Patterns of predation by natural enemies of the banana weevil (Coleoptera: Curculionidae) in Indonesia and Uganda
The banana weevil, Cosmopolites sordidus (Germar), is the most important constraint to banana production in East African highlands. For most resource-poor farmers in the region, biological control is the only viable option because it requires little or no cash investment. I investigated patterns of predation by natural enemies of the banana weevil in its presumed native range in Indonesia to determine if there were natural enemies that could be imported to control the pest in Africa. Work was also done in Uganda to determine if predatory insects already present in local banana farming systems could be better conserved to produce banana weevil control.
Through farm surveys in Indonesia, I confirmed existing reports that banana weevil damage is low in Sumatra and Java. However, I found no evidence of parasitism from 25,000 eggs and 3600 larvae collected from seven diverse geographical locations. The adult and larvae of the histerid, Plaesius javanus Erichson, were found to be important predators of C. sordidus. P. javanus larva entered tunnels of plants, presumably in search of banana weevil stages. This predator should be imported to Uganda for establishment as a classical biological control agent of the banana weevil.
Experiments in Uganda showed that destruction of crop residues in bananas, as recommended for weevil control and practiced by some farmers, reduced predator numbers on farms, reduced predator: prey ratios and had no benefit to the plant with respect to damage prevention. Instead, I demonstrated that residue presence, through maintaining high predator: prey ratios, offsets damage to plants that would otherwise occur from increased numbers of banana weevils.
Ants are the major predatory group now present in banana farming systems in Uganda. Surveys in banana farms found 55 species from pitfall traps, 17 from banana pseudostem residues and 34 from banana corms. Eleven species of ants came to banana weevil egg and larvae exposed in the field as baits and thirteen species were tested in the laboratory for their potential to attack the banana weevil. Two species—Pheidole sp. 2 (Myrmicinae) and Odontomachus troglodytes (Santschi) (Ponerinae)—caused significant banana weevil larvae mortality in crop residues and significant egg mortality in living plants in microcosm experiments and in the field.
0480: Plant pathology