A keystone ant species provides robust biological control of the coffee berry borer under varying pest density conditions
Species traits play an important role in how biodiversity functions and can ultimately determine the provisioning of associated ecosystem services. This is especially pertinent in agroecosystems where services like biological control are often tested only at snapshots in time and space. When pest populations fluctuate, farmers relying on biocontrol services need to know how natural enemies function in response to these changes. Here we test the effect of variation in coffee berry borer (CBB) density on the biocontrol efficiency of a keystone ant species (Azteca sericeasur) in a coffee agroecosystem. We performed an exclosure experiment with a paired-branch design to measure the infestation rate of CBB on coffee plants in the presence and absence of ants at different densities of CBB. CBB outbreaks were simulated by placing individuals on leaves on each branch. We measured infestation rate as the number of CBB bored into fruits after a 24-hour period, calculated biocontrol efficiency (BCE) as the proportion of infesting CBB removed by ants, and ant attack rate as the difference in CBB infestation between branches. This was repeated on the same plant over several days at four density treatments (10, 20, 40 and 80 individuals) and replicated on 20 coffee plants.
Infestation rates of CBB on branches with ants were significantly lower than on those without ants at every treatment level (all >71% reduction, p<0.001). Biocontrol efficiency was generally high (all >0.68) and did not significantly vary across the different pest density treatments. Additionally, ant attack rates increased linearly with increasing CBB density, suggesting a type I functional response (no satiation). These results demonstrate that robust biological control is possible despite variation in pest density, and that the functional response of natural enemies is an important factor to consider as another layer of complexity in the biodiversity and ecosystem service framework. Furthermore, functional traits should be considered as a mechanism for the insurance hypothesis of biodiversity. In this case, the potential ecosystem service value of ants may increase in the face of an exogenous pest outbreak, as non-satiating ants could potentially buffer such an outbreak. Considering how natural enemies respond to changes in pest populations will allow for more accurate biocontrol predictions and better-informed ecosystem service management in agroecosystems.