Abstract Malathion, 86.3% EC, and pirimiphos-methyl, 84.0% were used. The insecticides were diluted with water to 0.25, 0.50, 0.75, 1.0% solutions and applied to plywood panels to give intended deposits of 0.125, 0.250, 0.375, and 0.500 g (AI)/m2. The plywood panels were 13 by 13 by 0.6 cm, and 216 panels were sprayed using a Paasche air brush with a No. 5 nozzle at 69 kPa pressure. Malathion was applied at each of the four doses to 24 panels (96 panels), pirimiphos-methyl was applied at each of the four doses to 24 panels (96 panels), and 24 panels were treated with water (controls). The panels were stored at 21 ± 2°C and 30 ± 5% RH, in the dark. After 1 day, 1, 4, 8, and 16 weeks in storage, four panels from each insecticide-dose combination and the controls were bioassayed at the storage temperature and relative humidity with adult P. truncatus which had been reared at 30 ± 1°C, 70% RH on whole corn. The insects were confined for 24 h on the treated surface inside copper cylinders, 5.0 cm diam and 2.1 cm high, which had been coated with Fluon (polytetrafuorethylene) to prevent insects from crawling out. A fine screen cover was placed over each cylinder to prevent escape by flying. There were two copper cylinders per panel, eight replicates per treatment, and each cylinder contained 20 insects. A total of 1,440 insects were used on each bioassay date. Following the exposure period the number of beetles knocked down, or unable to walk, was counted. The insects from each cylinder were then placed in plastic vials, 1.25 cm diam and 3.0 cm high, containing about 3 g of ground field corn, cv. Pioneer 3993. Following a 3 to 5 day recovery period at + 1°C and 70% RH the number of living and dead insects was determined by removing the insects from the corn and prodding with a fine sable-hair brush. Fourteen concrete blocks, 40 by 20 by 3 cm, were divided in half with a piece of masking tape. The surface of 0.5 of each block, 20 by 20 cm, was covered with corn dust which would pass through a 20 mesh screen. The dust was ground into the surface for 1 min with a small square of plywood. Loose dust was then shaken off the block. This procedure was repeated three times. Two blocks were then sprayed with each of the following doses of malathion or pirimiphos-methyl as previously outlined: 0.5, 1.0, and 2.0 g (AI)/m2. Controls consisted of two blocks sprayed with water. Following treatment the blocks were stored at 21 ± 2°C, 30 ± 5% RH for 1 day, 1, 4, and 8 weeks and bioassayed with adult P. truncatus. Two copper cylinders, previously described, were placed on both the concrete and concrete covered with corn dust halves of each block. Twenty adult insects were placed in each cylinder at the storage temperature and relative humidity for 24 h. Knockdown was then determined followed by a recovery period of 3 to 5 days on crushed corn in plastic vials at 30 ± 1°C, 70% RH.