Impact of extrafloral nectar availability and plant genotype on ant (Hymenoptera: Formicidae) visitation to quaking aspen (Salicaceae) Jonathon R. Newman, Diane Wagner, 1 Patricia Doak Abstract—For quaking aspen (Populus tremuloides Michaux; Salicaceae) the rate of extrafloral (EF) sugar secretion is increased by defoliation and decreased by drought. Although wholesale blocking of EF nectar has been shown to reduce ant (Hymenoptera: Formicidae) visitation to aspen, the effect of more subtle and realistic variations in nectar availability on ant recruitment is unknown. Working in Alaskan boreal forest (United States of America), we reduced and supplemented EF nectar availability on potted aspen ramets of three genotypes and surveyed visitation by free-living Formica fusca (Linnaeus) (Hymenoptera: Formicidae). Ants were more responsive to a subtle increase in sugar availability than to a decrease. While nectar reduction had no effect on ant visitation, nectar supplementation increased ant visitation to one aspen genotype by 70% during an early summer trial. Average ant visitation to different aspen genotypes varied during the late summer, indicating that aspen genotype can influence attractiveness to ants. We conclude that natural induction of EF secretion in response to herbivory may benefit aspen through improved ant recruitment, though the response is dependent on aspen genotype and time of year. Differences among aspen genets in attractiveness to ants could influence the relative success of genotypes, especially in settings in which aspen regenerates from seed. Introduction Extrafloral (EF) nectar mediates defensive mutualisms in many plant populations (Koptur 2005). Meta-analyses support that, in general, ant visitation to plants expressing EF nectaries bene- fits plants by reducing herbivory (Chamberlain and Holland 2009; Rosumek et al. 2009: Trager et al. 2010). The quantity and quality of food rewards provided by the plant can affect the strength of defensive mutualisms. For example, ant visitation was positively associated with EF nectar availability across several experimental studies (e.g., Rudgers 2004; Rudgers and Strauss 2004; Kost and Heil 2005; Chamberlain and Holland 2008), and an increase in EF nectar secre- tion was associated with reduced leaf damage and increased fruit set by lima bean (Phaseolus lunatus Linnaeus; Fabaceae) (Heil 2004). Variation in the rate of EF nectar secretion occurs in response to environmental cues. Defoliation, application of jasmonic acid, and exposure to volatile organic compounds from damaged neighbours have been shown to increase the rate of EF nectar secretion across a broad range of EF nectary-bearing plant species (Heil 2011), and drought stress has been shown to reduce EF nectar secretion rate in a smaller number of species (Yamawo et al. 2012; Newman and Wagner 2013). Quaking aspen (Populus tremuloides Michaux; Salicaceae) expresses EF nectaries on a subset of its leaves (Doak et al. 2007). Extrafloral nectaries appear to function as an indirect defence against herbivory for boreal aspen, as EF nectary expres- sion by ramets is negatively related to damage by the leaf miner Phyllocnistis populiella Chambers (Lepidoptera: Gracillariidae) (Mortensen et al. 2011). EF nectar secretion by aspen plants varies in response to environmental stimuli. In a greenhouse experiment, 40% defoliation increased the EF sugar secretion rate by about 25%, whereas drought decreased the secretion rate of some aspen geno- types by a similar magnitude (Newman and Wagner 2013). J.R. Newman, D. Wagner, 1 P. Doak, Institute of Arctic Biology and Department of Biology and Wildlife, University of Alaska Fairbanks, Fairbanks, Alaska 99775, United States of America 1 Corresponding author (e-mail: diane.wagner@alaska.edu). Subject editor: Justin Schmidt doi:10.4039/tce.2015.38 Received 19 August 2014. Accepted 26 February 2015. Can. Entomol. 00:1–7 (2015) © 2015 Entomological Society of Canada 1