Dosage-mortality relationships for Rhyzopertha dominica (F.) (Coleoptera: Bostrychidae) exposed to heat in a fluidized bed

Abstract

Batches of wheat containing immature Rhyzopertha dominica were heated in a fluidized bed to examine the influence of air inlet temperature, bed depth and grain temperature on the exposure time needed for a given level of mortality. With a bed depth of 160 mm, raising inlet temperature from 60 to 80°C decreased the LT 99.9 for R. dominica from 16.1 to 3.25 min: corresponding values for 20 mm beds were 1.92 and 0.54 min. For a given bed depth, 1 LT 99.9 was linearly related to inlet temperature. With inlet temperatures of 80 to 90°C, LT values were linearly related to bed depth over 40 to 160 mm. By contrast, LT value and bed depth were not linearly related at lower temperatures. With beds of 40 to 160 mm, the grain temperature asociated with 99.9% mortality ranged from 57.1 to 64.0°C and varied with inlet temperature but not, generally, with bed depth. Heating grain to 56–57°C and allowing it to soak, unfluidized, before cooling increased mortality from about 50 to > 99%. Four other species of stored product beetle suffered complete mortality when subjected to a treatment expected to yield 99.9% mortality of R. dominica. The experimental results are discussed in terms of heating strategies, heat recovery and the choice of optimal bed depth.