Living rain gauges: cumulative precipitation explains the emergence schedules of California protoperiodical cicadas

Mass multi-species cicada emergences (broods) occur in California with variable periodicity. Here we present the first rule set that predicts the emergence of protoperiodical cicada communities. We tested two hypotheses with a dataset consisting of direct observations and georeferenced museum specimen records: first, that cicada broods are triggered to emerge by periodic ENSO events and second, that brood emergences occur after precipitation accumulates above a threshold value. The period of ENSO events does not explain the observed pattern of cicada brood emergence. Rather, broods emerged given two conditions: (1) that total precipitation exceeded a threshold of 1,181 mm, and (2) that a minimum 3-yr period lapsed. The precipitation threshold is obeyed over an 800 km north-south distance in California and across a variety of habitats. We predict the next brood emergence at one study site in arid Los Angeles County desert foothills to occur in 2020 or, if drought conditions continue, in 2021.