Delay Tolerant Network (DTN) is developed to cope with intermittent connectivity and long delay in wireless networks. Due to the limited connectivity, DTN is vulnerable to blackhole and greyhole attacks in which malicious nodes intentionally drop all or part of the received messages. Although existing proposals could accurately detect the attack launched by individuals, they fail to tackle the case that malicious nodes cooperate with each other to cheat the defense system. In this paper, we suggest a scheme called Statistical-based Detection of Blackhole and Greyhole attackers (SDBG) to address both individual and collusion attacks.