Forest Landscapes Influence Black Coffee Twig Borer, Xylosandrus compactus Eichoff Infestation in Adjacent Robusta Coffee Gardens: Management Implications

The black coffee twig borer, Xylosandrus compactus Eichhoff (Coleoptera: Curculionidae) is one of the major constraints facing the Robusta coffee industry in Uganda since its advent in 1993. Once in a new location, it spreads rapidly within and between coffee gardens. This is mainly driven by favorable climatic conditions, presence of alternate hosts, limited management by farmers and the fact that natural enemies present in the new eco-systems are yet to adapt to it. Its management is difficult due to its cryptic nature of spending almost its entire lifespan is spent inside the host galleries. X. compactus has >200 host plant species worldwide, whereas, >50 plant species have been proven to be hosts in Uganda including: - commercial and ornamental crops as well as shade and forest trees/shrubs. In addition, these trees provide shady conditions that promote infestation of this pest. A study was therefore conducted in central Uganda to elucidate the effects of forest landscapes on the distribution of X. compactus damage in the adjoining Robusta coffee gardens. New alternate host plants of X. compactus within the forests were also documented. This could inform further development of its management strategy Uganda. Results showed that the percentage of coffee suckers and primary branches infested by X. compactus increased significantly (p=0.0171 and p=0.0001 respectively) with increasing distance away from the forest edge towards the center of the forest. The percentage infestation however decreased significantly (p<.0001 and p=0.2367) for suckers and primary branches respectively with increasing distance from forest edge towards the center of the adjoining coffee plantation. These observations are explicable by the fact that the forest acts as a source of X. compactus infestation for the adjoining coffee plantation, commonly referred to as “pull-effect”, the nearer the coffee trees to the forest the greater the initial infestation. Nine alternative host plant species, namely: - charcoal tree Trema orientalis Linn. Blume (Ulmaceae), African celtis, Celtis mildbraedii Engl. (Ulmaceae), bastard-wild-rubber, Funtumia africana Benth. Stapf (Apocynaceae), velvet-leaved combretum, Combretum molle R. Br. Ex. G. Don. Engl & Diels (Combretaceae) and five unidentified tree species were recorded in the forest. These tree/shrub species have been added to the existing inventory of X. compuctus alternate host plants in Uganda. This study clearly demonstrates the influence of natural forest landscapes on incidence and damage of X. compactus infestations in adjoining Robusta coffee gardens. The results suggest that farmers with coffee gardens neighboring forested landscapes should take into account managing the source of X. compactus infestation from natural forests as well as that on coffee and alternate hosts in their gardens. NARO-BCTB traps should therefore be deployed along the forest boundaries in order to intercept the X. compactus from the forests before they enter the coffee gardens. However, there is need to fully elucidate the interactions between the ‘pull-effect’ and landscape and aggregation factors that influence incidence and damage of X.  compactus attacks so as to inform its management.