Knockdown, mortality and residual activity of pyrethrins and cyfluthrin were studied on the larger grain borer (LGB),Prostephanus truncatus(Horn). Pyrethrum marc with 0.4% pyrethrins and cyfluthrin dust (0.1% a.i.) when applied on filter paper at 0.72 mg/cm2gave 98.6% and 100% knockdown (KD) respectively after 6 h. Cyfluthrin dust had a faster KD effect with 100% KD compared to 93.1% for pyrethrum marc, ½1/2>h after treatment. After 24 h there was no recovery from KD in both treatments. After 72 h, 23.2% of the LGB had recovered from KD while mortality was 76.8% in the pyrethrum marc treatment. After the same period, cyfluthrin resulted in 100% mortality. Mortality ofP. truncatusexposed to different dosages of cyfluthrin dust in shelled maize was less than 100% at dosages lower than 1 part per million (ppm) but was 100% at 1 ppm and higher dosages 3 days after treatment. Progeny emergence was suppressed at all dosage levels above 0.25 ppm compared to a mean progeny of 68.8 in the check after 45 days. Cyfluthrin dust was more residually toxic at 0.5 ppm and higher dosages, with mortality of >90% at 5 months after treatment. This suggests that cyfluthrin dust at 0.1% a.i. mixed with shelled maize at 1 ppm will give protection and control of LGB infestation.