FIELD DYNAMICS OF THE PLANTAIN STEM BORER Cosmopolites sordidus (GERMAR) (COLEOPTERA: CURCULIONIDAE) LEADING TO ITS PEST STATUS IN GHANA

Plantain is grown mainly in the Southern parts of Ghana and also in backyard gardens around the country. A study was conducted to determine the behaviour of the banana weevil Cosmopolites sordidus, (Germar) (Coleoptera: Curculionidae) and the differential severity of its damage on plantain growing on ratoon fields. Three farmer fields within a 20 km radius and of three different cropping ages (plant crop, first and second ratoon fields) were selected for the study. One hundred plant stands per field were selected as target sampling subjects. Weevil population using split pseudostem traps and corm damage were analysed per month for one year between September and August of the following year. Weevils trapped from the fields were cultured in the laboratory to determine post-trapping longevity. The results indicate that the population and damage of the plantain weevil increased with ratooning. The attractiveness of the split pseudostem traps was highest at trapping day three and drastically reduced by day five. The mean larval damage on corm cross section in a plant crop was 3.1% and corm periphery damage was 6.2% whilst, the mean cross section damage was 8.6% and peripheral damage was 12.5% in the ratoon crops. It was observed that a higher proportion of damage was located in the outer cortex. The ratio of the damaged central cylinder to the outer cortex decreased from 1:4 in a plant crop to 1:2 in the ratoon crops. There was a differential larval damage with respect to the phenologies in a plant stand. The damage was lower in sword suckers of corm diameters <9 cm than the bigger corms with diameters > 19 cm. The ratio of cross-section damage in the inner cylinder to outer cortex was 1:1 in sword suckers, 2:3 in preflowering plants, 2:5 in flowered plants and 1:2 in harvested stools. The overall corm cross section damage ranged from 2% in a sword sucker from a plant crop to 14% in a harvested plant from a 1st ratoon crop. The monitored post trapping longevity of weevils after a month decreased with increasing ratooning from 63.8% for plant crop weevils, 63.6% in the 1st ratoon to 42% for the 2nd ratoon cultured weevils.