Well, there might be an ants nest at the base, in which case the plants might suffer when the hot weather comes. Or, more likely, there may be aphids on your raspberries - if so you will see the shiny, sticky honedew on the leaves - and the ants are coming to feed on the honeydew. You can blast off the aphids with strong jets of water. This early in the year is a good time to catch them, once you spray them off with water, the ants won't bother coming.
