Caenorhabditis elegans research experiment problem. Can someone help me?

I'm a junior in high school and an advanced research student and I'm trying to test how light effects the aging of C. elegans. I plan to have two groups, one control group that lives in darkness and one variable group that lives in a lit environment. I intend on using a microscope that can take pictures to take close up pictures of each group once a day. I know the main symptoms of aging, but how can I prevent them from breeding since 99.95% of the population is hermaphroditic. Please give a detailed explanation if possible.