Grub-killers won't stop grass seed from germinating. They won't do a very good job of killing grubs either this time of year.

Will grass grow back after grub damage?

If you have patches of dead grass that show up in the fall or spring, they are likely caused by grubs. Grub counts from six to 10 or more per square foot can damage and kill grass. ... It is best to repair your grub-damaged lawn in the fall or early spring to restore it to its green glory. Remove dead grass with a rake.

Can I apply GrubEx to newly seeded lawn?

If your area is prone to grub infestations, you need to apply a slow-release grub killer near the beginning of June after the new grass seeds are tall enough for normal mowing. Because the grass blades are strongly rooted, they withstand the chemical application.