The objective of this paper is to develop an index that indicates how and when flocking agents become disconnected from a swarm. Each flocking agent interacts with nearby neighbor agents and determines its motion relative to the positions and velocities of the neighbor agents. The neighbor relations are dynamic and it is difficult to predict an exact motion for each of them. Our objective is to consider the effect of a target agent on a swarm. First, we model the flocking in terms of a virtual spring and damper system and develop a disconnection index between the target and another agent using the model dynamics. We then consider the degree to which the target agent can affect a group of agents using a graph topology. We evaluate the index via numerical experiments.