Costs to defenders: Mortality of mutualistic plant ants attacking a specialist herbivore

Background/Question/Methods

Although the net benefits of mutualisms depend on the magnitudes of both benefits and costs to each partner, it is frequently easier to measure benefits than it is to measure costs.  In plant-animal mutualisms where the plant offers a resource to the animal consumer, studies have found measurable costs to the plant of resource production.  Much less is known about the costs to the animal of the services it provides to the plant.  I studied a symbiotic ant-plant mutualism between the tree Cordia alliodora and the ant Azteca pittieri to investigate the costs to ants of tree defense.  Trees provide nesting ants with hollow cavities and support phloem-feeding scale insects whose excretions feed the ants.  Ants defend the tree by recruiting to herbivores and biting them until they fall off the tree.  The caterpillar of the moth Cropia templada is a Cordia specialist and is highly aggressive when attacked by ants.  I characterized the aggressive behaviors of the caterpillar, and then asked whether the quality of ant defense of the tree was positively correlated with aggression by the caterpillar, and whether this led to higher ant mortality.  I also estimated the abundance of this specialist caterpillar relative to the tree's other herbivores and the potential impact of specialist herbivores on the system. 

Results/Conclusions

When bitten by an ant, late-instar C. templada caterpillars reached around to the place of attack with their heads, seized the offending ant between the mandibles, covered the ant in saliva, and ocassionally threw the ant to another part of the leaf or off the plant entirely.  Once seized by the caterpillar and covered in saliva, ants died.  More ant attacks caused more aggressive responses by the caterpillar, which led to higher mortality of defending ants.  On average, four ants died in successful efforts to chase the caterpillar from the plant.  Thus, higher-quality defense of trees by ants led to higher ant mortality in the presence of this specialist herbivore.  C. templada caterpillars are some of the most abundant herbivores of C. alliodora trees throughout Mesoamerica and can cause severe damage, occasionally defoliating entire trees.  Specialist herbivores of ant-inhabited trees may have a range of specialized behaviors to withstand or avoid ant attack, and these behaviors may increase the costs to ants of plant defense.