Host-plant mediation in ant-hemipteran mutualisms requires three conditions. First, hemipteran attractiveness to ants should vary with plant quality. Second, ants should preferentially tend those Hemiptera that produce the most nutritious attractant. Third, increased ant attendance based on a richer food reward should have a significant effect on some measure of hemipteran fitness. A field experiment is used to test these conditions. This is the first study to simultaneously test these three conditions, and the first to test the effect of plant flowering status on the ant-derived benefits for a honeydew-producing hemipteran. It is hypothesized that membracids (Guayaquila xiphias) feeding on plants (Didymopanax vinosum) with flowers ingest phloem sap of higher quality (higher sugar concentration), produce a higher-quality honeydew and, as a result, are attended by more ants and are better protected compared to those on plants without flowers. Total nitrogen content of the phloem sap of plants with or without flowers did not differ significantly, whereas the sugar concentration was higher in the sap of plants with flowers. Honeydew sugar concentration, honeydew production, and ant tending levels did not vary significantly with flowering status. Membracid survival increased, and natural enemy abundance decreased when ants were present. Plant flowering status did not affect the ant-derived protection afforded to treehoppers, but plants with flowers accumulated more natural enemies through time than plants without flowers. The results suggest that a trade-off between feeding on a higher-quality food and running increased risk of predation on flowering plants could underlie this ant-hemipteran interaction. Based on the parameters measured in this study it is concluded that host-plant mediation does not occur in the ant-Guayaquila system. The results suggest, however, that the way in which the host-plant could affect ant-Guayaquila interactions is complex and likely to involve other species.