Pesticides can negatively affect insect immunity. Although studies show that Formosan subterranean termites, Coptotermes formosanus Shiraki, are resistant to microbial infections, the effects of pesticides on disease resistance is not well studied. In this study, C. formosanus previously fed lufenuron was exposed to each of the three entomopathogenic bacteria, Pseudomonas aeruginosa (Schroeter) Migula, Serratia marcescens Bizio, and Bacillus thuringiensis Berliner subsp. israelensis. We found that termite mortality was significantly higher and synergistic in the combination of lufenuron and P. aeruginosa compared with treatment of lufenuron or P. aeruginosa alone. Other bacteria and lufenuron combinations were not quite as effective. Interestingly, only in treatments without lufenuron did termites show carcass-burying behavior. The results indicate that lufenuron, a chitin synthesis inhibitor, can suppress Formosan subterranean termite resistance to P. aeruginosa. Possible suppression mechanisms are discussed.