Tithonia diversifolia, Cyperus rotundus and Hyptis suaveloensis ethanol extracts combinatorially and competitively inhibit affinity purified cowpea storage bruchid (Callosobrochus maculatus) glutathione S-transferase

Cowpea, Vigna unguiculata L. (Walp) is an important crop in tropical countries, especially in West Africa, where it is a cheap source of dietary protein (Boeke et al. 2004b ). Cowpea storage bruchid Callosobruchus maculatus (Fabr.) (Coleoptera: Bruchidae) depredates stored cowpea (Jackai and Adalla 1997 ). The huge post harvest losses and quality deterioration caused by this insect are major problems of assuring food security in developing countries like Nigeria (Boeke et al. 2004b ). Effective control of storage insect pests are centered mainly on the use of synthetic insecticides. The continuous use of these synthetic chemicals pose problems: development of resistant insect strains, toxic residues in foods and humans; workers’ safety, and high cost of procurements (Adedire 2003 ). These problems have necessisated research on the use of alternative, eco-friendly insect pest control methods, amongst which are the use of plant products as bioinsecticides. Boeke et al. ( 2004a ) reported that plant materials and traditional methods are much safer than insecticides, and suggested that their use needed further exploration. The effects of some plants extracts on the biological parameters of the herbivourous insects (e.g. oviposition, mortality rate, adult emergence, developmental rate, mortality, fecudity and egg viablity) has been reported (Boeke et al. 2001 ). Earlier studies by Adedire and Lajide ( 1999 ) and Adedire and Akinneye ( 2003 ) showed that powdery and ethanol extracts of Tithonia diversifolia (Tree marigold), Cyperus rotundus (Tigernut), Hyptis suaveloensis (Labiatae) and Aframomum melegueta (Alligator pepper) have insecticidal activity against the cowpea storage bruchid, C. maculatus F. Understanding the diversity of the insect response to these plant powders and ethanol extracts in their local ecological context has been an important aspect of the bruchid control strategies and represents a key challenge in developing durable pest control. The responses include contact and ingestion avoidance, excretion, sequestration, degradation of the toxin and targets site mutation. Those ingested and absorbed enter the general circulation and can get accumulated into various tissues and organs (Despres et al. 2007 ). Their accumulation in tissues and organs can have severe physiological consequences such as cyst lesions, metabolic acidosis, and death (Valko et al. 2006 ). The responses of the insects to the plant secondary metabolites have been informative models for studying molecular mechanisms of resistance.