Laboratory assessment of the influence of various structure materialson the mortality of three cereal-flour insect pest species after variousexposure intervals to a temperature of 50°C

Abstract: A laboratory test was conducted whereby three species of common insect species incereal processing plants of different body size (Tenebrio molitor, Oryzaephilus surinamensis andTribolium confusum), maintained on three different structure material (galvanized iron, thin woodplate, and ceramic tile), were exposed to a fixed temperature of 50°C during increasing exposuretimes. During heat exposure, the temperature at the surface of each material was monitored byflat shape temperature sensors. The insect mortality was assessed on four replicates of 25individuals per kind of material immediately after the insects were withdrawn from the 50°Cheatedchamber at the end of the exposure time. The definitive percentage of killing wasconfirmed after 48 h incubation in optimal conditions for insect survival. The mortality ratevaried significantly both between the three species and between the different support materials.The lethal exposure was reached first on galvanized iron support, more slowly on ceramic tile,and very slowly on wood plate. The yellow mealworm (adult stage) was observed the mostsensitive species to the heat treatment, preceding the saw-toothed grain beetle and the confusedflour beetle. For this last species, a large part of adult stage population (about 50%) exposed to50°C in the present experimental conditions was not killed by prolonged exposure time whenmaintained on the wood plate support. It was confirmed by temperature records that thetemperature at the surface of wood plates was about one °C less than the temperature at thesurface of the two others materials. Consequently, it is emphasized that even though 50°C isconsidered as the lowest temperature level to be achieved in flour mill heat treatment schedules,it seems insufficient to kill the less sensitive insect species when crawling on a woody surface.These results indicated that i/ for an economical heat disinfestation schedule of flour mills,temperature must be carefully measured and controlled on the structural material with the highestinsulating capacity (such as wood or brick); ii/ the preliminary identification of the current insectspecies is of prime importance to determine its intrinsic heat tolerance; iii/ the relative level oftolerance of all the target insect species to be killed by heat treatment must be well known. Tofeed this later requirement, a new laboratory test enabling a rapid discrimination between insectspecies and even strains was studied in order to accurately determine upper lethal temperaturelevel (see related communication in these proceedings).