The research into moth-based B.O.W.s was a failure in this experiment. Mutating to a larger size normally did not keep them in proportion, making their wings strong enough only to hover over the ground. However, they did also develop the ability to spit poison on prey as a means of compensating for their flight problems.
If you aren't playing with Yoko, use the shotgun against it because you will be captured and poisoned if the moth isn't killed. Yoko also needs to kill the moth in harder difficulties because the poison drains your health quickly.