Extrafloral nectaries of associated trees can enhance natural pest control

Plant diversity may increase natural enemy populations because some plants can provide alternative food to natural enemies. Extrafloral nectar is such an alternative food and plants producing extrafloral nectar are known to suffer less from herbivory. Little is known about the effect of plants with extrafloral nectaries on herbivory of neighbouring plants. Here, we investigated whether extrafloral nectaries of an associated tree (Inga subnuda subsp. luschnathiana) can enhance natural pest control in coffee agroforestry systems. We assessed the effect of nectar availability from Inga trees on parasitism of coffee leaf miners (Leucoptera coffeella) and on damage caused by coffee leaf miners and coffee berry borers (Hypothenemus hampei). Most of the nectary visitors were either parasitoids or predators, with most predators being natural enemies of coffee pests. Coffee plants were sampled every metre along a transect of 10-15 m extending from each Inga tree. The distance of the coffee plants from the Inga trees did not significantly correlate with coffee leaf miner parasitism, proportion of mined leaves or with the proportion of bored coffee berries. We subsequently used abundance and species richness of those nectary visitors without known association with leaf miners and borers as indirect measures of nectar availability. Whereas species richness had no significant effect on natural pest control, leaf miner parasitism increased significantly with the abundance of nectary visitors (excluding natural enemies of the coffee pests), and the proportion of mined leaves decreased significantly with this abundance. The proportion of bored fruits decreased with increasing abundance of visitors, but this trend was not significant. Together, these results suggest that Inga trees provide alternative food to natural enemies of coffee pests, resulting in increased natural control. Thus, extrafloral nectaries of associated trees can enhance natural pest control in agroforestry systems.