Comparison of physiological and metabolic changes between phosphine resistant and susceptible strains of Rhyzopertha dominica (Fabricius) and Tribolium castaneum (Herbst)

The lesser grain borer, Rhyzopertha dominica (Fabricius) and the red flour beetle, Tribolium castaneum (Herbst) are serious insect pests of stored grain and a broad range of other stored products. Phosphine is widely used as a fumigant to protect the stored commodity from insect damage. However, phosphine resistance by stored grain insects has been occurred worldwide. The mechanism of phosphine resistance reported that it might include changes in respiratory and metabolic function. Lower respiration of the resistant insects may reduce the rate of phosphine entering insect bodies. Changes in the metabolism of resistant strain insects may prevent metabolic crises or supplies more energy. \n \nIn this thesis, the respiration rate, the emission of VOCs and lipid content of phosphine susceptible and resistant strains of R. dominica and T. castaneum were investigated. Two novel methods have been explored and validated, including the headspace solid phase microextraction (HS-SPME) and direct immersion solid phase microextraction (DI-SPME) technologies. The optimal conditions for both methods were investigated, and the optimized conditions were used for our study to determine the volatile organic compounds (VOCs) from insects and the insect cuticular hydrocarbons. \n \nThis thesis has confirmed that the respiration rate of the susceptible strain insects was significantly higher than the resistant strains. Phosphine can reduce the respiration rate of both susceptible and resistant strains when treated with average concentrations ranged from 2 to 20 ppm. However, the rate of respiration of the resistant strains was unexpectedly increased under a high level of phosphine (320 ppm) which indicated that resistant strain insect metabolism was elevated. \n \nMoreover, two characteristics were used as indicators to determine the metabolic differences between phosphine susceptible and resistant strains of R. dominica and T. castaneum including VOCs and lipid contents. The GC-MS response of most of the VOCs detected in susceptible strains of both species used in this study were higher than that from the resistant strains. However, 2, 3-Butanediol, [R-(R*, R*)] was significantly in abundance in the resistant strain of R. dominica; while dodecanal was only detected from the susceptible strain of T. castaneum. \n \nLipid contents including cuticular hydrocarbons, glycerolipids and phospholipids were analysed with different methods, such as cuticular hydrocarbons were analysed using our optimized (DI-SPME) method; glycerolipids and phospholipids were determined by HPLC. The results showed that resistant strains of both insect species containing higher levels of lipids than the susceptible insects. This higher content of lipids may have significant role in phosphine resistance. \n \nThis study concludes that respiration rate was found higher in the susceptible strain than the resistant; however, treating the resistant strain with a high concentration of phosphine caused an increase in the rate of the respiration. Some VOCs were found significantly different which indicated that metabolism is different according to the susceptibility of the strains. 2,3-Butanediol, [R- (R*, R*)] and dodecanal were suggested to be used as a biomarker to differentiate susceptible and resistant strains of R. dominica and T. castaneum respectively. The resistant insects of both species used in this study had a higher amount of lipids than the susceptible insects, shows that the higher lipid contents in the resistant strains may prevent the fumigant from entering the insect bodies, provide energy to the insect and participate in avoiding phosphine effect.