P. taeda plantings, in which A. crassispinus is the most frequent leaf-cutting ant, should be intense only at the beginning of planting, since the most severe attacks occur during this time.