A rapid laboratory method for comparative assessment of heat toleranceand lethal upper temperature level for different stored-product insect species by thermo-respirometry

Abstract: The effect of a fast temperature increase on the physiological condition of four insectpest species of grain and cereal processing plants (Sitophilus zeamais, Rhyzopertha dominica,Tribolium castaneum and Tribolium confusum) was evaluated through the monitoring of theirmetabolic rate during a temperature increase from 25-28°C (optimal temperature) up to 60°C, i.e.beyond the lethal temperature. The criterion used for the quantification of metabolic rate was thecarbon dioxide release rate by insect groups (200 individuals) in using an open-flow microrespirometer.The definite changes observed along the curve of CO2 released by the insectsduring a progressive heating process at a rate of 0.5°C min-1 enabled to accurately determine thetemperature levels at which significant events occurred corresponding with a knownphysiological foundation. This test enabled to accurately determine remarkable physiologicalchanges: the spiracle closing temperature, the heat stupor coma temperature (catharsis), and thelethal temperature. The different patterns of the CO2 release rate were established in averagingseveral replicates of the heating procedure with each species. The patterns obtained with differentspecies, stages or strains were compared and discussed. Given our experimental conditions, themost heat tolerant species was the lesser grain borer R. dominica for which the mean lethaltemperature was observed at 53.35°C. For S. zeamais, the lethal temperature (LT) was observedat 48.57°C and 49.98°C for a Portuguese and a French strain, respectively. The LT of the redflour beetle (T. castaneum) was observed at 49.83°C, when it was determined at 47.82°C for theconfused flour beetle (T. confusum). Additionally, a very different pattern of the thermorespirometriccurve between these two related species was observed: T. confusum displayed aregular increase in the CO2 production rate according to the temperature increase, while inT. castaneum the CO2 release rate pattern changed considerably at different time intervals. Inevery case, the capacity to develop a physiological reaction (especially in the capacity to closethe thoracic spiracles when oxygen uptake reached a potentially injurious level) to heat increaseobserved for the three “true” tropical species (S. zeamais, R. dominica and T. castaneum) mayprobably be the consequence of a more rapid start in the biosynthesis of heat shock proteins thanfor the last species (T. confusum).