Protection of grain storage from insect pests is an essential element of food security. Phosphine is an agricultural fumigant, used to protect stored grain from pest infestations. Global emergence of phosphine resistance in insects makes the understanding of phosphine toxicity and resistance mechanisms ever so urgent. Despite the widespread use of phosphine, the mode of action and mechanism of phosphine resistance, are still not well understood. In my thesis, I explore phosphine resistance mechanisms at three biologically different levels: The organism, the cell and the gene, and purpose models of phosphine toxicity and resistance from my findings. At the organism level, I used tissue specific expression of resistance genes to demonstrate cholinergic neurons play a key role in mediating phosphine resistance. At the cellular level, I show the specific subunit interactions of the phosphine resistance gene dihydrolipoamide dehydrogenase, with the pyruvate dehydrogenase complex, is responsible for phosphine resistance due to decreased oxygen consumption rates. At the gene level, I developed a novel genetic technique that allows identification of specific amino acid variants in the phosphine resistance genes that are responsible for their protective phenotype. I demonstrated the validity of my technique using three independent genes, two conveying phosphine resistance and one that does not. Overall, my findings suggest phosphine resistance in the genetic model organism Caenorhabditis elegans is caused by malfunctions in the cholinergic neuromuscular junction, characterized by decreased pyruvate metabolism, which can be caused by mutations to the subunit interaction domain or active site of dihydrolipoamide dehydrogenase. Thus, toxicity is proposed to result from over-activation of the cholinergic neuron-muscle junction. Insights from my research will be most beneficial for future development of supplementary treatments to enhance phosphine toxicity in resistant pests. Namely, my discovery suggests treating stored grain with phosphine and organophosphates or carbamates can increase fumigation efficiency and decrease resistance development. Fumigators who smoke or use nicotine-containing smoking alternatives should be cautious of the strong synergistic effect of phosphine and nicotine, which can endanger them in phosphine concentrations they believe to be safe.