Good afternoon, When is the best time to apply GRUBEX
Charles County Maryland
It's getting late, but you still could.
You only should though if you need to, which would be finding 8-12 in a square foot of turned soil. There are many things that can cause grass to die back, not just grubs.
They are pretty large now, and it may be better to monitor next summer and treat then if you have high numbers.
Here is more info: https://extension.umd.edu/hgic/grubs-lawns