1. Mobile organisms such as emergent aquatic insects can subsidise land with aquatic nutrients, creating a link between terrestrial and aquatic ecosystems.
2. Deposition of aquatic insects on land produces bottom-up effects in arthropod detritivore communities and may also affect plants and plant–herbivore interactions.
3. To investigate the effects of insect deposition on plant–herbivore interactions, we conducted a field experiment and surveys of tealeaf willow (Salicaceae; Salix phylicifolia Coste) and July highflyer caterpillars (Geometridae; Hydriomena furcata Thunberg) at lakes in Northeast Iceland with either high- or low-midge density and deposition to land.
4. It was found that willow at high-midge lakes had 8–11% higher nitrogen content compared with willow at low-midge lakes. In addition, natural caterpillar density was 4–6 times higher and caterpillars were 72% heavier at high-midge lakes than low-midge lakes.
5. A fully reciprocal caterpillar transplant experiment among willow at high- and low-midge lakes was performed to separate the influence of habitat and midge effects on caterpillar performance.
6. After transplant, pupae of July Highflyer caterpillars were on average 11% heavier at high-midge sites compared with low-midge sites. However, this difference was not statistically significant.
7. The present findings indicate that cross-ecosystem subsidies in the form of aquatic insects can increase plant foliar quality and the abundance of insect herbivores in recipient ecosystems.