In this paper, we propose a subnetwork key management strategy in which the heterogeneous security requirements of a wireless sensor network are considered to provide differing levels of security with minimum communication overhead. Additionally, it allows the dynamic creation of high security subnetworks within the wireless sensor network and provides subnetworks with a mechanism for dynamically creating a secure key using a novel and dynamic group key management protocol. The proposed energy-efficient protocol utilizes a combination of pre-deployed group keys and initial trustworthiness of nodes to create a level of trust between neighbors in the network. This trust is later used to allow secure communication between neighbors when creating a dynamic, high security subnetwork within the sensor network. Results of simulations of the protocol in Ns2 are presented and the complexity of the protocol is analyzed. The proposed protocol reduces delay by 50% and energy consumption by 70% over the existing dynamic group key management (DGKM) scheme.