Bacillus thuringiensis impacts on primary and secondary baculovirus transmission dynamics in Lepidoptera

Synergistic interactions between entomopathogenic micro-organisms can potentially be exploited to improve biological control of invertebrate pests but empirical data at the population level describing multiple-pathogen transmission dynamics is lacking. We examined how co-inoculation of Bacillus thuringiensis subsp. kurstaki (Btk) and the baculovirus Panolis flammea nucleopolyhedrovirus (PaflNPV) in an experimental field population of Lepidopteran Mamestra brassicae larvae impacted on viral transmission dynamics. We determined how the presence of Btk influenced primary and secondary PaflNPV transmission. When Btk was co-inoculated with PaflNPV, there was increased proportional viral mortality in primary transmission studies compared to plots with virus alone. A delay of up to 4days between applications of Btk and PaflNPV did not impact on primary viral mortality, indicating that a lag between inoculations was unlikely to affect the biocontrol potential of the two pathogens. Viral yields from cadavers in plots with Btk present were significantly lower than those from plots with virus only, and secondary cycling to introduced secondary transmission larvae was significantly reduced. Baculovirus transmission (in terms of the proportion of uninfected larvae in different treatments) was described by a 'refuge' model that allowed for heterogeneity in susceptibility and pathogen exposure. We discuss how transmission may be potentially affected by factors such as host feeding rate, spatial distribution of virus and interactions between pathogens within the insect host. This study improves understanding of the impact of pathogens within host populations and how mixtures of pathogens may be exploited for biocontrol of insect pests.