Simulated soil moisture and planting material health on the behaviour of Cosmopolites sordidus, Germar (Coleoptera: Curculionidae)
Description
study to confirm the inherent fecundity of Ghana weevils, soil moisture effect and planting material health status that may contribute to weevil behaviour
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For adult counting, split pseudostem traps were used to trap the weevils from the micro-plots. The adult weevils were trapped for 21 days which was the termination point (El-Sayed et al., 2006) when traps showed no catches for five consecutive inspections in a 90 x 180cm area. Plants were uprooted for corm damage assessment using Percentage Coefficient of infestation (PCI) and throughout the experiment, and for juvenile weevil count. The potential number of eggs embedded in gravid females was determined. Three commercial plantain-growing areas in the Eastern Region of Ghana (Akanteng, Dwenase and Pramkese) which are within 30km radius of the University of Ghana research station were selected for the study. Farm sizes were less than a hectare and were mixed cultivars of apentupa and apem for all locations. There were three treatments (location) and was replicated over twelve months. For each sampling month one hundred adult female weevils randomly selected from trapped weevils in backyard gardens in each of the three locations was used for the study. At fortnight intervals ten female weevils from each of the three locations were dissected along the lateral abdomen to expose the eggs. In all 240 females from each location were assessed during the 12-month study. The number of mature egg follicles present in their ovaries were extracted and with the aid of a compound microscope Leica DM 2500 were counted. The unused weevils (which was a security measure to take care or deaths and improve randomness) were discarded at the end of each month before new field samples were collected. Effect of pre-planting material A preliminary experiment with 15 replications indicated that the number of larvae increased on planting material when more adult weevils were available. Based on results from the preliminary experiment hot water treated suckers were each exposed to two or four weevils in 25-litre plastic containers lined at the bottom with moist soil at ambient conditions. These set ups were covered and kept at 25°C for 7 days to allow oviposition. The weevils were then manually separated from the suckers. These and a third treatment consisting of default suckers from farmers’ field formed the three initial infestation treatments under study. The infested suckers were planted in 25-litre plastic pots filled with topsoil (Figure 2). It was mulched and covered with 5 mm nylon mesh and fastened with a rubber band at the rim of the container. The potted plants were managed for 22 weeks when eggs would have reached adults. Plants were carefully uprooted for damage assessment using Gold et al., (1994). Adult weevil density within each pot was estimated by split pseudostem traps from the four weeks after planting till the 30th week when trap catches were zero for two consecutive weeks which was considered adequate for trapping effect per container used. The number of larvae and pupae embedded in the plantain corms were also counted after the destructive sampling.