Variation in plant resource quality and the transmission and fitness of the winter moth, Operophtera brumata nucleopolyhedrovirus

This study tested the ‘resource quality-pathogen fitness hypothesis’ which proposes that reduced food quality tends to increase susceptibility to pathogens. In addition, we tested whether food quality would affect other fitness parameters, namely viral reproduction and transmission. Larvae grown for a controlled period of time suffered lower mortality (13%) on a high quality plant species oak, Quercus robur, and comparatively high mortality (43%) on a poor quality host, Sitka spruce, Picea sitchensis. Per capita virus reproduction was higher on oak due to the higher yield of virus from larger cadavers, although the rapid growth of larvae on oak would have precluded additional cycles of infection. As older Q. robur foliage is less nutritious for winter moth, we used the within-species variation in bud-burst and foliage quality to test the same hypotheses. In contrast to our first experiment, pathogen reproduction was greatest on plants with the smaller larvae since the greater yield of virus per cadaver from older instars did not compensate for their reduced viral mortality. Budburst date did not affect larval growth rates but did affect viral transmission since viral mortality was increased for smaller larvae on early-flushing trees. We hypothesize that compensatory feeding on older, poorer quality foliage led to higher doses of ingested virus. In summary, reduced plant quality may increase the pathogen fitness in three ways: by increasing the opportunity for additional cycles of infection, by increasing the mortality of slower growing individuals and by increased ingestion rates on poorer quality hosts.