Bread-Bakers v107.n020.1

I agree with Bob; the most effective way to keep fruit flies out of 
your sourdough starter is by covering it with cheesecloth. However, 
to deal with the fruit flies try the following:

To get rid of pesky fruit flies take a small glass fill it 1/2-inch 
deep with Apple Cider Vinegar; add 2 drops of dishwashing liquid and 
mix well. You will find those flies drawn to the glass and gone forever!