Caracterização da tolerância de diferentes propágulos de Metarhizium spp. à radiação UV-B e avaliação da virulência de conídios expostos contra larvas de Rhipicephalus microplus

Abstract

Metarhizium anisopliae sl. is one of the most commonly used entomopathogenic fungi in the control of agricultural pests and has also shown promising results in tick tests, especially Rhipicephalus microplus. However, these agents may suffer from abiotic factors, such as high temperatures, fluctuations of humidity and UV-B radiation. In this way, the present work analyzed ten native isolates of Metarhizium spp., with the aim of: 1) verify the tolerance to the UV-B of the isolates, being in aqueous suspensions or oil-in-water emulsions; 2) to verify the tolerance to UV-B of the different propagules of these isolates; 3) to verify the viability of conidia in the soil after UV-B radiation; 4) to evaluate the mortality of R. microplus larvae after exposure of fungi to UV-B. The propagules (conidia, blastospores and microsclerotia) were exposed to UV-B radiation with a total dose of 4.0 kJ m-2 . Conidia suspended in water or oil-water emulsions were evaluated for germination 24 h and 48 h after exposure to UV-B. Conidia adsorbed on different soil types were evaluated for the presence of colony forming units (CFU) after seven days. Blastospores and microsclerotia were evaluated for the presence of CFUs (colony forming units) 72 h and 6 days after exposure to UV-B, respectively. We did not observe a pattern in the tolerance of the different propagules of Metarhizium spp., as the oil did not always protect the conidia of the irradiation. Our results suggest that the different types of soil tested provided UV-B protection to the isolates of Metarhizium spp., except for LCMS05, when adsorbed on soil type I. As for blastospores, LCMS05 was the only isolate that obtained moderate tolerance to irradiation (63.2% germination). Three isolates (LCMS05, LCMS08 and LCMS10) were more tolerant to UV-B when presented in the form of microsclerotia, with a CFC rate above 85%, but only the LCMS10 isolate was statistically equal to the non-exposed control, reaching the same number of CFUs. In the bioassay to verify the mortality of R. microplus larvae after exposure of the fungi to UV-B irradiation, the tested isolates (LCMS03 and LCMS08), although not showing statistical differences each other, obtained good results and showed potential to control R. microplus larvae. The data on UV-B tolerance of the same fungal isolate observed for different propagules, or the same fungal propagule exposed to UV-B in different circumstances, reveal important information not only on the relevance of the intrinsic tolerance of each isolate, but also different propagules of the same fungus. As far as we know, this is the first work analyzing the tolerance to UV-B of different propagules of the same fungal isolate. This study aims to support future research on the discovery of promising fungal isolates and propagules for biological control.