Why is the only British cicada so rare?
A Cicadas are warmth-loving animals, and the New Forest cicada Cicadetta montana is on the margins of its European range here. It most likely edged itself north after the retreat of the last ice sheet some 15,000 years ago, but probably before rising sea levels cut off the British Isles about 6,000 years ago. As in all boundary zones, low numbers and poor genetic diversity make such outlier colonies highly susceptible to random climatic changes, disease and parasites. The subterranean cicada nymphs suck root sap and take six to ten years to reach maturity, so, in contrast to insects that enjoy the resilience of yearly life-cycles, there’s plenty of time for reproductive success to be hampered. Adults are short-lived, surviving just a week or two in June or early July, again making them vulnerable to a poor summer.
The species has not been recorded in Britain for 20 years. If the population has crashed to a point where it is just not viable here, recolonisation from Europe may not now be possible. On the other hand, if these hot summers continue, any remnant colonies may find conditions improved enough to claw their way back from the brink.