Op: unless you have a major major infestation, i doubt they could eat enough to make a noticeable dent in efficiency, but that's a complete assumption.
I have a similar situation so I'll add on to this recent thread instead of starting my own. I just brewed my first batch in exactly a year.
I had about a whole sack of grain sitting in the garage during that time and when I went to measure out my base malt i noticed small bugs crawling in it. I guess they're weevils?
Most reports on these forums say that they're not harmful, most food products contain acceptable amounts of animal/insect parts, and the boil sterilizes everything (duh).
But I can't get over the fact that it's gross. There wasn't a major infestation or anything, but what do most people do? Do most of you throw the buggy grain away? Toss the batch? Or brew on?