Abstract
Shortly prior to death, many species of Lepidoptera infected with nucleopolyhedrovirus climb upwards on the host plant. This results in improved dissemination of viral occlusion bodies over plant foliage and an increased probability of transmission to healthy conspecific larvae. Following applications of Spodoptera exigua multiple nucleopolyhedrovirus for control of Spodoptera exigua on greenhouse-grown sweet pepper crops, necrophagy was observed by healthy S. exigua larvae that fed on virus-killed conspecifics. We examined whether this risky behavior was induced by olfactory or phagostimulant compounds associated with infected cadavers. Laboratory choice tests and olfactometer studies, involving infected and non-infected cadavers placed on spinach leaf discs, revealed no evidence for greater attraction of healthy larvae to virus-killed over non-infected cadavers. Physical contact or feeding on infected cadavers resulted in a very high incidence of transmission (82–93% lethal disease). Observations on the behavior of S. exigua larvae on pepper plants revealed that infected insects died on the uppermost 10% of foliage and closer to the plant stem than healthy conspecifics of the same stage, which we considered clear evidence of baculovirus-induced climbing behavior. Healthy larvae that subsequently foraged on the plant were more frequently observed closer to the infected than the non-infected cadaver. Healthy larvae also encountered and fed on infected cadavers significantly more frequently and more rapidly than larvae that fed on non-infected cadavers. Intraspecific necrophagy on infected cadavers invariably resulted in virus transmission and death of the necrophagous insect. We conclude that, in addition to improving the dissemination of virus particles over plant foliage, baculovirus-induced climbing behavior increases the incidence of intraspecific necrophagy in S. exigua, which is the most efficient mechanism of transmission of this lethal pathogen.
Highlights
Baculoviruses are dsDNA viruses that infect insects, the larvae of butterflies and moths (Lepidoptera) [1]
Larvae were obtained from a laboratory colony of S. exigua, started in 2012 using larvae originally collected from maize fields close to Monterrey, Mexico
DNA extracted from these occlusion bodies (OBs) and analyzed using the restriction endonucleases BglII and PstI indicated that this strain of Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) was identical in terms of restriction profiles to that of SeMNPV-US2, which is the principal active ingredient of the biological insecticide Spod-X (Certis USA LLC, Columbia, MD)
Summary
Baculoviruses are dsDNA viruses that infect insects, the larvae of butterflies and moths (Lepidoptera) [1]. Larvae usually become infected by feeding on foliage contaminated with virus occlusion bodies (OBs). Prior to death, infected larvae migrate to the top of the plant, where they die in a characteristic form hanging from the pseudopods, a behavior induced by the baculovirus [3]. A virus gene (egt) was identified as being responsible for this behavior in Gypsy moth larvae, Lymantria dispar L. Increased locomotor behavior prior to death is a similar but distinct baculovirus-induced behavior, that is modulated by another virus gene (ptp) in Spodoptera exigua and Bombyx mori [7,8,9]
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