Abstract
Demands for chemical-free treatments for controlling insect pests are increasing worldwide. One such treatment is microwave heating; however, two critical issues arise when using microwaves as a heat source: intensive labor and excessive energy-consumption. Optimization is thus required to reduce energy consumption while effectively killing insects. Currently, the lethal effect of microwaves on insects is considered to be due to the temperature of the irradiated materials. This study examines how the conditions of irradiation, such as resonance or traveling mode, changed the conversion of electromagnetic energy into heat when 2.45 GHz microwaves penetrated the body of the termite, C. formosanus. Our results indicated that it is possible to heat and kill termites with microwaves under resonance condition. Termites were however found to be very tolerant to microwave irradiation as the permittivity of the insect was low compared with other reported insects and plants. Electron spin resonance revealed that termites contained several paramagnetic substances in their bodies, such as fe 3+ , cu 2+ , Mn 2+ , and organic radicals. Interestingly, irradiation with traveling microwaves hardly produced heat, but increased the organic radicals in termite bodies indicating non-thermal effects of microwaves. The use of electromagnetic waves such as microwaves and radio frequency waves (RF) for pest control through heating is not fundamentally new and was first tested and reported in the 1930s 1. These authors reported a possible mechanism through body water contents and biological factors 2 ; however, results were speculative. RF can penetrate non-metal substances efficiently, and thus, high-water content products like fruits tend to be treated with RF. Microwaves are also effective for producing heat energy in a short time and are thus applied to low-water content products such as rice or cereals 3-6. Many reports support the use of microwaves in controlling pests including termites 7-14. Although microwaves allow efficient and rapid heating deep inside treated materials, the high-energy consumption required for optimal results is a major drawback 15,16. Microwave heating has been traditionally used in the spot control of dry wood termites as they are classified as a single-piece infester of timbers 17. However, this treatment method has had mixed results and some damage to test boards was noted 10. Microwave heating has been characterized as a non-chemical method for insect control. Its mode-of-action is based on the dielectric heating and conversion of absorbed electromagnetic energy to thermal energy throughout the irradiated substrate 17. An advantage of microwave irradiation is the rapid temperature increase in the interior of the irradiated material regardless of its thermal conduction properties. Energy consumption for heat treatments has the potential to be considerably reduced when using microwaves 4. During the last decade, several studies have supported the efficiency of microwave heating of wood 18-25 and wood-invading insects 7,10,11,26. However, some other studies reported reduced or no lethal impact on pests 24,27,28. According to Nakai et al. 27 , direct exposure of the termite body to microwave radiation did not show a lethal effect. The high temperature of the timber obtained through microwave irradiation killed wood pests 24,28. To be lethal, the complete heating of the relevant material is required up to a minimum temperature of 55 °C for a duration of at least 60 min. Owing to the low heat conductivity of wood, the heating of its interior is relatively time intensive. However, in the treatment of agricultural products, pest insects generally die in much shorter time frame 2,29. It may be possible that factors