Ants, aphids, and extrafloral nectaries: a test of the Becerra-Venable hypothesis.

Abstract

Many ecologists have speculated on the evolution and function of extrafloral nectaries in plants. Becerra and Venable have recently proposed that extrafloral nectaries evolved and function as a defense strategy against the potentially highly injurious ant-Homoptera mutualisms. Extrafloral nectar production would lure ants away from tending the homopterans and thus reduce population size of these sap-feeders. From this hypothesis I predicted that sugar solution additions to nectariless plants hosting ant-Homoptera mutualisms would result in a decrease in Homoptera population size. I tested the prediction using the Formica-Aphis-Asclepias system in northern lower Michigan. Sugar additions resulted in a positive, not negative, effect on Homoptera populations, thus contradicting the prediction. Extrafloral nectaries would not be an effective defense against the insect mutualism in the system studied. Alternative explanantions for the results and perspectives for future research are discussed.