Answer: Lettuce is not included in the lists of plants usually attacked by root maggots or seed corn maggots, however these pests do potentially attack a wide range of plants. They can be more prevalent during periods of cool damp weather, so to some extent it may be weather related. I would strongly suggest you work with your local county extension to determine if in fact that is what damaged the lettuce and decide how to proceed.
Here is some more detailed information on maggots with several suggestions for control -- including simple methods for exclusion. (You may need to cut and paste the complete url into your browser to make it work correctly.)
and information on the seed corn maggots including controls
I'm so sorry you are having this problem.
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