A food-for-protection mutualism alters the aggressiveness and predatory behavior of two dominant ant species

Food-for-protection mutualisms between ants and sap-feeding insects can have cascading consequences for arthropod communities by changing ant abundance or behavior. Our previous work in forest arthropod food webs showed that ant predation of caterpillars increased in the presence of sap-feeders. Here we investigated two predatory ant species and the mechanisms by which membracid treehopper mutualists might alter ant predation of caterpillars. In this system, Camponotus ants recruit heavily to sap-feeders, but Formica is the numerically dominant ant in this community.  Therefore, we used Camponotus and Formica ants to investigate two hypotheses for increases in ant predatory behavior in the presence of sap-feeders. First, the nutrient deficit hypothesis predicts that honeydew causes ant workers to increase predatory behaviors as a compensatory response to a carbohydrate-biased diet. Ants exhibiting this behavioral change are expected to increase aggressiveness toward prey, but not competitors. Second, the induced aggression hypothesis predicts that ants consuming honeydew alter their behavior to defend sap-feeder mutualists. Ants exhibiting these behavioral changes are expected to show aggressiveness toward potential competitors, as well as prey. We tested these hypotheses by comparing the behavior of Camponotus and Formica ants found naturally in the presence or absence of treehopper mutualists and also using simulated honeydew. A field behavioral assay tested their responses to prey (waxworm caterpillar) and competitors (conspecific ants and ladybird beetles). Aggressiveness was scored based on the presence or absence of attack behaviors in each trial. Camponotus ants showed more aggressive behaviors than did Formica. We did not find support that sap-feeders induce behavioral changes towards prey or competitors. Instead, sap-feeders appear to be recruiting the aggressive Camponotus chromaoides to host plants, increasing Camponotus abundance, and thus increasing the predatory effect of ants on caterpillars.