Indirect effects of an unspecialized endophytic fungus on specialized plant - herbivorous insect interactions

The effects of Acremonium alternatum Gams (Ascomycotina, Clavicipitacea) on the development and nutrition of diamondback moth larvae Plutella xylostella L. (Lepidoptera, Plutellidae) were studied in the laboratory. All experiments were conducted before the endophyte reached the green parts of the plants; thus P. xylostella, a folivore, was not in direct contact with the endophyte. Larvae feeding on leaves of previously inoculated plants suffered from increased mortality, especially during the first 10 days of development. Likewise, during early development surviving larvae had a reduced relative growth rate (RGR), which, however, did not result in reduced pupal or adult weight. We found sexual differences in the food utilization efficiency; female P. xylostella progeny reacted more sensitively to endophytic infection of cabbage than male larvae. Female larvae feeding on leaves of endophyte-infested plants responded to reduced efficiency of conversion of ingested food (ECI) by increasing their relative consumption rate (RCR). The underlying mechanisms for these results are discussed in relation to changes in plant phytosterol metabolism which could account for reduced larval growth on inoculated cabbage plants. Our data suggest that unspecialized, soil-borne endophytic fungi, even when their association with the host plant is weak, can influence aboveground herbivore development and should be considered when investigating plant-insect interactions.