Effect of some vegetable oils on survival and progeny emergence of Callosobruchus maculatus Fabricius (Coleotera: Bruchidae) on stored cowpea in Zaria, Nigeria

Vegetable oils (groundnut, soybean and palm) at two levels of 5 ml and 10ml per 650g seed of cowpea (IAR 48 white) were tested against Callosobruchus maculatus Fabricius in the Department of Agric. Education entomology laboratory in 2008. The experiment comprised of forty eight plastic pots arranged in randomized completely block design with three replicates. Each treatment was carefully pipette into the plastic container, manually mixed and spread on the bench to dry. After drying, the treated seeds were reintroduced to the labeled containers, infested with the weevil at ratio  of 3 male and 7 females and covered with cheese cloth and perforated lid to avoid insect escape. The experiment was repeated in two locations and means taken. Data were collected on insect survival and mortality, oviposition, insect emergence and grain perforation at weekly interval and subjected to analysis of variance (ANOVA) while means were separated using least significant different (P = 0.05). The efficacy of the oils was determined by weevil perforation index (WPI). Results showed that the oils significantly suppressed insect population at both 5ml and 10ml.  However, groundnut oil gave the best result as it consistently and significantly (p = 0.05) suppressed population and inhibited progeny emergence (0.67 and 0.33 at 5ml and 10ml respectively) at 5th week as well as inhibited oviposition to 1.0 (5ml) and 0.33 (10 ml). Weevil perforation index (WPI) of 4.7% (5 ml) and 2.07% (10 ml) confirmed the efficacy of groundnut oil in the management of C. maculatus. Soybean and palm oils also inhibited development up to the 3rd week but significantly encouraged progeny emergence at the 5th week with very high WPI. The control was significantly higher in all the parameters. It is therefore recommended that groundnut oil (5-10ml/650g grain) should be integrated with adequate drying in an air tight condition to prolong shelf life of the grain and prevent weevil infestation.