Index modulation is applied to cluster-based wireless sensor networks (WSNs) in this study. The aim is to design an efficient decision gathering scheme for target-detection cluster-based WSNs. The proposed scheme implies that only a single node reports all the local decisions from cluster-members toward the central entity. Unlike conventional clustering, the reporting node is not fixed as it varies based on the obtained local decisions. Consequently, the local decisions of all cluster members are divided into two groups. The first one is modulated and the other one determines the index of the reporting node. The decision error rate of the proposed scheme is analyzed and an upper bound is derived. The derived bound is shown to precisely match simulation results at pragmatic signal to noise ratio values. Furthermore, the detection threshold for the global decision is optimized for minimizing the decision error rate, and formulated in a closed-form expression.