The influence of nutrient availability on an insect food web in a tropical coffee agroecosystem

In temperate systems bottom-up forces such as, nutrient availability and plant quality (nutrition and defense) strongly effect trophic interactions between insects feeding on plants, their enemies, and their mutualists. However, little is known about how bottom-up effects influence complex insect food webs in tropical systems, such as coffee agroecosystems. We conducted lab and field experiments to determine how changes in nutrient availability affect (a) coffee growth rate, (b) caffeine (secondary metabolite) concentration in coffee leaves and phloem, (c) growth rate of a coffee pest, the green scale (Coccus viridis), (d) scale attendance by an ant mutualist (Azteca instabilis), and (e) abundance of a coccinellid scale predator Azya orbigera. In growth chambers, we manipulated plant nitrogen availability with Hoagland’s solutions (0, 0.5, 3 mmol L^-1 NO₃NH₄). In the field, at an organic coffee plantation in Chiapas, Mexico, we manipulated the ratio of organic compost (0, 25, 75%) available to potted coffee seedlings placed near A. instabilis nests. Increased nitrogen/nutrient inputs stimulated coffee growth and increased the concentration of caffeine in phloem extracts, but not leaf tissues. In the field, high nutrient plants had greater scale populations and a tendency for higher A. instabilis attendance of scales. The abundance of A. orbigera larvae on experimental plants did not differ across treatments. These results indicate an importance of nutrient availability on plant quality and insect interactions for a tropical agroecosystem. We propose different effects of the nutritional and defense characteristics of coffee host plants as plausible explanations for the observed patterns.