Is it worth the risk to apply to REUs? How likely am I to be accepted? I am applying to biology REUs(Research Experience for Undergraduates) this summer. Would a white female junior from a small R2 university, 3.9 gpa, and one summer of research experience be likely enough to get into one these programs? I've heard that it's a long shot if you're not a minority, the issue is while female counts as minority for other REUs I don't know if it counts much for bio these days...
I'm asking because there's a very very good study abroad program (a very good university to study at for bio) I want to apply to that goes until June (which overlaps with 99% of REUs and other internships). So I either pick the top study abroad program and simply do research at my university over the summer or get a job, or I pick my second choice study abroad program in order to apply to REUs (as well as other internships such as NOAA), but risk not getting into any of them anyway and going right to my backup plans for the summer.
I don't know where you are getting the idea that it is a longshot specifically for non-minorities. In general, REUs do have low acceptance rates for everyone. There are a lot of students who want to do them and not that many REUs. 

Now, to answer your question: yes, being female will likely help somewhat, but it likely won't make a big difference. What will matter more is the GPA which will help out a fair bit. The previous research experience may hurt or help; at least in math some REUs try to admit undergrads without experience to spread them around, but that's not all of them  by any means, and for the most part it would be helpful. I suspect the situation in biology is similar.

Since you are a junior, you should already have some connections within the biology department, either your adviser or faculty whose classes you have taken. They can likely give you more personal advice since they know you better. But at least from what you've said to us, I suspect you have a higher chance of getting admitted to an REU than the average candidate.