Trees with extrafloral nectaries (EFNs) and their association with arboreal ants and their parasitoids in a coffee plantation
Understanding the mechanisms that favor the maintenance of natural enemies -- such as ants and parasitoids -- in coffee plantations, is fundamental because they provide biological control of major pests. In natural systems, extrafloral nectaries (EFNs) on plants provide sugar resources for ants and adult parasitoids when hosts and pray are not highly available on the canopy. EFN resources may also play important roles in driving diversity in agroecosystems. We investigated the use of EFNs by ants and parasitoids on four abundant shade trees species (Inga micheliana, I. lauriana, I. vera and Alchornea latifolia) in shaded coffee agroecosystems in SE Mexico. Specifically we asked: 1) Do ant and parasitoids visits differ by tree species or time of day? 2) Does community composition of visitors differ by tree species? 3) Does visitation frequency by specific ants vary with tree species? 4) Do parasitoids of ants visit EFNs? We observed ant and parasitoid visits to EFNs during 15 min observations on 49 trees. We observed each tree during three different times of day (8-10AM, 10AM-12PM, 12-2PM) during June-August 2014. We collected all ants and parasitoids for identification.
We found 30 species of ants (from 15 genera) and 10 morphospecies of parasitic wasps visiting EFNs. Mean number of EFNs visits by ants was significantly different between species (P<0.05), however there were no differences in the number of visits during the times of observation. Differences in mean number of ant visits could be explained by nectar composition and amount produced, morphology and location. I. micheliana has large pit nectaries on stem (~7.4 mm3), while A. latifolia has 1-2 mm flat nectaries located on the lower leaf surface. I. micheliana received significantly higher number of ant visits in 15 min. (9.05±1.41) than A. latifolia (3.19±0.77) (P<0.05). Community composition of ants and parasitoids did not differ between shade tree species or time of observation (P>0.05). However, individual ant species visited more frequently certain shade trees. Among the parasitic wasps collected, one species belongs to Eucharitidae, one of the most abundant groups of parasitoids of social insects. These results suggest that EFNs are a widely used resource for ants and parasitoids in coffee plantations. Because ant parasitoids feed on EF nectar, EFN resources may favor encounters between ants and their parasitoids, an interaction that has been poorly studied.