The document proposes iTrust, a probabilistic misbehavior detection scheme for secure routing in delay-tolerant networks (DTNs). iTrust introduces a periodically available Trusted Authority (TA) that judges nodes' behaviors based on collected routing evidence and probabilistically checks nodes. The TA models iTrust as an inspection game and sets an appropriate investigation probability to ensure security at reduced cost. Detection probability is correlated with node reputation, allowing a dynamic probability determined by user trust. Analysis and simulations show iTrust effectively and efficiently detects misbehavior.