In this study, we analyze security and quality of service (QoS) issues in cluster-based wireless sensor networks (WSN). Taking spatial resolution as the main QoS metric and limiting the security definition to data integrity and authentication, we present a control strategy to maintain desired QoS and security levels during the entire operation of a cluster-based sensor network. Besides, our proposed strategy considers some other WSN QoS attributes such as coverage, packet collision and system lifetime. It provides sufficient coverage by statistical means and minimizes packet loss due to collisions by employment of a slotted MAC (media access control) scheme. It also tries to maximize the operational lifetime of the sensor network by a power conserving scheme which make all sensors participate equally. In this study, we also determine the best tradeoff between security and spatial resolution.