Recognition and duration of the larval instars of banana weevil, Cosmopolites sordidus Germar (Coleoptera : Curculionidae), in Uganda

Analysis of head capsule widths of banana weevil larvae was carried out to determine the number and recognition of instars. The analysis involved mode l fitting to frequency distributions of head capsule measurements of labora tory-reared and field-collected larvae. The laboratory population revealed developmental polymorphism, with larvae passing through 5-7 instars. The mo del fitted Gaussian curves with five peaks for laboratory samples and six p eaks for field samples. Mean head capsule widths for the first four instars showed close agreement among both laboratory and field-collected populatio ns. Variability appeared greater among field populations, resulting in a hi gher probability of misidentification. The method of analysis was not suffi ciently sensitive to separate later instars. Plotting mean head-capsule wid ths against instar number showed a geometric curve to be the best fit, but with only approximate conformity to Dyar's rule. The duration of banana wee vil immature stages was determined under ambient conditions in three experi ments. Most eggs hatched within 5.5-8.0 days. Using a developmental thresho ld of 12 degrees C, thermal requirements (92.8-95.9 degree-days) appeared s imilar to those established for a West African population. Larvae passed th rough 5-8 instars, with 74 % pupating after six instars. Larvae completed d evelopment in 20-41 (most less than 30) days and spent 3-5 days in each ins tar. The pre-pupal period averaged 4.6 days, while the pupal stage averaged 7.0 days. Overall, the egg to adult period lasted 6-8 weeks. Rearing metho ds influenced the number of instars and the length of the larval period. Ap plications of these data for life-table studies are discussed.