PS 44-46
Indirect benefits of an ant-hemipteran mutualism in a coffee agroecosystem: Scale-tending ants attack the coffee berry borer
In complex agroecosystems, mutualistic interactions between ants and ant-tended hemipterans may result in both direct negative effects on plants and indirect positive effects on plants if ants suppress other herbivores that damage crop plants. We studied whether the density of scale insects tended by ants in a coffee agroecosystem is associated with higher or lower frequency of coffee fruit damage by the coffee berry borer (CBB, Hypothenemus hampei) a major insect pest of coffee. The ant, Azteca sericeasur, is a mutualist of the green coffee scale (Coccus viridis), and scale abundance is strongly correlated with ant attendance. Azteca ants also limit CBB activity and damage caused by the CBB to coffee fruits. During Aug-Sept 2014, we conducted an experiment to examine the effects of scale density (5-15, 16-26, or >26 scales per branch) and Azteca presence on CBB damage caused within 24h. We selected 15 coffee plants, and selected 6 branches per plant (2 per scale density). We excluded Azteca from 3 branches per plant (one of each scale density). We then removed all damaged fruits, placed 20 CBB onto branches, and returned after 24 hours to count the number of damaged fruits per branch.
Results/Conclusions
The proportion of berries with CBB damage on branches with Azteca (0.082±0.01) was lower than on branches from which Azteca was excluded (0.12±0.0.15) (χ2=17.163; df=1; p<0.0001). CBB damage did not differ on branches that differed in scale insect density (χ2=2.037; df=2; p=0.361). The influence of Azteca on CBB tended to be stronger on branches with high scale insect density (Azteca: 0.05±0.01; No Azteca: 0.13±0.03 ), than on medium (Azteca: 0.08±0.01; No Azteca: 0.11±0.02) or low density branches (Azteca: 0.09±0.02; No Azteca: 0.12±0.02), but this interaction was not significant (χ2=3.526; df=2; p=0.17). Our study reinforces the idea that Azteca provides important biological control services to coffee by significantly reducing the damage to the coffee from the CBB. In other studies, plants with higher scale insect density sustain lower CBB damage. Our data suggest that this may be due to stronger patrolling effects by Azteca on branches with more scale insects, thus supporting the idea that mutualistic interactions between ants and scale insects may provide benefits to coffee plants. Our study is the first experimental attempt to understand the importance of scale insect density in the indirect benefits of an ant-hemipteran mutualism for coffee.