The physiochemical responses of stored grain insect pests to synthetic amorphous silica (SAS) powders

Xin Du

Fumigation is widely used for the disinfestation of stored grain products. Every loss of grain during storage is a loss of all the inputs that produced the grain in the first place. In many situations, fumigation is the only feasible process for pest control. Currently, phosphine is the only fumigant accepted by international trade for the disinfestation of grain and oilseeds. However, phosphine resistance now occurs worldwide and has challenged the continued use of phosphine in the grain industry. Food-grade synthetic amorphous silica (SAS) can act as a phosphine resistance breaker in storage systems. This thesis explored the mechanisms of SAS powder for controlling two phosphine-resistant stored grain insects, red flour beetle (Tenebrionidae: Tribolium castaneum (Herbst, 1797)) and lesser grain borer (Bostrichidae: Rhyzopertha dominica (Fabricius, 1792)). Grain protection during storage is essential. Both contact grain protectants and fumigants leave toxic residue issues to humans and the environment. The world wants residue-free grain, especially countries where grain is a substantial proportion of the diet. A high sensitivity headspace-solid phase micro-extraction gas chromatograph-mass spectrometer (HS-SPME-GCMS) method was optimised and validated to determine the residues of eight fumigants simultaneously, including phosphine, methyl bromide, cyanogen, sulfuryl fluoride, ethylene oxide, propylene oxide, ethyl bromide and ethyl formate. A 2 cm long 50/30μm divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS) coated SPME fiber was chosen based on its absorption performance. The food matrices included grain, oilseed, dried fruit, and nuts. The limits of detection (LODs) of the fumigants ranged between 0.03 to 1.99 ng/g. Responses to a range of diluted authentic standards gave significant (r2 > 0.9983) linear regressions and the relative standard deviations (RSDs) were ≤ 8.7% at the 3 ng/g level of aged spiking standard, except for sulfuryl fluoride with a LOD of 1.99 ng/g and an RSD value of 39.7% (6.64 ng/g). The performance of the HS-SPME-GCMS method was more sensitive than the gas syringe method for all fumigants, except sulfuryl fluoride. Due to residue issues, the world is increasingly demanding residue-free treatments. The main components of insects' cuticular lipids are hydrocarbon compounds. SAS powders may change the hydrocarbons on the cuticle, impacting an insect’s self-protection mechanism(s) against toxic gas chemicals, possibly by acting as a barrier between the insect and the surrounding phosphine environment. X-ray micro-computed tomography (Micro-CT) scanning of SAS treated and untreated T. castaneum indicated that the SAS powder penetrates the tracheal system of T. castaneum and potentially blocks it, leading to asphyxiation. Micro-CT 3D reconstruction model of R. dominica showed the internal body fluid was completely depleted and the internal organs shrank. Based on metabolomics, several energy metabolites and derivatives were found to alter after applying food-grade SAS powders to adult T. castaneum and R. dominica. Phosphine-resistant adults are known to downregulate or slow the consumption of energy substances to survive phosphine fumigation. Fortunately, the food-grade SAS powders accelerated the carbohydrate metabolism leading to the depletion of monosaccharides, and the blocking of the β-oxidation pathway causing the accumulation of free fatty acids (FFAs). The excess FFAs, including saturated and unsaturated FFAs, possibly induce the lethal toxicity of the fatty acids. The associated bioassay results show that hydrophilic (HL) SAS and hydrophobic (HB) SAS controlled the larvae and adults of T. castaneum and R. dominica; however, HB-SAS was more efficient than HL-SAS when the moisture content and relative humidity were high. HB-SAS stimulated T. castaneum to increase respiration and produce benzoquinones and derivatives, leading to its death within two hours of treatment. The respiration rate of the insects was monitored by Mass Spectrometry (MS), and varied with HL-SAS and hydrophobic HB-SAS treatments. Volatile organic chemicals were identified and quantitated from adult T. castaneum by headspace solid-phase microextraction gas chromatography-mass spectrometry (HS-SPME-GCMS). Three benzoquinone derivatives, ethyl p-Benzoquinone, methyl p-Benzoquinone, and ethyl 1, 3-Benzenediol, were increased significantly by 133.1, 43.1 and 41.9 folds, respectively. Importantly, these benzoquinone derivatives can be used as biomarkers to identify phosphine-resistant strains of T. castaneum two hours after SAS treatment by HS-SPME-GCMS. The smaller particle size allowed the two SAS dusts to pass through open spiracles during air exchange. Due to their light weight, SAS particles are carried along with airflow into the tracheole tubes, which lie within the haemolymph and internal tissues. Small amounts of biofluid in the tracheole tubes evaporated due to the SAS treatments leading to the overwhelming loss of oxygen and water near the muscle cells. Therefore, the irritation of the SAS powder particles provides high insecticidal efficacy, even against phosphine-resistant individuals. In conclusion, food-grade SAS powders kill phosphine-resistant insect adults, T. castaneum and R. dominica, by depleting sugar energy and inhibiting the β-oxidation of FFAs to energy substances. Consequently, SAS powders offer a viable, pesticide residue free alternative to phosphine for managing and eradicating stored product insects.

The physiochemical responses of stored grain insect pests to synthetic amorphous silica (SAS) powders

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