In Burkina Faso, farmers commonly use insecticidal plants for crop protection. To understand how insecticidal plant works (their mode of action), we carried out a bioassay on Clavigralla tomentosicollis, the cowpea pod sucking bugs with three insecticidal plants, Cassia nigricans V., Cymbopogon schoenanthus S. and Cleome viscosa L. Three modes of exposures (1) direct contact application, (2) stomach poisoning activity (3) and inhalation toxicity activity, were tested. The results showed a potent contact and stomach toxicity on 1st instars larvae regardless of the three crude extracts. But the plant extracts was less effective with older stages of the insects. A highest effectiveness was recorded with inhalation of vapours of crude extracts regardless of insect stages and type of plants. Implications of these findings are discussed regarding the use of plant extract for controlling pod sucking bugs in cowpea fields.