Complex interactions between a plant pathogen and insect parasitoid via the shared vector-host: consequences for host plant infection.

Plant viruses modify the development of their aphid vectors by inducing physiological changes in the shared host plant. The performance of hymenopterous parasitoids exploiting these aphids can also be modified by the presence of the plant pathogen. We used laboratory and glasshouse microcosms containing beans (Vicia faba) as the host plant to examine the interactions between a plant virus (pea enation mosaic virus; PEMV) and a hymenopterous parasitoid (Aphidius ervi) that share the aphid vector/host Acyrthosiphon pisum. Neither PEMV-infection of V. faba, nor the carriage of PEMV virions by A. pisum, affected the growth or morphology of the aphid, or the oviposition behaviour and development of A. ervi. The presence of developing Aphidius ervi larvae within Acyrthosiphon pisum did not affect the ability of the aphids to transmit PEMV. However, by reducing their longevity, parasitism ultimately decreased the time viruliferous aphids were able to inoculate plants. In terms of virus dispersal, parasitized aphids exhibited more movement around experimental arenas than unparasitized controls, causing a slight increase in the proportion of beans infected with PEMV. Exposure to adult Aphidius ervi caused Acyrthosiphon pisum to rapidly drop off bean plants and disperse to new hosts, resulting in considerably higher plant infection rates (70%) than that seen in control arenas (25%). The results of this investigation demonstrate that when parasitoids are added to a plant-pathogen-vector system, benefits to the host plant due to reduced herbivore infestation must be balanced against the consequences of parasitoid-induced aphid dispersal and a subsequent increase in the level of plant infection.