Fatal attraction : do virus-infected cadavers of Spodoptera littoralis larvae attract healthy conspecifics?

Abstract: Baculoviruses manipulate insect larval behaviour by inducing increased activity and by enhancing the movement to the top of the host plant. The Spodoptera littoralis nucleopolyhedrovirus (SpliNPV) may have a large impact on larval populations and is currently used as a control method. Dispersal of NPVs is fairly limited since foliage or infected larvae need to be ingested for transmission. S. littoralis is cannibalistic under high population densities or low food availability conditions. We hypothesized that healthy larvae recognize and are attracted to virus-infected cadavers. Dual choice experiments in petri dish arenas, involving infected and uninfected larval cadaver suspensions revealed no significant differences in neonate larval attraction. In contrast, fourth instar larvae fed significantly more on infected larval cadavers than on cadavers of uninfected larvae. Although mortality of larvae that fed on infected cadavers was increased, high mortality in both groups suggests vertical transmission of the virus in this species. Our results show that that virus-killed cadavers increase attraction and feeding by healthy larvae. Consumption of conspecific cadavers increases viral dissemination. Our results suggest that larvae killed by NPV infection emit chemical cues, which make them more attractive to conspecific larvae, thereby promoting in viral transmission.