Elucidating the effects of nectar carbohydrate composition on ant behavior

Background/Question/Methods

Many plants produce food rewards to attract ant defenders. The tree cholla cactus (Opuntia imbricata) of the Chihuahuan Desert produces extrafloral nectar, a food resource that is high in total carbohydrate concentration but variable in composition. The cacti are predominantly tended by the ant guards Crematogaster opunitae and Liometopum apiculatum. These two species are found primarily on one of the two cholla life stages, vegetative (nonreproductive) and reproductive, respectively. We used artificial nectaries to investigate the effect of four carbohydrate compositions (relative abundance of monosaccharide vs. disaccharide sugars), spanning O. imbricata’s natural variation, on ant visitation and aggression (measured as discovery and lethality).

Results/Conclusions

Out of five ant species analyzed, three showed significant preference for particular carbohydrate composition. Four of the five exhibited significant differences in aggression, both in discovery and lethality.  L. apiculatum and C. opunitae, the natural visitors, differed in their responses to carbohydrate composition. Aggression also varied significantly among the five species. However, aggression and nectar composition preference were independent of each other. L. apiculatum showed pereference for high discaccharide nectar composition, characteristic of reproductive cholla nectar, while C. opuntia showed preference for a lower proportion of disaccharide nectar composition, associated with vegetative cholla. Our results suggest a correlation between nectar carbohydrate composition preference and cactus life stage preference, which can explain why C. opunitae and L. apiculatum are the dominant ant species found on tree cholla cacti. We conclude that the composition of nectar rewards may be an important determinant of the dominant ant species in ant-plant mutualisms.