Cascading indirect effects in a coffee agroecosystem: Effects of a parasitic phorid fly on ants and the coffee berry borer in a high-shade and low-shade habitat

Background/Question/Methods   The Pseudacteon sp. phorid fly parasites have a negative direct effect on Azteca instabilis ants, thereby creating a positive indirect effect on ant prey. However, it is unknown of the influence of habitat factors on ant-phorid interactions or to what effect phorid flies have on the coffee berry borer, a pest to coffee farms. For two months, we performed research on the three species in two field sites in Finca Irlanda, a high shaded area and a low shaded area in Chiapas, Mexico. We conducted lab and field experiments to test the following hypotheses: 1) The phorid fly effects on A. instabilis ants differ between sun coffee and shade coffee habitats and 2) The presence of phorid flies near A. instabilis colonies has a positive, indirect effect on the coffee berry borers by allowing them more access to coffee berries. To test hypothesis 1, we observed the influences of phorid flies on ant activity in both field sites. We randomly selected Alchornea latifolia trees with A. instabilis nests, and placed five tuna baits on the tree. We then recorded the number of A. instabilis ants present at each bait every two minutes for thirty minutes and the phorid arrival time. We performed a lab experiment to determine hypothesis 2. We set up twenty-six insect arenas consisting of three treatments. Treatment one contained a coffee branch with ten berries and twenty CBB. Treatment two contained a coffee branch with ten berries, twenty CBB, and twenty ants. Treatment three contained the same components as treatment two, but two phorid flies were added. The insect arenas were left unattended for 24 hours, and then we counted how many berries were occupied by the CBB.

Results/Conclusions   Our field results indicate that the phorid flies have a bigger impact on the A. instabilis in the low shaded area. Also, the phorid fly arrives to the nest in the low shaded area almost twice as fast as the restoration area. Our lab results show that treatment 1 and treatment 3 are significantly different from treatment 2. Our results prove that the A. instabilis ants control the CBB population, and that since the phorids have a greater impact on the ants on a low shaded farm, the ants will be more effective in controlling the CBB in densely shaded areas.