Garlic due to its antimicrobial activity, has a high potential in controlling of compatible bacteria in gut of pest insects. Developing the use of these compounds can be effective in plant pest management. In this study, the effect of Allicin from garlic extract on reducing the population of compatible bacteria of two moths larva Plodia interpunctella and Ephesti kuehniella gut was investigated. Initially, the two pests were bred in sufficient numbers. The Larva were used to isolation of guts bacteria. The bacteria were isolated and grouped based on phenotypic characteristics and comparison of protein electrophoresis pattern. Larvae were fed with feed containing tetracycline for three days to minimize guts bacteria, than a suspension of isolated bacteria was added to the larva diet. A series of dilutions of garlic extract were prepared with ratios of 1:1, 1:2, 1:4, 1:8, 1:16, 1:32, then 50 microliters of each dilution was added to each well in the plate and the bacterial suspension added to it. The growth inhibition zone diameter was measured and the results showed that garlic extract has antimicrobial activity against all bacteria. Antimicrobial activity of garlic was dependent on concentration and the highest activity was observed in7.816 mg/ml concentration. Bacillus safensis and Bacillus zhangzhouensis showed the highest and Serratia marsecesence showed the lowest susceptibility to allicin garlic extract. The highest and lowest MIC were related to Bacillus safensis and Serratia marcescence respectively