Between the dark, dank days stretching from October to March, it may seem that things quieten down for our butterflies.
But this is actually an extremely busy time for conservationists as this is the key period when sites are managed to make sure conditions are just right for when the weather warms up and the butterflies start to emerge.
Management for one of our rarest butterflies, the previously extinct Large Blue, is no different and is made more complicated because of its predatory nature and complex life-cycle. Upon hatching, the tiny caterpillar feeds on flower heads of Wild Thyme, but after a few weeks it will be taken underground into the brood chambers of a certain type of red ant (Myrmica sabuleti). The caterpillar cleverly attracts this ant with secretions from a special ‘honey’ gland. The caterpillar hibernates and in spring, feeds on ant grubs to achieve its final body weight. The caterpillar pupates underground and the butterfly crawls to the surface before unfurling its striking black-blue wings.
It is this fascinating life-cycle which also makes this butterfly complicated to manage for. Not only do we have to make sure conditions are right for the butterfly, but they must also be right for the ant and for the Wild Thyme on which the caterpillars depend.
The Large Blue was re-introduced to the Polden Hills, Somerset, in the 1990s after much research and through the hard work of a number of individuals and organisations. The butterfly is now established in the Poldens but more work is required. Last September, Butterfly Conservation was awarded a £50k grant from Biffa Award to help us ‘Secure the Future of Large Blue’ in the Polden Hills landscape’.
One step to protecting this butterfly is to increase the area of suitable habitat on occupied, potential and previously occupied Large Blue sites. In the first six months of the project four hectares of scrub has been cleared. Work included substantially increasing the area of limestone grassland habitat along with smaller-scale bramble and scrub clearance to maintain habitat.
Contractors helped clear larger areas of scrub, especially on very steep slopes but help from volunteers was essential. Members of the public braved drizzle and brambles to help clear more than a hectare of scrub during five work parties on project partner sites. Luckily for most work parties we did have glorious sunshine and a plentiful supply of cake.
The last two summers have been so warm that the Wild Thyme suffered and dried out, especially on the hot south-facing slopes, preferred by the Large Blue. As part of the project Wild Marjoram has now been sown onto four sites. This plant is more drought resistant and flowers later than Wild Thyme so has potential to be an important foodplant resource for Large Blue if we continue to experience these hot summers with the threat of climate change.
The right type of grazing is important when managing for Large Blue as it helps create the warm ground conditions required for ants to forage. Too much or too little grazing can be damaging to both the ant and the caterpillar foodplant. Ecologist, David Simcox has been working on Large Blue for over 30 years, so his expertise and advisory work is a crucial element of the project. David has been measuring sward (grass length), checking for foraging ants and monitoring the amount of foodplants which all provides important feedback. Over winter his advice on management and grazing levels has helped make sure habitat conditions are suitable for the butterfly. This gives this year’s emerging Large Blue the best chance of breeding success, helping increase populations and dispersal in future years.
The weather has not been particularly kind over the last few years for Large Blue. Heavy rain in 2013 and 2014 and a cold spring in 2013 cooled ground temperatures, making it too cool for ants to forage and in this situation they will eat their own grubs along with any imposters in their chamber. The hot summers affected the Wild Thyme at a crucial stage for caterpillar feeding. As a result Large Blue numbers have been low in the Poldens and there has not been much migration between sites.
The work of site managers, the ecologist, contractors and volunteers means that the sites are in great condition and ready for the upcoming 2015 flight season, but we are now in the dubious hands of the great British weather.
If you want to learn how to identify the Large Blue or if you or your local groups can spare time to help us manage scrub in the winter or to plant Wild Liquorice to help the rare Liquorice Piercer moth, please email me [email protected] or visit our Large Blue Action Group.
By Rachel Jones, Conservation Officer
Follow me on Twitter: @RachJBC
The East Poldens Large Blue Landscape Partnership consists of Butterfly Conservation, J&F Clark Trust, Millfield School, the National Trust, University of Oxford, Somerset Wildlife Trust and South Somerset District Council, with support from Natural England.