Population Dynamics of the Plantain Stem Borer (Cosmopolites Sordfdus Germar) and Factors Affecting the Severity of Damage to Plantain (Musa Spp. Aab Group) in Ghana.

Abstract

A study was conducted to determine the behaviour of the banana weevil Cosmopolites sordidus Germar and the differential severity of its larval damage on four stages of plantain' (suckers, preflowering, flowered and recently harvested) in three different crop cycles. The results indicate that the population of the adult weevil increased in the ratoon crops, and that 3-day old traps were more attractive than 5-day old traps. The longevity of weevils decreased with increasing ratooning from 63.8% in the plant crop, 63.6% in a 2nd cycle crop to 42% in a 3rd cycle crop. Larval damage on the cross section and periphery of the corm indicates that the mean damage on conns in a plant crop was 3.1% and 6.2% respectively and a mean cross section damage of 8.6% and 12.5% peripheral damage in the ratoon crop. It was observed that a higher proportion of damage was located in the outer cortex. However 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. When all the plant growth stages were available, the larval damage was lower in sword suckers of corm diameters<9cm than the bigger mature corms with diameters>19cm. 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 plants. The overall corm cross section damage ranged from 2% in a sword sucker from a 1st cycle crop to 14% in a harvested plant from a 1st ratoon crop. In a soil moisture or plant vigour trial, the results suggest that, in confined situations where weevils have no choice for corm size, plants under moisture stress had a higher percentage damage than vigorously growing plants with larger conns. A lower number (3.1) of insects was however associated with plants under stress than vigo rously growing plants (5.1) during the 65-day experimental period. Studies on the potential fecundity of weevils in Ghana revealed that the number of mature egg follicles present in the ovaries of a female was up to 18. Thirty percent of the weevils collected had no eggs. In general the mean number of mature egg follicles was 4.03/female. A study on the effect of initial infestation of planting material on subsequent adult population build up and damage, indicates that as the level of initial infestation increased subsequent adult weevil population and damage also increased. An initial planting material of 0.2 and 0.3 larva per corm of mean diameter 4.6cm led to 0.3 and 2.3 adults respectively after 22 weeks of plant growth in pots. Damage to these plants ranged from 8.6% to 33.6% respectively in the 0.2 and 0.3 larva initial infestations.