The characteristics of mobile ad hoc networks cause a number of challenges to security design such as lack of infrastructure, shared wireless medium, stringent resource constraints and highly dynamic topology. Key management is a central aspect for security in mobile ad hoc networks. While central servers generate and distribute the keys in traditional wired networks, distributive key management systems are used in mobile ad hoc networks. Trust establishment and management are essential for any security framework of MANETs. Threshold cryptography has proved to be an effective scheme for key management and distribution. In this paper we propose a trust based threshold cryptography key management scheme for MANETs. In the proposed scheme, threshold cryptography supports security association establishment between mobile nodes in a web of trust. It enables the source and the destination nodes to successfully complete a certificate chain discovery even if the number of trusted nodes in the neighborhood of the requesting nodes is very low. The advantages of the proposed scheme are justified through extensive simulations.