Hexane, acetone, and methanol extracts from Gnidia kaussiana Meisn (Thymeleaceae), each at two dosages (0.2 and 1 ml/50 g grains corresponding, respectively to 1 and 5g/kg), and neem seed oil (NSO), used as standard insecticide were evaluated for repellence, toxicity to Callosobruchus maculatus (F.) (Coleoptera: Chrysomelidae) adults, F1 progeny inhibition, persistence and as grain protectant during storage. Experiments were laid out at complete randomized design with five replications for repellence test and four for others. All the extracts were effective in protecting stored Bambara groundnut (Vigna subterranea (L.) Verdcourt) from insect attack; however, their bioactivities were inversely correlated with solvent polarity. No adult survival was recorded in treated grains with hexane extract at 5 g/kg dosage within 2 d exposure. Also at 5 g/kg, all extracts hindered adults emergence, grain damage and weight loss after 4 months storage. Moreover, hexane extract was more repellent and exhibited averagely repellency. The insecticidal effectiveness of hexane extract did not decreased provided that the exposure time of insects to the product was high (7 d). The potency of acetone and methanol extracts decreased with storage time, although not linearly and remained significantly toxic to C. maculatus up to 60 d of storage. Therefore, hexane and acetone extracts are good candidates for incorporation in integrated pest management programs for control of cowpea weevils in stored grains by poor-resourced farmers and store keepers in Cameroon and other developing countries.

No abstract

Experiments were conducted to determine the host preference of cowpea weevils Callosobruchus maculatus F. (Coleoptera: Chrysomelidae) using three pulses (Vigna unguiculata (L.) Walp., Vigna subterranea (L.) Verdc and Glycine max (L.) Merr.) and the susceptibility of these seeds to the beetle. Susceptibility was assessed using parameters like fecundity, developmental period and progeny production of bruchids. C. maculatus cultures were maintained separately on the three commodities for the host preference experiments and on cowpea for the susceptibility experiments. Seeds of each of the pulse species were introduced into partitioned petri dishes for the former and into glass jars for the latter experiments, and infested with C. maculatus. In the fecundity experiment, the maximum number of eggs was recorded on soybean and the minimum on cowpea. The developmental period of insects was two times longer in soybean than in cowpea and Bambara groundnut. Egg-toadult survivorship was highest for eggs laid on cowpea and lowest for those ones laid on soybean. Thus, Cowpea and Bambara groundnut are more susceptible to C. maculatus, whereas soybean is far less susceptible. In the choice experiment involving the three pulses, although there was an improvement in oviposition by C. maculates on their natal host. In stores containing cowpea and soybean, control measures for C. maculatus should target mainly cowpea. However, in storage facilities containing a mixture of pulses, control strategies should consider all the commodities since C. maculatus has the ability of better adapting to Bambara groundnut from cowpea and to soybean from Bambara groundnut.

The response of pests to the effects of a botanical insecticide can vary spatially and temporally. To test whether efficacy of botanicals differed spatially, the insecticidal efficacy of Gnidia kaussiana and Ocimum canum against Callosobruchus maculatus was investigated in two different agro-ecological zones of Cameroon, i.e. Maroua and Ngaoundéré (sudano-sahelian and sudano-guinean zones, respectively). Experiments were, therefore, conducted to determine the insect mortality, progeny production, grain damage and weight loss in cowpea. G. kaussiana was more effective against C. maculatus in Maroua (LD50 = 0.12 g/kg at 6th day of exposure) than in Ngaoundéré (LD50 = 4.35 g/kg at 6th day of exposure). Moreover, it was more toxic than O. canum extract and neem seed oil (reference), irrespective to the agro-ecological zones. Overall, the performance of O. canum did not vary significantly between the two zones, and was slightly more active (LD50 = 4.66 g/kg) than the reference insecticide neem seed oil (LD50 = 4.89 g/kg) in Ngaoundéré in contrast to the results recoded in Maroua (LD50 = 1.44 g/kg and 2.60 g/kg, respectively for neem seed oil and O. canum at 6 days post exposure). In view of the above, there were some discrepancies in extract performance between Maroua and Ngaoundéré. Thus, the establishment of dose rates of insecticidal products formulated from G. kaussiana must be specific to an environment in contrast to those from O. canum.

No abstract