Cotswolds Blues

1st July 2018
Daneway Banks in the Frome Valley

The large blue butterfly is not an obvious conservation icon; it is not a great looker nor is it very large or showy as it dashes low over chalk grasslands usually to settle inconspicuously. But the large blue presented one of the greatest and longest of wildlife conservation puzzles. Why for most of the 20th century had it steadily declined to extinction, finally exiting the stage in 1979 despite great efforts to stop it taking its final bow? That belligerent refusal to respond, in retrospect to sometimes misguided conservation actions, was driven by a lack of appreciation of its complicated breeding ecology and consequently how to manage its habitat.

Two great lepidopterists almost cracked the secret life of the large blue at the start of the 20th century; F W Frohawk and Dr T.A. Chapman both spent summers on the remote north Cornwall coast tracking the fate of the butterfly larvae; the former worked out they were housed in ants’ nests and the latter that they fed on ant larvae.  The conclusion was that four species of meadow ant, which were in any case all very similar, were larval hosts and no-one considered a more specific relationship.

Research eventually showed that it was totally reliant for its overwintering, larval stage on just one species of meadow ant.  All four species of ants, attracted by sugary secretions by the larva, took it into its burrow but only one was fooled and permitted it to remain to eat its larva until it emerged the following year. The three others at some point realised the con and killed the interloper. Only when a young PhD student, Jeremy Thomas, looked again in the 1970s, over more years of meticulous observation, was Myrmica sabuleti recognised as the sole host. This species, unlike the others, required sun-warmed, hard-grazed, short grasslands for effective foraging together with a carpet of thyme flowers for the young larvae to feed on after hatching. This discovery provided the basis to develop and refine effective, habitat management prescriptions. The problem was that the species went extinct during the research programme so reintroductions were required. The old haunts on Dartmoor and across the Polden Hills in Somerset received the scrub clearance and spring and autumn grazing treatment, then ‘gapping up’ the thyme and finally donations of large blue eggs from Sweden.  The response was a rapid increase in Myrmica sabuleti, wild thyme and large blue populations. There was a lag in trumpeting this success because the reintroductions had to establish and stabilise and of course some didn’t, and sites were sensitive to disturbance so everything was low key for a decade or so. A key paper by the large blue conservation scientist team was published in 2009, including a beautiful figure showing the intricate life cycle, on the decline and fall and subsequent renaissance, through translocation from a climatically similar population on the Swedish island of Öland is here. The work quietly revolutionised insect ecology and conservation across the globe.

One might blame the First World War and the loss of farm workers from the hills, the advent of the oil economy and the disappearance of horses and hay meadows; or the myxomatosis epidemic and disappearance of rabbit-grazed turf; or the decline of mixed farming and disappearance of sheep-grazed hills for causing the decline of the preferred hard-grazed, grassland habitat during the 20th century. But the problem was also one of impatience; too much haste to buy reserves and implement management measures and thus to do so blindly; it was in retrospect a gamble that did not pay off.  Perhaps butterfly collecting proved too easy to label as rampant and hence to blame, which it never was; or perhaps the powers that be considered butterflies to be simple creatures requiring simple measures and all that was needed was nature reserves in the right place; after all the modern science of conservation was an uncomfortable straitjacket for naturalists that were required to test hypotheses, sample endlessly and then run complex analyses to produce dull statistics and put up models just so that they could be shot down. But the heart of the problem was that the life cycle turned out to be so fiendishly non-obvious and one of the most complex of any animal species.

After restoration of populations and establishment of meta-populations on Dartmoor and the Polden Hills, the large blue conservation team looked north and tried in vain to reconquer the Cotswolds.  The problem was that the Cotswolds were colder and eggs did not hatch early enough to catch the flowering thyme, the larval food plant up until the 3rd instar.  Two things changed that; the Somerset population adapted over 20 years to hatch earlier and climate change has steadily warmed the soils. So today, after successful recent reintroductions, the ants need a longer sward to maintain the perfect foraging temperature and the larvae do better on its alternative, southern European, food plant, the taller marjoram than the diminutive thyme. On the oolitic limestone escarpment at Daneway Banks there are now thyme-feeding and marjoram-feeding large blue larvae depending on slope, aspect and temperature.  One can imagine that in 100 years or so, as climate change continues, large blues will be flying over the magnesian limestone grasslands of County Durham.

Poor Mr. Frohawk and Dr. Chapman; they never knew their brilliantly intimate and painstaking observations effectively camouflaged the truth about the life cycle of the large blue for over 50 years and led the conservation world such a merry dance.


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