One of the most challenging issues in Mobile Ad-Hoc Network (MANET) is that the insiders are untrustworthy in packet forwarding stage. All the nodes which loss data-packets over a preset threshold can be generalized as Suspicious Insiders. To intimidate these suspicious insiders and reduce packet dropping, surveillance is the most direct and easiest method. In this paper, we propose a novel surveillance mechanism to monitor the packet dropping of suspicious insiders. On one hand, our work provides the monitor with optimal strategies which will maximize its long term utility; On the other hand, our work presents an on-demand monitoring scheme which will balance the tradeoff between security and resource consumption. First, we utilize a reputation scheme to distinguish suspicious insiders from legitimate members, and quantify the threat level of the suspicious insiders. Then taking into consideration security and resource consumption, we utilize game theory to analyze the interaction between monitor and suspicious insider. Finally, optimal mixed-strategy is computed to identify the best way for the monitor to respond to the suspicious insider.