Control of three stored-product insect species in wheat using superheated steam and hot air

Abstract

Various heating methods have been studied to control stored-product insect pests. The present study compared the effect of superheated steam (SS) and hot air on the germination of wheat and mortality of three important species with adult insects outside kernels; Tribolium castaneum (Coleoptera: Tenebrionidae), Cryptolestes ferrugineus (Coleoptera: Laemophloeidae), and Sitophilus oryzae (Coleoptera: Curculionidae), and mortality of adults and immature stages of Sitophilus oryzae inside kernels. Thirty adults of each species or 30 infested wheat kernels mixed with sound wheat kernels at 12.5, 14.5 and 16.5% moisture content were subjected to 105 ± 0.3°C hot air or SS both at velocity of 0.7 m/s with different treatment times (1 to 90 s in hot air and 1 to 3 s in SS). The mortality of adults placed outside kernels was evaluated at 0, 24, and 72 h. Lethal time values were estimated by using probit analysis. In both processing media, insect survival and seed germination decreased with increased treatment time and there were no significant differences among survival of adults outside of kernels evaluated at various times. The mortality for insects outside and inside of kernels was higher at lower initial moisture content of wheat kernels in hot air, whereas initial moisture content of wheat did not influence mortality in SS. Adults of S. oryzae outside kernels had higher mortalities than adults inside kernels in both media. In hot air, all the adults of the three species outside kernels had 100% mortality when the treatment times were 60 s for 12.5 and 14.5 % initial wheat moisture content, and 75 s for 16.5%; whereas for SS treatment, the time to reach 100% mortality was 1 s at any moisture content. The adults and immature stages of S. oryzae inside kernels required 90 s to reach 100% mortality in hot air, while 3 s was needed in SS at any moisture content. Overall, in hot air, T. castaneum and S. oryzae adults outside kernels had similar heat intolerance, while the adults of C. ferrugineus were more heat tolerant than the other two species. However, there was no significant difference in heat tolerance of three species in SS. Mortality among adults and immature stages of Sitophilus oryzae had lack of clear cut differences in hot air treatment, however pupae were found to be most heat tolerance. In SS, similar heat intolerant of different stages of insects was observed. The treatment times to reach 100% mortality of insects caused a drop in seed germination of 20% in SS and 81% in hot air. Therefore, SS at 105℃ could be used to control insects, as hot air was impracticable. Saturated steam will also have similar results of SS at 105°C because of condensation.