Temperature and Geotaxis Preference by<i>Cryptolestes ferrugineus</i>(Coleoptera: Laemophloeidae) Adults in Response to 5°C/m Temperature Gradients at Optimum and Hot Temperatures in Stored Wheat and Their Mortality at High Temperature

Cryptolestes ferrugineus (Stephens) is often found in abundance in association with heating stored grain. Their mortality at high temperature and their distribution at optimum and hot temperatures are important information for insect control and for models of their distribution in grain bins. The lethal exposure times of the adults were determined at 42 ± 0.2–50 ± 0.2°C and 75 ± 5% RH. Insect mortality increased with increasing temperatures and exposure time. For each temperature, there was a cumulative period of thermal stress, and after the critical exposure time an additional few hours or minutes at that temperature would kill all of the adults. The mortality was 100% at 45°C in 78 h, at 47°C in 18 h, at 49°C in 4.5 h, and at 50°C in 3 h. At 50°C, insect mortality determined at 0 h was significantly different than that determined 12 h later after the insects had been moved to room temperature. A regression equation predicted insect mortality better than published models when temperatures were above 45°C. The net displacement of the adults in both vertical and horizontal directions at 27.5–52.5°C was determined in 100 by 100 by 1,000-mm wheat columns at 14.5 ± 0.3% moisture content with or without a 5°C/m temperature gradient. The adults responded to temperature gradients and the preferred temperature was from 30 to 36.5°C. There was no obvious boundary between preference and nonpreference temperatures for the adults. In horizontal wheat columns without a temperature gradient, the adults moved in both directions, and the distribution pattern gradually became more uniform when temperature increased but was under 42°C. At hot temperatures, adults could locate and move to the cooler area in <12 h; however, the adults could not move at 50°C. Geotaxis, temperature gradient, and the interaction between these two factors affected insect distribution and movement direction; and the geotaxis was more influential than temperature gradient at any condition in the vertical columns. A pattern for adult movement was suggested.

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