1. Larvae of Macromia illinoiensis Walsh are often colonised by the zebra mussel, Dreissena polymorpha Pallas, a recent invader to North America. To determine how mussel attachment affects an individual's foraging behaviour, we quantified capture of Hexagenia limbata Hexes mayfly prey and the distance moved by newly-molted final instars before and after an individual's colonisation with zebra mussels.
2. In night trials, larvae sprawled above the sand, and caught more mayflies than individuals in daytime trials, but the estimated distance travelled did not differ. When resting under a layer of sand with only its eyes exposed during the day, an individual could capture a mayfly prey using a sit-and-wait ambush strategy. When sprawled above the sand, some larvae caught prey that rested on their legs.
3. When mussel-free, individuals captured more prey than they did when carrying zebra mussels, although mussel attachment per se did not affect the estimated distance that a larva moved.
4. During day trials, but not night ones, the increasing mussel load of colonised individuals decreased prey capture and the distance moved in an apparent step-wise function. Although the number of mussels carried did not differ, night foragers carried a heavier load. Independent of time of the day, the distance an individual travelled when mussel-free was predictive of the number of prey it caught when colonised, suggesting that the greater general activity of some individuals helped mitigate negative effects that mussel attachment had on prey capture.
5. Our results add to a growing number of negative effects of zebra mussel colonisation on sprawling and hiding dragonfly larvae. Although the impact of these costs on dragonfly populations remains to be determined, a decrease in this guild of predators whose life cycle spans aquatic and terrestrial habitats might have cascading effects across ecosystems.