Few empirical studies have examined the importance of plant quality in complex tropical food webs where species diversity is high and top-down forces are strong.  For example, in coffee agroecosystems, interactions between species, such as predation and mutualism, dominate and structure insect food webs.  Here, we investigate whether coffee plant quality influences herbivores and their ant mutualists.  In a field survey, we compared plant nutritional and defensive traits across coffee bushes that substantially varied in the density of Coccus viridis (honeydew-producing scale insects), but not distance to nest sites of their ant mutualists (Azteca instabilis).  In additional experiments, we introduced coffee seedlings grown under three fertilization treatments (high, intermediate, and low) to coffee fields and examined changes in plant quality, C. viridis population growth, and recruitment by ant mutualists.  
Results/Conclusions

In areas with A. instabilis ant nests, coffee bushes with high density of C. viridis had a higher percent nitrogen and calcium than coffee bushes with low scale density.  At the leaf and phloem level, caffeine secondary metabolites did not vary across coffee bushes with high and low C. viridis densities.  Step-wise multiple regressions revealed variation in A. instabilis activity and percent nitrogen in host plants explained 45% of the variation in C. viridis density.  Fertilization experiments suggested C. viridis population growth increased with fertilization.  Further, responses of scale-tending ants to fertilization varied with species.  Ant tending of scale insects by Pheidole sp. increased overall and per scale insect on high fertilized seedlings relative to low.  However, A. instabilis tending of scales did not vary with seedling fertilization treatment.  These results suggest host plant quality may affect the spatial distribution and growth of C. viridis at local levels and may mediate ant-scale mutualisms for some ant species in the agroecosystem.  Variation in ant recruitment caused by plant quality may have important consequences for C. viridis fitness if parasitization and predation of scales decline with increasing ant attendance.