Radiofrequency heating for the management of rice weevil Sitophilus oryzae (L.)(Coleoptera:Curculionidae) and lesser grain borer Rhyzopertha dominica (Fabricius)(Coleoptera:Bostrichidae)

Abstract

India is the leading country in the export of rice. The infestation of storage pests, notably the rice weevil, Sitophilus oryzae and lesser grain borer, Rhyzopertha dominica causes a significant loss in the quantity and quality of stored grains. Phosphine use effectively manages stored pests, but insect resistance, residue concerns, strict import regulations, and rising demand for organic products highlight the need for chemical-free alternatives like radiofrequency (RF) heating to protect high-value commodities.. Hence, this study was carried out to assess the effectiveness of RF heating for the management of rice weevil, S. oryzae and lesser grain borer, R. dominica, and to study the impact of RF disinfestation on the quality parameters of rice grains. Eggs, grubs (15 ± 2 day old), pupae, and adults (one week old) of S. oryzae and R. dominica were exposed to RF dryer (40.68 MHz, 10 kW) at three different levels of electrode height (190, 195, and 200 mm) and conveyor speed (6, 8, and 10 m/h) and compared with an untreated control. The order of susceptibility among different life stages of lesser grain borer at 195 mm electrode height and 8 m/h conveyor speed was adult>grub>egg>pupa. S. oryzae was highly susceptible to RF heating, leading to complete mortality of all the stages except pupa at all the treatment combinations. The susceptibility of grubs, eggs, and pupae of R. dominica were comparable, whereas adults were highly vulnerable to RF heating. The mortality of immature stages was assessed based on the per cent decrease in adult emergence over control. Lowering the electrode height of RF dryer increased the mortality of all life stages of S. oryzae and R. dominica. Highest mortality of eggs, larvae, pupae, and adults was obtained at the lowest electrode height of 190 mm. Similarly, the mortality of eggs, larvae, pupae, and adults of S. oryzae and R. dominica increased significantly by decreasing the conveyor speed. The lowest conveyor speed of 6m/h significantly reduced adult emergence and recorded the highest mortality of all the developmental stages of S. oryzae and R. dominica. The analysis of the interaction effect of electrode height and conveyor speed of RF dryer demonstrated that the mortality of different developmental stages of S. oryzae and R. dominica increased with a decrease in electrode height and conveyor speed. There was no adult emergence when the eggs, grubs and pupae of S. oryzae were exposed to RF heating. Meanwhile, the adult emergence of R. dominica was completely inhibited only at the lowest conveyor speed of 6 m/h at all three electrode heights. RF heating resulted in the complete mortality of adults of rice weevil at all the treatment combinations. Lesser grain borer adults were more tolerant to RF heating, with complete mortality observed at all the electrode heights at a conveyor speed of 6 m/h and at the electrode heights of 190 and 195 mm at a conveyor speed of 8 m/h. All other treatment combinations did not cause complete mortality of R. dominica adults. Per cent weight loss was significantly reduced over control when R. dominica and S. oryzae eggs were exposed to RF heating. Meanwhile, weight loss was significantly reduced over control only when S. oryzae grubs were subjected to RF heating. The impact of RF heating on the quality parameters of rice grains was analysed after exposure to RF heating. Moisture, amylose, and cooking time of rice grains decreased with the decrease in electrode height of RF dryer. The moisture content at all the treatment combinations was significantly lower over control. Amylose content and cooking time did not vary significantly over control by RF heating at various combinations of electrode height and conveyor speed. Similarly RF heating did not adversely affect the colour of rice as there was no significant colour change over control after RF heating. Three combinations of electrode height and conveyor speed (190 -6 mm m/h, 195-6 mm-m/h,200- 6mm-m/h) resulted in the complete mortality of all life stages of S. oryzae and R. dominica. Also, there was no significant difference in amylose, cooking time, and colour values of rice grains at these treatment combinations over control. The lowest temperature recorded after RF treatment was 56.9°C in the 200 mm-6 m/h treatment combination. This indicates that a temperature of 56.9°C generated through RF heating can effectively control all life stages of S. oryzae and R. dominica by retaining the quality parameters of the rice grains. Hence, radiofrequency heating at an electrode height of 200 mm and a conveyor speed of 6 m/h can be commercially employed in the complete disinfestation of all the stages of S. oryzae and R. dominica without compromising the quality of rice grains.