The uncommon plant that its caterpillars depend on has suffered a catastrophic die-back due to fungus diseases, putting Britain’s largest butterfly in danger.
Only 16 locations in Britain, all on the Norfolk Broads, where milk parsley grows, are known to have swallowtail breeding activity. But at one of its breeding sites last summer, more than 90% of the milk parsley plants wilted and died, preventing the plant from setting seed. The distinctive subspecies of the swallowtail seen in Britain would go extinct if milk parsley vanishes.
A recognized fungal pathogen may have combined in an unusual way with another pathogen to generate the “milk parsley droop,” which was observed in the Wheatfen nature reserve near Norwich. Due to the wilt’s typical onset in July and August, nature reserve managers throughout the Broads are currently on high alert for any signs of the disease spreading this year.
The milk parsley population at Wheatfen has decreased by six, according to Mark Collins, chair of the Swallowtail and Birdwing Butterfly Trust. In a typical wild setting, these pathogens wouldn’t be a problem because the milk parsley and swallowtail would be in other locations because they are subject to natural population dynamics, but there are currently only 16 swallowtail breeding sites left, and these locations are becoming more and more isolated in an area that is heavily agricultural. We are concerned.
Even though the swallowtail caterpillar might be able to reach adult size before the late-summer droop sets in, the perennial milk parsley plant might quickly become even more scarce in the Broads if the infections prevent it from setting seed.
The Wheatfen warden, Will Fitch, is actively watching the area for further outbreaks. We already face enough issues with climate change and salt surges, he declared. Overall, milk parsley isn’t doing well, and now there are these additional issues. However, we have plenty of caterpillars on the plants and a healthy population of swallowtails this year, so perhaps it’s not the end of the world.
Plant pathologists are working rapidly to determine what is causing the droop, which has been connected to a fungal infection that has long been recognized in umbelliferous plants, including economic commodities like carrots.
Unknown factors may have contributed to the exceptional die-back last year, but one possibility is that a second fungal pathogen collaborated with the first. Flooding seawater inundations could also be a factor, harming milk parsley plants.
The primary fungus was proven to be one of the fungal infections belonging to the Diaporthe genus by Dr. Fay Newbery, a plant pathologist at the Royal Horticultural Society, who also discovered a second pathogen inside the plant. In an effort to demonstrate that the pathogens are working together to induce the wilt, she is cultivating milk parsley plants in a lab.
Diaporthe has most likely been present in Wheatfen for generations, according to Newbery. Why is it now becoming such a problem? If this issue is to be resolved, that question must be addressed.
The swallowtail is only found on the low-lying Norfolk Broads, making it “vulnerable” to extinction in Britain according to the most recent IUCN red list. Some of the freshwater marshes that the swallowtail and its food plant currently inhabit will be lost due to projected sea level rises, and milk parsley will be wiped out by the intrusion of saltwater into freshwater reedbeds and fens.
Collins claims that the new disease threat makes it more likely that the swallowtail butterfly and milk parsley will be relocated to new wetlands farther inland, such the sizable fens being restored in Cambridgeshire. The swallowtail must be reared and transported since it cannot fly far enough across hostile countryside to reach a new appropriate environment on its own.
In addition to the many other issues, such as sea-level rise and saltwater intrusion into rivers, this is another another challenge facing the pair of species, the milk parsley and the swallowtail butterfly, said Collins. “The swallowtail is so restricted to these locations, all of which are well-managed, but they have nowhere to hide. The butterfly will need to relocate if it is to survive these diseases, but those new prospective habitats are currently few and far between.
There may be a need to relocate butterfly populations so they may avoid suffering from rising sea levels and the inflow of saltwater, according to the researcher.
Lead Image: The swallowtail is only found breeding at 16 sites in Britain, all on the Norfolk Broads where milk parsley grows. Photograph: Iain H Leach/PA.
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