Britain’s largest butterfly may be at risk from fungal pathogens that have caused a drastic die-back of the rare plant on which its caterpillars feed.
The swallowtail is only found breeding at 16 sites in Britain, all on the Norfolk Broads, where milk parsley grows. But last summer more than 90% of the milk parsley plants at one of its breeding sites wilted and died, preventing the plant from setting seed. If milk parsley disappears, the unique subspecies of the swallowtail found in Britain will become extinct.
The “milk parsley droop”, which may be caused by a known fungal pathogen combining in a novel way with another pathogen, was spotted on Wheatfen nature reserve near Norwich. Nature reserve managers across the Broads are now on high alert for signs of the disease spreading this year, with the wilt typically taking hold in July and August.
Mark Collins, the chair of the Swallowtail and Birdwing Butterfly Trust, said: “This is not an unknown pathogen but it has knocked the milk parsley population at Wheatfen for six. These pathogens are subject to the natural population dynamics and in a normal wild situation this wouldn’t be a problem because the swallowtail and the milk parsley would be in other places but there are only 16 swallowtail breeding sites left and these spots are increasingly isolated in a very agricultural landscape. We are worried.”
Although the swallowtail caterpillar may be able to grow large enough to pupate before the droop takes hold in late summer, if the pathogens prevent milk parsley from setting seed then the perennial plant could rapidly become even rarer in the Broads.
Will Fitch, the warden of Wheatfen, is closely monitoring the site for further outbreaks. He said: “We’ve got enough problems as it is with climate change and salt surges. Milk parsley on the whole isn’t thriving, and now we’ve got these extra concerns. But we’ve got a good population of swallowtails this year and caterpillars on the plants so it’s not the end of the world, we hope.”
Plant pathologists are urgently seeking to identify the cause of the droop, which has been linked to a fungal pathogen identified in umbelliferous plants including commercial crops such as carrots for many years.
The trigger for the unprecedented die-back last year is unknown but it may be due to a second fungal pathogen working with the first. Another potential cause could be saltwater inundations from flooding weakening the milk parsley plants.
Dr Fay Newbery, a plant pathologist for the Royal Horticultural Society, has confirmed the primary fungus to be one of the Diaporthe genus of fungal pathogens but has also identified a second fungal pathogen inside the plant. She is growing milk parsley plants in a laboratory to seek proof that the pathogens are combining to cause the wilt.
“Diaporthe has probably been at Wheatfen for centuries,” said Newbery. “Why is it suddenly causing such a problem? That question needs to be answered if this problem is going to be solved.”
The swallowtail is classified as “vulnerable” to extinction in Britain on the latest IUCN red list because it is restricted to the low-lying Norfolk Broads. Projected sea-level rises will result in the loss of some freshwater marshes that the swallowtail and its food plant currently inhabit, with milk parsley killed by the incursion of saltwater into freshwater reedbeds and fens.
According to Collins, the new disease threat strengthens the case for the translocation of the swallowtail – and milk parsley – to new wetlands further inland, such as large fens being restored in Cambridgeshire. The swallowtail cannot fly far enough across inhospitable farmland to reach a new suitable habitat on its own, so specimens would have to be bred and moved across.
“This is yet another difficulty facing this pair of species, the milk parsley and the swallowtail butterfly, among the many other problems, including sea-level rise and saltwater getting into rivers,” said Collins. “The swallowtail is so confined to these sites which are all being well managed but they’ve got nowhere they can retreat to. To survive these sorts of diseases, the butterfly will need to find other locations but other potential sites are currently scattered and small.
“There could be a situation where we will need to translocate populations of the butterfly to other places where they will not suffer from sea-level rise and the incoming saltwater.”