The Effects of Aquatic Insect Nutrient Subsidies on Plant Quality and Insect Herbivore Growth

Cross ecosystem nutrient subsidies such as mobile organisms have been shown to directly enrich plants which may indirectly influence herbivores and predators. For example, herbivorous insects may grow larger or faster on plants that have been fertilized by emergent aquatic insects. We conducted a survey of July Highflyer larvae (Geometridae: Hydriomena furcata) on willows (Salicaceae: Salix phylicifolia) at 4 sites near Icelandic lakes with high or low midge (Diptera, Chironomidae) emergence. After accounting for differences between individual plants and sites, caterpillar leaf shelter density was 4 times higher at the high-midge than low-midge sites. We hypothesized that cross-ecosystem subsidies in the form of midge carcasses fertilize nearshore plants at high-midge sites, resulting in larger herbivores. We conducted an experiment, with field and lab components, where caterpillars were transplanted from and reared on willow from the high and low-midge sites. Larval mass did not differ between caterpillars reared in the lab regardless of the origin of caterpillars or willow. In our field experiment, pupae transplanted to willows at high-midge sites were heavier than those at low-midge sites. Our findings indicate that cross-ecosystem subsidies in the form of aquatic insects can influence the prevalence and mass of insect herbivores in recipient ecosystems