Effect of Lambda-Cyhalothrin and Dimethoate on the Growth Response of Cowpea Plants and the Surrounding Soil

Abstract Insecticides are used widely to control insect pests affecting cowpea and their residues are often deposited in the soil. This may impact the physico-chemical characteristics of the soil and the plant health. This study was designed to investigate the growth response of cowpea treated with lambda-cyhalothrin and dimethoate and the insecticides influence on the physico-chemical characteristics of the soil. Three cowpea seeds were planted per polyethylene bag containing top soil, and later thinned to two plants. The insecticides were assayed singly or combined. The insecticides were applied at the pre-flowering (30th day), flowering (50th day) and at podding (70th day) stages of cowpea growth. The physico-chemical parameters of treated soils were assayed. Treated soils with lambdacyhalothrin and dimethoate and the control were slightly acidic with pH varying from 5.78 to 6.55. Soil organic matter and soil nutrients such as nitrogen, phosphorous, potassium and sodium had lowest values in untreated soil; they had highest values in soil treated with combined insecticides. Lambda-cyhalothrin and dimethoate treatments elicited diverse response from the cowpea plant. Generally, there were slight increase in the vegetative parameters, yield and pod dimensions of treated plants when compared with the control. These insecticides also increased the root nodules, root length and dry weight of cowpea at different concentrations. The combination of these insecticides had positive effect on cowpea without adverse influence on the yield and as well as the soil physicochemical properties.