Sugar ants on a plant can also be a sign of scale or aphids. The ants seek out these pests and feed on the nectar exuded by the pests, so are an early warning sign of a problem. There are passive ant baits that you can use which have no impact on your plants. The worker ants collect the bait (which often contains Borax as the active ingredient) and carry it back to the nest and the queen, killing the whole colony. So yes, sugar ants are an early warning of a problem with pests.