Protease inhibition in cowpea pod-sucking bug species (Hemiptera: Heteroptera) using cereal and legume grain extracts

Selected grain extracts were assayed against digestive proteases of Anoplocnemis curvipes, Clavigralla tomentosicollis and Mirperus jaculus with a view to identifying suitable sources of protease inhibitors against the cowpea pests. Laboratory bioassays showed that grains of a local soybean variety Kishi and a cowpea landrace NG/SA/07/0098 contain potent inhibitors although the former produced a consistently higher level of inhibition in each of the three pests. Protease activity was highest in C. tomentosicollis (1.06 μmol/mg) followed by M. jaculus (0.94 μmol/mg) and A. curvipes (0.69 μmol/mg) but inhibition was highest in M. jaculus. The optimum temperature range for the inhibitors was 40-90°C while optimum inhibitory activity occurred in weak acidic to neutral media, pH 4-7. There was total protease inhibition in the pests when soybean and cowpea extracts were combined against gut extracts in soybean-biased volumetric ratios of 0.7:0.3 and 0.8:0.2. This synergistic efficacy was superior to those of two commercial products. The inhibitors were stable for 10-50 min at 80°C but got denatured within few minutes of exposure to higher temperatures. Obtained results showed that the soybean and cowpea are suitable sources of protease inhibitors in breeding exercises for development of transgenic cowpea varieties. Key words: Grain extract, optimum pH, optimum temperature, protease inhibition, thermostability, transgenic cowpea.