Host plant species can influence the fitness of herbivore pathogens: the winter moth and its nucleopolyhedrovirus

Plants can have a significant impact on the fitness and efficacy of natural enemies. These interactions are widespread and suggest that the influences on the population dynamics of insect herbivores cannot be simply divided into bottom up and top down. Several questions remain little studied in this field. Firstly, to what extent can plants affect the interactions between insects and their pathogens? Secondly, what are the effects of variation within natural enemy species on host/enemy/ plant interactions? Finally, if plant/pathogen interactions can occur, do pathogens have increased fitness on the locally abundant food plant of their host? This study explored the influence of three host plant species of the polyphagous winter moth, Operophtera brumata, on infections caused by two geographic isolates of the winter moth nucleopolyhedrovirus (NPV) collected from distinct winter moth habitats. Insects were infected on excised leaf tissue of common oak, Quercus robur, Sitka spruce, Picea sitchenis, and heather, Calluna vulgaris. Parameters fundamental to the basic reproductive rate of the pathogen were estimated: these being infectivity, speed of kill and the yield of virus per insect. Leaf nitrogen and phenolic content were measured as indicators of host plant quality for the three plant species: oak had the highest levels of nitrogen and also the highest levels of phenolic compounds. Heather had higher levels of phenolic compounds than Sitka spruce. Host plant did not affect the infectivity of either isolate but insects that ingested virus on oak foliage died sooner and yielded more virus than insects that ingested virus on Sitka spruce or heather. The effect of host plant species on pathogen yield varied between the two isolates of the NPV but not as predicted by our adaptive hypothesis. The interactions between virus and food plant are discussed in relation to host and pathogen population dynamics.