An examination of stress-related activation of SeMNPV in covertly infected Spodoptera exigua

Abstract: The aim of this study was to evaluate the effect of different stress factors on covertlyinfected Spodoptera exigua larvae in terms of nucleopolyhedrovirus (NPV) activation. For this,adult survivors that had ingested occlusion bodies of S. exigua multiple nucleopolyhedrovirus(SeMNPV) were mated and the subsequent generation (F1) tested for virus activation in thesecond instar in both laboratory and field conditions. In the laboratory, a number of treatmentswere tested including chemical stressors, inoculation with heterologous NPV species and Bacillusthuringiensis spores and crystals. Both, parental and F1 adults were confirmed to harbor theinfection by qPCR. Virus activation was observed in insects treated with 0.1% copper sulphate,1% iron sulphate, and 1 ppm sodium selenite, resuling in 12%, 15%, and 41% mortality due toSeMNPV, respectively, whereas no larvae with symptoms of viral infection were registered invirus-free controls. No effect on NPV-induced mortality was detected after inoculation withheterologous virus. Field trials were carried out by artificial infestation of pepper crops inexperimental greenhouses using sublethally infected S. exigua larvae to evaluate copper sulfateand sodium selenite as activation factors. Very little NPV-induced mortality (< 5%) was observedin those larvae treated in field conditions.