Virulência de Nomuraea rileyi (Farlow) Samson e sua relação com a estrutura e composição cuticular de lavar de lepidópteros-praga

Abstract

Nomuraea rileyi is an entomofungus that mainly infects lepidopteran representatives of the Noctuoidea superfamily, which includes key-pests of various crops of large economic importance. Nevertheless, the susceptibility of species in relation to this agent is variable, even among representatives of the same family or genus. One of the factors responsible for this differentiated susceptibility might be the lipid composition of the larval cuticle. The aim of this study was to assess the cuticular structure and composition of lepidopteran larvae and their relationship with the susceptibility to N. rileyi fungi. The species Anticarsia gemmatalis, Mythimna sequax, Rachiplusia nu and Spodoptera cosmioides and two strains of the fungus (CH87551 and UB93150) were used. Second instar larvae were obtained from the mass creation of the Pest Control Laboratory at the University of Caxias do Sul and subjected to suspensions of 107, 108 and 109 conidia/mL for 24 hours and subsequently transferred to 200mL containers containing specific artificial diets. The bioassays were maintained in controlled conditions and assessed daily for 14 days. In order to perform an analysis of the cuticular lipid composition, exuviae derived from the larval-pupal metamorphosis were subjected to extraction with hexane and with dichloromethane-methanol afterwards. For obtaining the M. sequax exuviae, the larvae were fed with Lolium multiflorum. The obtained extracts were analyzed by means of slender layer chromatography and their components were separated using open column chromatography components, whose resulting fractions were analyzed through gas chromatography attached to mass detector (GCMS). Images of scanning electron microscopy (SEM) were obtained from second instar larvae of each species, subjected to a suspension of 108 conidia/mL and analyzed 24, 48 and 72 hours after exposure to the pathogen. The results showed that the assessed species were differently affected by N. rileyi fungi or even by different assessed strains. The LC50 (log dose) obtained with the CH and UB strains was 6,73 and 7,97 for A. gemmatalis and 8,26 and 9,16 for R. nu respectively. S. cosmioides was susceptible only to the UB strain, but the mortality was limited to 11.5% in the highest concentration and M. sequax was not susceptible to any of the assessed strains. The cuticular lipid composition showed qualitative and quantitative differences between the larvae. The main components identified in A. gemmatalis, M. sequax and R. nu were hydrocarbons, accounting for 72,3, 74,5 and 83,6% respectively, with lower diversity of compounds for M. sequax. Linear and branched fatty acids (C14 to C20) have been identified, with different compositions in each one of the species. It was found an aldehyde and a fatty alcohol in A. gemmatalis and other fatty aldehyde in R. nu. In S. cosmioides, the major identified component was vitamin E, with more than 70%. In the SEM images, it was possible to observe conidia of N. rileyi attached to the cuticular surface of the four assessed species, especially in the abdominal region and in pseudopods, even after 48 hours of contact, mainly in S. cosmioides and M. sequax, species in which many observed conidia showed germ tube growing in tandem with the surface. The species that were more likely in these bioassays were the ones that presented, in addition to large amounts of alkanes, the greatest diversities of these compounds. The less susceptible species showed little quantity and/or variety of alkanes or even some antioxidant compound, as in the case of the vitamin E. The obtained results suggest that, besides differences in the lipid composition and cuticular structure of the larvae, the variability of N. rileyi might be related to the differentiated susceptibility observed in species, however, the potential usage of this agent as a tool for holding an integrated pest management is sizeable and needs to be better understood and explored.