The biology and ecology of the kola weevil,Sophrorhinus gbanjaensisD. & T. (Coleoptera: Curculionidae)

Abstract

Summary Aspects of the biology of the kola weevil, Sophrorhinus gbanjaensis, were studied under the laboratory temperature of 26–28°C and three temperature regimes of 25±1°, 27·5±1° and 30±1°C. Both mated and unmated females had a pre-oviposition period of 13·3±0·5 days. Eggs were laid singly and in special oviposition holes made inside the nut by the female. Mated and unmated females oviposited for an average period of 56·0±2·2 days and 30·8±1·7 days and laid an average of 48·6±3·1 eggs and 10·2±0·5 eggs, respectively. Under the three temperature regimes of 25°, 27·5° and 30°C, a significantly higher number of eggs (1304) were laid at 27·5°C when compared with the numbers laid at 25° (686) and 30°C (996). The mean incubation periods of the eggs were 7·9±2·3, 5·9±1·4 and 5·2±0·7 days at 25°, 27·5° and 30°C, respectively. There were five larval instars. The mean larval periods at 25° and 27·5°C were 29·3±4·2 days and 26·5±3·3 days, respectively. The mean total periods of development from egg to adult emergence from the nut in S. gbanjaensis at 25° and 27·5°C were 48·4±3·5 days and 41·2±4·2 days, respectively. Adults of S. gbanjaensis that were allowed to feed and mate freely lived for an average of 71·6±3·4 days with a minimum and maximum of 33 and 112 days, respectively. The sex ratios of the laboratory-reared adults of S. gbanjaensis and those that emerged from field-infested nuts were very close to unity. The natural enemies observed were a hymenopterous larval parasite (Ichneumonidae), Phorotrophus mameti; an entomophagous fungus, Botrytis (Beauveria) bassiana, and an unidentified mite. Field experiments on the time and sequence of infestation of kola nuts by the kola weevils showed that S. gbanjaensis larvae were first observed 12 weeks after fruit-set when the nuts were already filled with solid materials. Few larvae of B. kolae were found in the field until 16 weeks after fruit-set when many of the pods had been damaged by other insect pests. Results of laboratory and field studies showed that the ability of B. kolae to infest kola nuts through undamaged husks is highly limited and is unlikely to be of economic importance.