Proper etiquette for withdrawing from an undergraduate research lab? I am currently involved in two very different Biology research labs in two different areas at my university, as an undergraduate volunteer. Lab A has been a long-term commitment of mine, and Lab B is a newer addition to my schedule but also a valuable experience, I think, but I have a choice coming up:

Before I graduate, I must complete one (and only one) senior capstone/thesis/etc lab project, that will almost certainly take up all the time I can dedicate to research outside of my regular coursework. I originally sought out and joined Lab B because it was looking like Lab A would not be able or willing to advise a capstone project, but now that is no longer the case - so I want to complete all my senior work at Lab A.

How do I politely tell Lab B that I do not plan to complete a capstone there after all, and probably won't even remain a volunteer in future semesters? (I don't recieve any payment or grades from them, but I know it does take time and effort to train new lab members so I feel bad about leaving without contributing a paper or presentation or something.)
Just tell them, politely and with appreciation for what they've offered you. Sooner rather than later, so that they can plan for your departure and maybe have you prepare a useful contribution in your remaining time there - wrap up whatever you've been doing, document your work, train another student to continue your project, or whatever else they need.

It is understood that students are balancing a lot of competing interests and opportunities; as long as you handle yourself professionally (give plenty of notice, remain engaged and work hard during your remaining time there even though you know you won't continue, etc.), there is nothing to feel bad about.