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. 2020 Mar 23;10(1):5197.
doi: 10.1038/s41598-020-61902-6.

Physical Assessments of Termites (Termitidae) Under 2.45 GHz Microwave Irradiation

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Free PMC article

Physical Assessments of Termites (Termitidae) Under 2.45 GHz Microwave Irradiation

Aya Yanagawa et al. Sci Rep. .
Free PMC article

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 Fe3+, Cu2+, Mn2+, 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.

Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1
Figure 1
Patterns of microwave energy transduction in the insect body under two irradiation conditions. (A) Mortality of the termite under each microwave irradiation condition. (B) Energy absorption of an insect body under the irradiation with a resonating wave. (C) Energy absorption of 30 insects (80.5 mg) under the irradiation with traveling waves. Vertical bars represent standard errors (SE).
Figure 2
Figure 2
Microwave irradiation device to create resonance condition. (A) Device connection, (B) Device photo. Arrows indicate the microwave direction.
Figure 3
Figure 3
Temperature of arena during 10 measurements under microwave irradiation with traveling waves. Vertical bars represent standard errors (SE).
Figure 4
Figure 4
ESR spectra of the termite, C. formosanus. *1 indicated the spectrum of iron (Fe3+). *2 indicated the spectra of manganese (Mn2+). The S-shape curve over Mn spectra indicated the presence of cupper (Cu2+). (A) ESR spectrum of the termite, C. formosanus (34.5 mg). (B) ESR spectrum of kim towel (control, 47.9 mg). (C) Interactions with microwave suggested by ESR spectrum. If the relative intensity increased proportionally, it suggested that there was no interaction with microwaves. The intensity at −150 °C was set as standard (=1) for each paramagnetic substance to compare the intensity with other temperatures. (K) temperature in kelvin unit, (D) Relative intensity of paramagnetic substances at −150 °C when Fe3+ was set as standard (=1) to compare the intensity with other paramagnetic substances. The spectra at −166 was used as measurement was at this temperature.
Figure 5
Figure 5
ESR spectra of organic radicals under 2.45 GHz microwave irradiation. g-value indicates the dimensionless quantity that characterized the magnetic moment and angular momentum of a composite particle, a particle, or nucleus. (A) ESR spectra of organic radicals showing the increase of free radicals in the termite, C. formosanus (815.2 mg) under the microwave exposure of 2.45 GHz at −8.5 dBm (black) and 0 dBm (control, grey) (B) ESR spectra of organic radials showing no increase of free radicals in the control substance, kim towel (47.9 mg) under the microwave exposure of 2.45 GHz at −8.5 dBm (black) and at 0 dBm (control, grey).
Figure 6
Figure 6
Travelling wave irradiation device with reflection. (A) Device connection, (B) Device photo, (C) photo of samples in monitoring device. Arrows indicate the microwave direction.

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