Costs of cannibalism in the presence of an iridovirus pathogen ofSpodoptera frugiperda

Abstract

Abstract. 1. The costs of cannibalism were examined in larvae of Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) in the presence of conspecifics infected by a lethal invertebrate iridescent virus (IIV). The hypothesis of a positive correlation between insect density and the likelihood of disease transmission by cannibalism was examined in laboratory microcosms and a field experiment.2. Transmission was negligible following peroral infection of early instars with purified virus suspensions or following coprophagy of virus-contaminated faeces excreted by infected insects. In contrast, 92% of the insects that predated infected conspecifics acquired the infection and died prior to adult emergence in the laboratory. Diseased larvae were more likely to be victims of cannibalism than healthy larvae.3. The prevalence of cannibalism was density dependent in laboratory microcosms with a low density (10 healthy insects þ one infected insect) or high density (30 healthy insects þ one infected insect) of insects, and field experiments performed on maize plants infested with one or four healthy insects þ one infected insect.4. Cannibalism in the presence of virus-infected conspecifics was highly costly to S. frugiperda; in all cases, insect survival was reduced by between & 50% (laboratory) and & 30% (field) in the presence of the pathogen. Contrary to expectations, the prevalence of disease was not sensitive to density because cannibalism resulted in self-thinning. As infected individuals are consumed and disappear from the population, the prevalence of disease will be determined by the timescale over which transmission can be achieved, and the rate at which individuals that have acquired an infection become themselves infectious to conspecific predators.

Family Iridoviridae: Poor Viral Relations No Longer

“…Cannibalism or predation of infected individuals appears to be the principal mechanism of transmission in populations of mosquitoes Nielsen 1968a, 1968b), isopods (Federici 1980;Grosholz 1992), tipulids (Carter 1973a(Carter , 1973b, crickets (Fowler 1989), and cannibalistic Lepidoptera (Williams and Hernández 2006). IIV particles can be transmitted from infected to susceptible insects by hymenopteran parasitoids and entomopathogenic nematodes (Mullens et al 1999;Lopez et al 2002).…”

Predatory cannibalism in Drosophila melanogaster larvae

“…The rapid response to artificial selection for reduced incidence of intraspecific predation in Spodoptera caterpillars 23 demonstrates that it is a heritable trait with substantial genetic variation. Although its benefits are unclear, cannibalism in Spodoptera carries a cost in terms of disease transmission 24 . Nonetheless, a comprehensive study of predatory cannibalism has been hindered by its apparent absence in genetically tractable model species.…”

Natural invertebrate hosts of iridoviruses (Iridoviridae)

“…This has led to a lack of interest in the study of these viruses and a resulting paucity of information concerning their biology and survival in invertebrate populations. Indeed, the mechanisms of transmission of most IIVs remain unclear, although cannibalism and wounding have been shown to be viable mechanisms in some species (Carter 1973, Grosholz 1992, Undeen & Fukuda 1994, Marina et al 2005, Williams & Hernández 2006. Nematodes and hymenopteran endoparasitoids can also transmit IIVs by introducing virus particles into susceptible hosts during the act of host penetration or oviposition, respectively (Mullens et al 1999, López et al 2000.…”