Açao do difluorobenzon no potencial reprodutivo e no processo da muda larval de Spodoptera frugiperda (Smith & Abbot, 1987) (Lep., Noctuidae)

Abstract

Resumo: O objetivo desta pesquisa foi investigar o efeito do difluorobenzon no potencial de reprodução dos adultos de Spodoptera frugiperda resultantes de larvas que foram alimentadas, durante 48 horas, com folhas de milho tratadas com esse produto, a concentração de 1 ppm, e estudar quais as zonas do tegumento das larvas que foram afetadas e quais as estruturas que foram alteradas, por meio de cortes histológicos, na área de microscopia óptica e ultra-estrutural. O tratamento das larvas provoca uma mortalidade maior do que a da testemunha, durante o subsequente desenvolvimento dessa espécie; que foi tanto maior quanto mais jovens eram as larvas tratadas. No concernente aos aspectos relacionados com a reprodução, constatou-se que o tratamento com o difluorobenzon afetou mais as larvas que deram origem a machos do que as larvas que deram origem a fêmeas. Através da microscopia óptica constatou-se que a espessura da endocutícula nas larvas tratadas era menor do que nas larvas normais. Nos estudos ultra-estruturais, verificou-se uma antecipação na muda das larvas tratadas, em todos os instares estudados (4º, 5º e 6º), com relação a testemunha. Nas larvas normais do 4° e do 5º instares esse processo começou às 48 horas, e nas larvas do 6º instar às 40 horas ainda não se observara qualquer alteração; o processo completou-se às 9º horas, nos três instares. Nas larvas tratadas no 4º instar o processo começou antes das 48 horas; nas larvas tratadas no 5º instar esse processo iniciou-se às 24 horas; nas do 6º instar às 40 horas já se tinham iniciado os processos para a passagem ao estado de pupa. Todo esse processo estava terminado às 72 horas, nos três instares. Em nenhum dos instares testados foi constatada qualquer alteração a nível celular, pois somente se observaram modificações resultantes da mitose celular e do mesmo processo de muda. A epiderme das larvas tratadas no 4º instar apresenta células claras ao lado das escuras, numerosos microvilos de forma regular na região subcuticular, com engrossamentos densos na parte apical e com secreção prematura de cuticulina. A camada de cuticulina e de proteína epicuticular se formou às 60 horas após a muda para o 6º instar larval na larva tratada no 5º instar. Na larva tratada do 6º instar encontrou-se que a parte apical da membrana plasmática apresentou expansões ("membrane blebbing") que não eram mais do que acúmulo de secreção proveniente das vesículas secretoras.

Abstract: The aim of this research was to investigate the effect of difluorobenzon in the reproductive potential of adult Spodoptera frugiperda born from larvae fed for 48 hours with corn leaves treated with the product, at a concentration of 1 ppm, and also to study by means of histological procedures - optical microscopy and ultra-structural analysis - which zone of larval tegument were affected and which structures were altered. The treatment of larvae causes a mortality which is higher in the treated group than in the control group, and the mortality being higher the younger the larvae were at the time of treatment. As regards reproductive aspects was observed that the effect of difluorobenzon was stronger in the group of larvae that gave origin to males than in those that gave origin to females. , Through optical microscopy was observed that those larvae that underwent treatment had a less thick endocuticle than the control ones. By means of ultra-structural studies was observed an earliness in the moulting of treated larvae in all the instar studied, compared to the control group. For the normal larvae in the 4th and 5th instars this process started at 48 hours, and those in the 6th instar at 48 hours were had not yet observed any alteration; the process was complete at 96 hours for all instars. Those larvae treated at the 4th instar the process began before 48 hours, but for those in the 5th instar the process began at 24 hours; for those in the 6th instar, at 40 hours the process for passing to the pupal stage had already started. The whole process was finished at 72 hours for all instars. In no instar tested was observed any alteration at the cellular level; the only modifications observed resulted from cellular mitosis and from the same moulting process. The epidermis of treated larvae in the 4th instar shows clear cells beside dark cells, many regular microvilli in the subcuticular region with dense thickening at the apical part and premature secretion of cuticulin. The cuticulin andepicuticular protein layer were formed at 60 hours after moulting for the 6th larval instar in the larvae treated at the 5th instar. In the larvae treated in the 6th instar was observed expansions at the apical region of plasma membrane (Membrane B1 ebbing) which were no more than deposition of secretion form the secretor vesicles.