Effects of carbamate and pyrethroid insecticides on cowpea bradyrhizobial population assessed in sand culture and on nitrogen fixation in soil
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Abstract
The effects of two insecticides, Lannate (a carbamate) and karate (a pyrethoid) on nitrogen fixation by cowpeas (vigna unguiculata) were evaluated after establishing the toxic effects of these insecticides on the microsymbiont bradyrihobia in vitro. In this (in vitro) experiment conducted in modified Leonard jars (LJ), seedlings from pre-germinated, surface sterilised cowpea seeds were inoculated with 1 ml each of five - fold (s-1- 5-6) dilute soil suspensions. Aqueous solutions of lannate or karate, each at a final concentration of 0.5 µg ml-1 were incorporated into the N-free solution in the LJ nutrient reservoirs. After 28 days, the plants were examined for nodulation on the basis of which, the most probable Number technique was used to estimate the cowpea bradyrhizobial count in the original soil from which the inoculant soil suspensions were prepared. Then, the effects of these insecticides each at 0.5 and 1.5 ppm were assessed on tbe cowpea Bradyrhizobium symbiotic relationship in potted soil. Cowpeas were grown and harvested after 35 days. Nodule numbers and dry weight as well as shoot dry weight and nitrogen content were determined. Results ofthe Leonard jar investigations revealed that bradyrhizobial counts were reduced by the application of these insecticides, the effect being more pronounced with lannate than with karate. However, nodulation and nitrogen fixation by cowpeas grown in soil, were not significantiy affected by any of these insecticides at any of the concentrations tested. It was concluded from this study that while these two insecticides appear to have no profound effects on the cowpea-Bradyrhizobium symbiotic association in the experimental soil, the very severe depressive effect of lannate on the microsymbiont population in vitro, may warrant further investigations into the effects of these insecticides on the Bradyrllizobittm - cowpea symbiosis in other soils.