Length of Exposure and Quantal Responses of Four Strains Ofprostephanus Truncatus (Horn) (Coleoptera: Bostrichidae) Strains to Pyrethroid Insecticides

Abstract

Comparative studies on exposure time to two pyrethriod insecticides, deltamethrin (12.5g/L EC) and bifenthrin (10% WP) on the larger grain borer, Prostephanustruncatus (Horn) (Coleoptera: Bostrichidae) strains were investigated under laboratory conditions 28-33^oC and 70-85% r.h. Serial concentrations of the toxicants ranging from 2.5 to 50µg/ml for deltamethrin and 1.2 to 31.3µg/ml forbifenthrin including a control were used. Mortality was recorded at 3-hourly intervals for periods 3, 6, 9, 12 and 24 hours post-treatment. Data wereanalysed using log₁₀dose versusprobit-regression and analysis of variance (ANOVA). Results showed that dosage-related mortality responses were noted at different time intervals and these were significant (P< 0.05). The mean mortalities for the Enugu strain significantly (P< 0.01) increased after every three hours of exposure to deltamethrin, although the increase between 9^th and 12^th hours was not. However, the mortality responses of this strain to bifenthrin were not significantly (P> 0.05) different during the first 3 to 6 hours of exposure (LSD=13.56). Significant mortalities (P < 0.05) occurred after 9 and 12 hours of contact with insecticides. TheP. truncatus strain from Ibadan appeared to be more susceptible with LD₅₀ value of 3.19µg/ml, than all the other strains compared. The Benue strain on the other hand exhibited greater tolerance to insecticides with LD₅₀ of 58.39µg/ml, than the others being about 17 times more tolerant than the Ibadan strain.