I don't think the ants are the problem. It could be a root rot or some type of shoot canker disease. If anything has changed in the soil moisture of that area such as redirecting drainage, root gutters, etc. this could be a contributing factor. If there is any service in your area that provides diagnostic assistance this would be a good thing to do. It is not possible for me to give you a definite diagnosis and course of action without seeing the plants. However the progressive nature of the problem points primarily to some soil issue, most likely related to poor drainage or possibly damage from certain types of weed killers. Some times doing a major cut back on a plant and getting new growth is a good idea especially if the problem is canker related as cutting it back to the ground remove all topgrowth and thus the infection and results in very vigorous regrowth with fresh new shoots.
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