The list of insects and diseases that can attack your corn fields is long. Thanks to DuPont Pioneer, we have a great chart at which corn growth stages to scout for what. The roundup offers all the common bugs, worms and diseases and at which leaf and growth stage to scout. For example, look for seedcorn maggot and wireworms at VE to V3. Scout for Goss's wilt from V3 to V9.
Please note: This information is just a guide. It should not be the only factor in making decisions. You can access more information about the insects and diseases, as well as the calendar from DuPont Pioneer.