Ant‐Herbivore Interactions: Reasons for the Absence of Benefits to a Fern with Foliar Nectaries Vanessa K. Rashbrook

Abstract

We tested the hypothesis that ants, attracted to the foliar nectaries of bracken fern (Pteridium aquilinum) confer protection to the plant in South Africa. Using ant—exclusion experiments with unmanipulated and artificially augmented ant densities, we were unable to detect a positive effect of ants on bracken, as neither the number of herbivores nor the level of herbivory was significantly greater on ant—free fronds. We then examined possible explanations for these results by performing a number of laboratory experiments using the ant Cremtogaster peringueyi and two lepidopteran species, the major bracken—feeding herbivores. We showed that their eggs were susceptible to ant predation, but gained protection from their oviposition sites. We also demonstrated that bracken can benefit when densities of ants are high, such as when fronds are infested with honeydew—producing homopterans, as the lepidopteran larvae were more vulnerable to attack under these conditions. We propose that low ant density on the fronds is the primary factor limiting the occurrence of mutualism between bracken and ants in nature.