Spotted Wing Drosophilia is a very small and troublesome pest. Scouting or monotoring insects is one of the few and best ways to know if you do or don’t have a pest. Once these counts are taken you can find out if the economic threshold has been met or guage the effectiveness of your control.
I have based these traps off of Michigan State University studies. I have chosen a red cup and have drilled 1/32 sized holes at the top, thus preventing any larger insects from entering the trap. There are a few different bait solutions out there to use. I’ve had good luck with yeast based solution. Checking the traps weekly and rotating the bait is very important.