Biofilms are multicellular structures, cells immobilized at a substratum, aiding in their colonization of hosts, efficient nutrient capture, and expression and enhancement of toxins. Biofilm establishing is important to a successful gut microorganism. We found Candida corydali is a dominant yeast in the gut of four Euploea spp., migratory butterflies. They are nectar feeding insects, which is a good model system for the study of gut’s microbiome, due to the simple eating patterns comparatively. We build an easy biofilm population measuring method, to test how biofilm formation affected by environmental conditions. The aim is to observe the role of killer effect as interference competition on gut’s microbiome and on biofilm formation of sensitive yeasts. We found 21 strains of C. corydali forming biofilm. The optimal biofilm forming temperature of 13 and 8 strains of C. corydali is 16-24°C and 24-32°C, respectively; 6 sensitive yeast species is 16-24°C. Compare with sterilized cell-free culture filtrate control treatment, cell-free culture filtrate of two killer yeast strains decreases to 10% biofilm cell population of two sensitive yeasts in 24 hours. We prove that extracellular killer toxins of C. corydali providing the interference competition ability to inhibit the biofilm formation of sensitive yeast. Our study suggests that C. corydali enters gut from nectar, colonizes, forms biofilm, and produces killer toxins to inhibit the biofilm-forming of other yeast, to be dominant yeast in the gut of nectivorous insects. Interference competition is the key to establish success population in the gut.