Find matches between lists

I have 2 lists of data, one in column A and the other in column B and neither list is any specific order. I'm looking for a way to search Column B for a match to each piece of data in Column A and then use conditional formatting to highlight the cells in Column A that did not find a match.

Example:
A B
1 tree flower
2 grass tree
3 water dirt
4 flower rock

I'd like for cells A2 and A3 to use conditional formatting to say have a red background because they do no have a match in Column B. I'm OK with how to do conditional formatting, just having trouble getting the formula working within the conditional formatting rule setup.

Thanks for your help.