How do cicadas know what season it is?

Episode Summary

Executive Summary: The episode explains how cicadas know when to emerge and sing each year or after long underground cycles. Using experts from Spain, the US, and Japan, it shows that cicadas likely track seasonal changes through plant xylem sap, then use temperature and moisture as final emergence cues, though the exact internal counting mechanism remains unresolved.

Main Topics: Listener question and cultural significance of cicadas (Priority: 5/5): Ryusuke from Japan asks how cicadas know the season, noting their strong association with summer, media soundtracks, and worry when they appear late. Cicada biology and life cycle (Priority: 5/5): The program explains what cicadas are, how males sing, and how nymphs spend years underground feeding on plant roots before emerging to mate. Field research in Spain on annual cicadas (Priority: 4/5): Pere Ponch describes Catalonia’s cicadas, how scientists collect shed skins to monitor populations, and how local species emerge across spring and summer. Environmental triggers for emergence (Priority: 5/5): Temperature is shown as a key short-term signal, with soil warmth indicating the right season to come up; moisture can also matter in some regions or species. Xylem sap as a clock for long-term timing (Priority: 5/5): Research suggests cicadas may track annual cycles through changes in plant xylem sap, with experiments on peach trees altering cicada emergence timing. Open scientific questions about internal counting (Priority: 4/5): Scientists suspect cicadas may use biochemical markers, possibly involving DNA modification, to count years, but the precise mechanism is still unknown.

Key Arguments: Cicadas do not simply sense summer directly; they likely use a combination of cues, including plant chemistry, soil temperature, and moisture. Annual cicadas in places like Japan and Spain emerge reliably in warmer months because underground conditions gradually shift toward summer. The xylem sap of host plants changes seasonally, and cicadas feeding on it may use those changes as a time signal. Rick Carbon’s peach-tree experiment suggests cicadas can be tricked into emerging a year early if the tree goes through two annual nutrient cycles in one year. The exact yearly counting mechanism is still unknown, but scientists suspect an internal marker system, possibly linked to DNA modifications. Cicada shed skins can be used as a practical scientific tool to estimate abundance and compare populations across time and space.

Data Points: Cicada species worldwide: More than 3,500 - Professor Chris Simon describes global cicada diversity Continents inhabited: All continents except Antarctica - Cicadas are nearly global in distribution Catalonia cicada species: 13 - Pere Ponch says 13 species occur in Catalonia Common Spanish species season: May to September - Cicada orni heard across this period Peak season in Spain: July and August - Most abundant species peaks in midsummer Underground duration: 4 to 10 years - Annual cicadas studied in Spain spend this long below ground Periodical cicada cycles: 13 or 17 years - North American Magicicada emerge after fixed long cycles Soil temperature observed: 10–11 degrees Celsius - Measured underground in Girona during research visit Soil temperature expected for emergence: 20–25 degrees Celsius - Warm soil indicates summer emergence conditions Rainfall threshold mentioned: 1181 millimeters - Some periodical cicadas reportedly require this amount of rain to emerge Experiment outcome: 16 years vs 17 years - Cicadas on manipulated peach trees emerged after 16 years; controls after 17 Two annual cycles in one year: 2 - Peach trees were forced through two cycles to test emergence timing

Pivotal Quotes: "How do sikeras know what season it is now?" — Ryusuke: The listener’s central question about cicadas and seasonal timing "We think there could be some kind of internal marker that accumulates over time." — Chris Simon: Hypothesis for how cicadas count years underground "When the sun is hitting the soil, reaches a certain temperature, then they know they have to emerge." — Pere Ponch: Explanation of the immediate environmental trigger for emergence

Implications: The episode suggests cicadas use multiple environmental signals and may rely on plant chemistry to count years, but the full mechanism is still unsolved. This leaves room for future research on insect timing, plant-insect interactions, and climate change impacts on seasonal life cycles.