Diversity and abundance of twig borers of Robusta coffee and their associated ambrosia fungi in Uganda

Abstract

Robusta coffee (Coffea canephora) is of great economic importance to Uganda. However, its production is recently impeded by the ambrosia twig borer Xylosandrus compactus. A survey was conducted in 10 randomly selected districts in the main Robusta coffee-growing regions of mid-eastern, central and mid-western Uganda to determine (1) diversity, incidence and abundance of twig borers attacking C. canephora in Uganda and (2) ambrosia fungi associated with these twig borers. In each district, 10 randomly selected farms were inspected for twig borer species attacks. Up to 20 twigs infested by twig borers per farm were sampled and dissected to morphologically identify borer species inside the twigs and enumerate different stages of the borers per twig. Ambrosia fungi from galleries of each distinct twig borer were isolated and identified using molecular techniques. Five twig borer species, X. compactus, Xylosandrus sp., Hypothenemus sp. and two unidentified twig borers (K1 and K2), were found attacking C. canephora. Whereas the rest of these species were detected in only single coffee twigs from single farms, X. compactus occurred in nine of the 10 study districts. The proportion of X. compactus-infested farms in each of these districts ranged from 0.1 to 1 and the number of X. compactus individuals per twig per district ranged from 2.5 +/- 1.9 to 29 +/- 28.1. Fusarium solani was isolated from galleries of both X. compactus and K1, whereas Glomerella cingulata was isolated from a gallery of K2.