Laboratory Evaluation of Malathion-Treated Sawdust for Control of Stored-Product Insects in Empty Granaries and Food Warehouses

Abstract

Spruce sawdust impregnated with malathion was assayed by chemical and biological methods to determine whether it was an effective and economical formulation to control stored-product insects in empty storage structures. The size of sawdust particles (fine: 0.250–0.425 mm; medium: 0.425–0.840 mm; coarse: 0.840–2.0 mm) did not affect the residual activity of malathion on a given surface during 8 weeks at 25°C and 50% RH. However, malathion on the sawdust decomposed more rapidly on concrete surfaces than on wood surfaces. Sawdust containing 0.4, 0.8, and 2.0%(AI) malathion was stored on concrete, wood, or galvanized steel for 16 weeks at 25 and 50% RH. The sawdust and surface beneath the sawdust were bioassayed with Tribolium castaneum (Herbst) adults for exposure periods of 1, 3, or 24 h. Sawdust initially treated with 0.4% malathion (4,148 ppm) was relatively effective for up to 4 weeks after a 3-h exposure of insects only on steel; wood and steel surfaces under the treated sawdust were very toxic to insects for 8 to 12 weeks, respectively. Sawdust initially treated with 0.8% malathion (7,430 ppm) was ineffective on concrete surfaces, and gave 100% mortality of T. castaneum exposed for 24 h for 12 weeks on steel and 16 weeks on wood. Sawdust initially treated with 2% malathion (20,442 ppm), gave nearly 100%mortality for 4 weeks (3-h exposure) on concrete and 16 weeks (3-h exposure) on wood and steel. For up to 12 weeks, sufficient malathion had been transferred to the concrete surface to kill 100%of insects in 24 h; 100% mortality occurred on wood and steel surfaces for up to 16 weeks.