Merit is the trade name for the chemical, imidacloprid. It is manufactured by Bayer and used as the active ingredient in various grub control products. It is available in both granular and liquid form. I have only used the granular form, purchased under the name of Bayer Advanced Lawn Grub Control. I'm not sure that the liquid is called.

Check with your local farm service (feedstore) or garden centers. You might even find it in the garden dept of the large retailers like K-Mart, WalMart, Lowe's, etc. I have to special order mine through my feedstore. Just make sure that the product you use contains ONLY imidacloprid and no other active ingredients.

I would use it now. I don't think it is too late. Emerging borers are almost microscopic, and they continue to emerge in waves.I have not yet done mine. Should do it within the next week or so however.
Laetitia