Effects of shade cover and availability of midge breeding sites on pollinating midge populations and fruit set in two cocoa farms.
The abundance of several species of Forcipomyia, known to be pollinators of cacao, increased markedly in 1 cacao farm in Costa Rica when discs of rotten banana stems were added to the ground litter, but not in another one. The addition of other potential midge breeding habitats, cacao pod husks, cacao leaves and artificial bromeliads, produced no increases in midge populations in either farm. The increase in numbers of midge larvae and pupae associated with rotten banana stems occurred in 1 farm that had a shade cover consisting of an open canopy of bananas mixed with various wild trees, produced many cacao flowers and had a large number of midge species. The other farm had a uniform and homogeneous shade cover of rubber trees (Hevea), produced few cacao flowers and had only a few species of the midges. There was marked improvement in fruit set for cacao trees near sites in the farm where midges were increased, presumably a result of increased pollinator activity. These observations are probably of general application. A cacao farm with a heterogeneous shade cover can be expected to have a larger resident pool of pollinating midge species. Rotten banana stems will usually be limited as breeding sites for pollinating midges, but the abundance of the midges is a limiting factor in the pollination of cacao.