Relative toxicity of phosphine against the bean bruchid Acanthoscelides obtectus (Say) (Col., Bruchidae)

The toxicity of phosphine against adult Acanthoscelides obtectus (Say) was studied over a range of concentrations, viz. 0.01, 0.0125, 0.015, 0.02, 0.04, 0.08, 1.0, 2.0, 3.0, 4.0, 5.0 and 6.0 mg/l and exposure periods, viz. 0.25, 0.50, 1, 2, 6, 12, 24 and 48 h at 25degreesC and 65% relative humidity. Results show that both the parameters exhibited significant effects on the mortality of A. obtectus. Data indicate that 100% mortality only occurred in A. obtectus when 1-, 2- and 12-h exposure times were involved in the respective highest individual ct product. The ct products decreased as the exposure times increased for both the mortality levels of 50 and 99%. The minimum ct products for 50 and 99% kill were 0.19 and 0.29 mg h/l, respectively, at an exposure of 12 h. The influence of thresholds on regression equations of the form c(n)t = k were 0.59 and 13.80 (mg/l)(n)h for the fixed concentration for 50 and 99% mortality levels, respectively while they were 0.45 and 12.45 (mg/l)(n)h for the six exposure periods for the same mortality levels.