Terrestrial deposition of aquatic insects increases plant quality for insect herbivores and herbivore density

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.