Impacto do desastre de Mariana (MG) na seleção de folhas e na escavação de ninhos por Acromyrmex subterraneus (Forel, 1893) (Hymenoptera, Formicidae)

Abstract

The rupture of the mining waste dam in Mariana-MG has brought socioeconomic and environmental damage. A mitigating action for the recovery of soil and biodiversity is the revegetation. One of the main obstacles to revegetation is the presence of leaf-cutting ants, since they cut a large amount of fresh leaves for the cultivation of the symbiotic fungus. On the other hand, leaf-cutting ants should not be recognized only as pest species. Its digging activity interferes with the enrichment of soil, thus contributing to the success of revegetation programs. Digging provides nutrients (by organic matter deposition), prevents erosion, contributes to development of plant roots close to the nest and promotes soil decompression and aeration. Leaf-cutting ant digging has an intrinsic relationship with carbon dioxide (CO2), whose gradients concentration are used as clues of ideal locations and depths for this activity. In this way, leaf-cutting ants play a dual role in the recovery process of areas affected by environmental disasters. Thus, the present study aimed to evaluate the effect of different concentrations of mining waste on the process of (i) leaf selection and (ii) nest digging by Acromyrmex subterraneus. For that, specimens of Bauhinia rufa (pata-de-vaca), were grown in soil containing different concentrations of mining waste (0, 25, 50, 75 and 100%). B. rufa leaves were cut, marked according to each concentration of mining waste concentration and offered in a foraging arena, connected to the colony by a glass bridge. Then, the frequency of transported discs was recorded, until all discs from one of the tested mining waste concentrations were completely transported by workers. To prove the novelty effect, the selection test was performed again 60 days after the first contact with B. rufa. In addition, the leaves were evaluated for thickness, water content, starch concentration and total soluble carbohydrates (TSC). It was observed that discs of plants grown in 100% of mining waste were the most transported by workers, at the first contact and in the tests carried out after 60 days. However, the probability of loading discs with lower concentrations of mining waste was increased, indicating a reduction of preference. Leaves’ plants grown in higher concentrations of mining waste showed less thickness and water content and higher content of starch and TSC. It was also verified the variation of CO2 flow on the soil in loco and in the laboratory using a probe with infrared sensor. Likewise, CO2 flow varies depending on the digging activity carried out by workers. For this, mining waste samples mixed with previously prepared fertile substrate were used to obtain the different concentrations (0, 25, 50, 75 and 100%). An airtight digging chamber (containing soil) with a CO2 probe attached was used. In the first three hours, the CO2 emission from soil was measured, and in the next three hours, CO2 emission from soil was measured with the presence of 30 workers from different colonies (n = 50). The flow of CO2 in loco (Barra Longa - MG) did not vary in relation to the distances to the Gualaxo river. The same was verified in CO2 flow did not vary depending on the amount of soil excavated (g), for any concentration of mining waste, despite the weight of soil excavated by workers of A. subterraneus was lower in soil with 100%. From this results, it is possible that A. subterraneus could prejudice the revegetation project due to herbivory and build nests close to the places affected by the disaster. To ensure the maintenance and success of revegetation projects, it is recommended to monitor and control these leaf cutting ants and also to use fertilizers in the soils affected by the disaster.