Effects of soil and larval sterilization on the mortality of white grubs in bioassays

Laboratory and greenhouse bioassays of soil insects often require soil as a bioassay media in addition to insects. Soil and insects used in such assays are often collected from the field and thus may be infested with various entomopathogenic microorganisms that may affect the results of the bioassay. Sterilization of soil and insects to be used in soil bioassays could help mitigate such problems. Using white grub soil bioassays as a model, we carried out a greenhouse study to determine if the sterilization of soil and insects had an effect on percent mortality of the white grubs. Field soil was sterilized at 72°C for 3 hr in a steam chamber. Mature third instar grubs were surface sterilized by dipping and rinsing them in 70 % alcohol, distilled water, 1 % sodium hypochlorite and distilled water. Our study showed that soil sterilization significantly reduced grub mortality by 20 % compared with unsterilized soil. However, surface sterilization of white grubs did not have a significant effect on grub mortality. This study suggests that use of sterilized soil could help reduce the high control mortality of white grubs in bioassays.