When is the best time for me to apply Merit insecticide around my Rose bushes and shrubs in the Mt.Airy, Md area
Carroll County Maryland
You did not mention the insects that you are trying to manage on your shrubs.
There is a new Pollinator Protection Act of 2016 which went into effect January 2018 that restricts the sales and use of neonictinoids. Merit, imidicloprid is a neonictinoid pesticide. Only farmers and certified pesticide applicators (or people working under their supervision) can apply neonicotinoid pesticides outdoors. So while neonicotinoid products may appear on store shelves in Maryland they cannot be applied outdoors by gardeners. Many bee species and other pollinators have suffered population losses in recent years. Here is more about it and the law http://extension.umd.edu/hgic/topics/pollinators-and-pesticides
You can let us know what insects and the shrubs you are dealing with so we can give you more information. Always select the least toxic control if you decide to use a pesticide. It is possible you may be able to use cultural controls.