Besides humans, the blue whale's enemies are killer whales and large sharks. Adult blue whales are much larger than any individual killer whale or shark, so these animals tend to attack calves. Few creatures are capable of subduing a healthy adult blue whale, which is the largest creature on earth. Blue whales feed mainly on krill.
Since blue whale calves stay close by their mothers, the strategy of the killer whale pod is to harass the mother and calf until they are exhausted. The killer whales then isolate the calf and prevent it from surfacing to breathe until the calf drowns. However, there have been cases in which adult blue whales have been badly injured by killer whale attacks. The killer whales can chase a blue whale for hours, surround it and even leap onto its back to defeat it. All the while, the members of the pod bite chunks out of the whale to weaken it.
A great white shark might approach a blue whale calf from behind or below to avoid detection. Very large sharks that are13 feet or longer tend to target marine mammals, such as blue whales, because of the nutrition to be had from their blubber.