Contrasting effects of shade level and altitude on two important coffee pests

The diversity and abundance of natural enemies of insect pests is often higher in agroforestry plantations than in sun-exposed monocultures, and it is often assumed that this will lead to improved pest suppression. The effect that incorporating trees in cropping systems will have on pest populations, however, also depends on the habitat requirements of the pests themselves. In Eastern Uganda, we studied how shade level (full > 50 trees per acre, moderate 21-50 trees per acre, and low 0-20 trees per acre) and altitude (high 1,717-1,840 m.a.s.l. and low 1,511-1,605 m.a.s.l.) influenced the abundance of the white stem borer Monochamus leuconotus and the coffee berry borer Hypothenemus hampei. We found that the effect of shade trees differed between the two pest species. The coffee berry borer was more common on sun-exposed plantations, whereas the white stem borer was more common in shaded plantations. Furthermore, the effect of shade level on the white stem borer depended on altitude, with the differences between shade levels being most pronounced in plantations at low altitudes. This implies that the impact of agroforestry on pest regulation both under current conditions and in a global warming scenario will be highly context dependent; it will depend on the identity of the most important pests in the area, and on environmental factors such as altitude.