A powdery mildew infection on a shared host plant affects the dynamics of the Glanville fritillary butterfly populations

While biotrophic fungal pathogens have generally been considered to have negative effects on phytophagous insects sharing the same host plant, very little is known about whether fungal infection may affect the dynamics of natural insect populations. This study was designed to determine the effects of fungal infection by Podosphaera plantaginis, a powdery mildew, of a shared host plant, Plantago lanceolata, on the larvae of the butterfly Melitaea cinxia. Larval responses were assessed in a no-choice feeding assay involving infected and healthy leaves, as well as in a behavioural experiment in which larvae had an opportunity to move among infected and uninfected plants. In the no-choice feeding assay larvae developed more slowly and weighed less at diapause when feeding on fungal-infected than on healthy leaves. In the behavioural experiment larval groups tended to leave the original host plant when it was infected by P. plantaginis. This tendency was associated with splitting of larval groups into smaller subgroups. These effects observed in an experimental setting were also confirmed to act under natural conditions. An analysis of 167 M. cinxia populations showed that over-winter survival of larval groups was 26% lower in host populations infected by the mildew than in non-infected host populations. Smaller, more slowly developing larvae may not be ready to enter diapause at the onset of fall, causing the observed increase in mortality. This is the first study to demonstrate that the negative effects of a biotrophic fungal infection may extend to the dynamics of entire insect populations.