Dielectric properties of bean weevil, grain moth and their hosts (common bean and amaranth) using the resonant cavity technique

Abstract. Dielectric properties are important parameters to develop radiofrequency or microwave disinfestation treatments, with potential to replace the use of pesticides in the globe. These properties are useful to estimate the penetration depth and heating uniformity in the proposed treatments. In Mexico, bean weevil (Acanthoscelides obtectus Say) attacks common beans (Phaseolus vulgaris L.) during their storage. Angoumois grain moth larvae (Sitotroga cerealella Olivier) have been observed in amaranth seeds (Amaranthus hypochondriacus L.) and also in pop amaranth seeds during storage. Thus, the dielectric properties of bean weevil, grain moth and their hosts (common beans and amaranth seeds) were determined. The employed method was the resonant cavity at 2450 MHz, a high precision technique, adequate for samples with low losses. The cavity was designed and built in our lab. Alive insects or grains were milled and introduced in a plastic pipe (6 mm) and placed within the aluminum cavity. Dielectric constant (e’) and loss factor (e”) of the samples were determined at 20^oC. Adults of bean weevil had e’=3.91 and e”=2.89, while Negro, Bayo, Flor de Mayo beans had values of 2.60-2.77 and 0.51-0.60 for e’ and e”, respectively. Grain moth larvae had e’=12.74 and e”=1.90, while their hosts, amaranth seeds (e’=2.81 and e”=0.54) and pop amaranth (e’=1.33 and e”=0.22) had lower values. Dielectric loss factor of insects is clearly higher than those for beans or amaranth seeds. This suggests that differential heating will occur in further microwave treatments, reaching higher temperatures in the insect and lower temperatures in the grains.