Periodical cicadas exhibit life cycles with durations of 13 or 17 years, and it is now accepted that large prime cycles arose to avoid synchrony with predators. Less well explored is how, in the face of intrinsic biological and environmental noise, insects within a brood emerge together in large successive swarms from underground during springtime warming. Here, we consider the decision-making process of underground cicadas experiencing random, spatially correlated thermal microclimates such as those in nature. Introducing short-range communication between insects leads to an Ising model of consensus building with a quenched, spatially correlated random magnetic field and annealed site dilution, which displays the kinds of collective swarms seen in nature. These results highlight the need for fieldwork to quantify the spatial fluctuations in thermal microclimates and their relationship to the spatiotemporal dynamics of swarm emergence.