Use of the response of two growth stages of cowpea to different population densities of the coreid bug, Clavigralla tomentosicollis (Stål.) to determine action threshold levels

The effects of increasing the population density of the cowpea coreid, Clavigralla tomentosicollis Stål., up to a maximum of 32 pairs of insects per cage, infested on cowpea at either early flowering or mid pod-fill, were studied in potted plants and field cages. Overall, the results show that increasing insect density resulted in a corresponding increase in damage to the crop (measured as abscission of fruiting bodies and as damage to pods and seeds). Yield reduction was approximately proportional to the bug population when low, with highly significant correlation coefficients. Damage to the crop and yield reduction were more drastic when infestation was made during flowering as opposed to infestation at podding; although there was some compensation of damage to fruiting bodies during the former growth phase, this was not sustained with increasing pest density. Suggested pest densities at which control measures should be initiated are two insects (fourth instar and over) per 10 plants at flowering and four insects at podding. Further studies on damage thresholds in multiple pest scenarios are suggested.