what would mothra have to eat in order to become an adult?

Interesting idea, but I'm not sure the environmentally savy Mothra would devour ancient redwoods as a food source.  That said, I'm not sure where else whe could get food!  Sea life, perhaps?  Maybe she absorbes energy like Godzilla absorbs radiation!  Who knows?  Only Mothra, and she ain't talkin'... :)

I believed her to be a plant eater that only rarely eats meat and usually she would scavenge off a dead carcass.

I think she migth eat the plants the natives use to make the ceremonial drink

Who said she had to eat anything to become an adult? She didn't have to eat anything during 1961 to become an adult nor in any other godzilla movie.

well the new movies are going to be a lot diffrent compared to the older ones i imagine so i assume mothra will be somewhat different in the way she acts. more similar to an animal 

DINOBOY22 - That's a very neat idea! I believe it could be feasible for Mothra to dine on massive plant life in the event she requires material food to reach maturation. What an excellent hypothesis! :)

I agree, Mothra probably won't be a God-like creature in the second film. So it's either this or she absorbes radiation.