Banana weevil, Cosmopolites sordidus (Germar) (Coleoptera : Curculionidae): tests for suspected resistance to carbofuran and dieldrin in the Masaka District, Uganda

Damage by banana weevil, Cosmopolites sordidus (Germar), contributed to the decline and disappearance of highland cooking-bananas (Musa spp., type AAA -EA) from the central region of Uganda. Commercial growers in the southwest use chemical insecticides for the control of the weevil although resistanc e to dieldrin was suspected as the cause of weevil outbreaks in the mid-198 0s. Carborfuran, which replaced dieldrin, has also been reported by some fa rmers to be ineffective against banana weevil. In this study, banana weevil s from three farms in the Masaka District, Uganda, were screened for possib le resistance to carbofuran and dieldrin. Test populations of banana weevil s were not resistant to carbofuran: LD50 values were 3.3-4.5 times higher f or males and 1.3-1.7 times higher for females than those for a reference po pulation assumed to be susceptible. This is within the range of normal vari ability among field populations. Slopes of probit regression lines were ste eper for test populations than those of the reference population, while LD9 9 values were similar among all populations. Two of the test populations ha d LD50 values for dieldrin twice that of the reference population, while th e third had an LD50 value similar to that of the reference group. All three test populations had low slope values for mortality, with LD99 values 92 t imes higher than those of the reference population. There is a strong possi bility that banana weevils had developed resistance to dieldrin and that th is was the cause of earlier control failures.