A complex trophic interaction in a coffee agroecosystem: effects of a parasitic phorid fly on ants and the coffee berry borer

Phorid fly parasites (Pseudacteon sp.) directly negatively affect Azteca instabilis ant activity. However, it is unknown whether phorid-ant-prey interactions involving the coffee berry borer (CBB, Hypothenemus hampei), a significant coffee pest, differ between habitats. We examined tri-trophic interactions in a high- and low-shade coffee farm to determine whether: 1) Phorid effects on A. instabilis differ between habitats and 2) presence of phorids near A. instabilis colonies creates positive, indirect effects on the CBB by allowing them access to coffee berries. To test hypothesis 1, we observed A. instabilis recruitment in each habitat with and without phorids, noting when phorids first attacked, if ever. To test hypothesis 2, we performed arena experiments to examine the number of CBB boring into coffee fruits under three experimental treatments: a) 20 CBB alone, b) 20 CBB with 20 A. instabilis, and c) 20 CBB with 20 A. instabilis and 2 phorids. CBB were given 24 hours to colonize fruits. In the low-shade area, phorids reduced ant activity by 52%; in the high-shade area reduction was only 23% (P=0.04) and phorids arrived twice as fast in the low-shade habitat (P=0.031). Thus phorid effects on ants differ with habitat type. In arenas, A. instabilis significantly lowered the number of colonized fruits (1.88). However, the number of fruits colonized did not differ between treatment a (4.00), and c (3.46, P=0.0007). These results indicate A. instabilis control the CBB, and that phorids inhibit A. instabilis function as biological control agents, but only in high-shade farms.