Effect of Imidacloprid Gaucho-Seed Coating on Mosaic Virus Transmission and the Dynamics of the Main Insect Pests of Common Bean (<i>Phaseolus vulgaris L.</i>) Cultivation in Rwanda

Abstract

The common bean (Phaseolus vulgaris L.) is one of the principal food crops of Rwanda and Central Africa. It is cultivated by 92% of the  family farms and constitutes the principal source of proteins for the majority of the Rwandan population. Since 2000, there has been a  considerable decrease of the common bean yields and among the main causes are diseases and insect pests in particular the black aphid  (Aphis fabae) vector of the Bean Common Mosaic Virus (BCMV), the bean stem maggots (Ophiomyia spp.), Cutworm (Agrotis segetum) and  Bean leaf beetle (Cerotoma trifurcata). For contributing to the search of affordable methods and less polluting for the protection of  common bean against insect pests, three doses of imidacloprid for seed coating (2g, 4g and 6g of active ingredient per kg of seeds) were  tested and compared to the control (untreated) in a Completely Randomized Block Design (CRBD) with 5 replications in 2016B agricultural  season. The overall objective of the trial was to protect the bean plants from insect attacks for the first eight (8) week period  of young age when the plants are vulnerable to disease and insect attack.The imidacloprid seed coating has shown positive effect up to 8  weeks after the seed emergence; very few virus-infected plants (less than 3%) were recorded until 8 weeks after plant emergence; the  percentage of infected plants increased up to 42% for the dose of 2 g of active ingredient per kilo of seeds and 25% and 23.5% maximum,  respectively for the doses of 4 and 6 g of active ingredient per kilo. However, the symptoms of common mosaic appeared before the  presence of wingless aphids on common bean plants. This suggests that winged forms of the black aphid stung the plants before the  wingless forms of the black aphid settled. The used product has also controlled other insect pests, namely the bean stem maggots  (Ophiomyia spp.), Cutworm (Agrotis segetum) and bean leaf beetle (Cerotoma trifurcata). In view of these results, we can recommend to  bean producers, the coating of bean seeds at the dose of 6g of active ingredient per kilo of seeds to protect common bean plants against  the mosaic virus and other insect pests.