Invasive butterfly bush threatens Swiss butterflies

Butterfly bush is rapidly displacing native plants and threatening Swiss butterfly species, SRF reports. Although the plant provides nectar for adult butterflies, it can destroy the food plants needed by their caterpillars.

The plant, which originates in China, is considered an invasive neophyte in Switzerland. Its sale and distribution have been banned for the past two years.

Its rapid spread was evident at a drained pond in Blattig, an area of Kriens in the canton of Lucerne. The pond was emptied last summer for renovation work. One year later, the area was covered with butterfly bush.

“We pulled out more than 5,000 young plants,” said Guido Huser, who leads a volunteer group that regularly removes invasive plants. Six weeks later, the group returned and removed hundreds more.

A large butterfly bush can produce about three million seeds, Huser said, with tens of thousands in a single flower cluster. The seeds are dispersed by the wind. They should be removed and disposed of before they mature.

Marie-Louise Kieffer of the Lucerne environmental consultancy said butterfly bush often colonises species-rich habitats and forms monocultures. It particularly threatens open areas such as stream banks, gravel banks and forest clearings.

“Among other things, food plants for endangered butterfly species disappear,” Kieffer said. Many butterflies depend on a specific food or host plant. While adult butterflies can feed on butterfly bush, their caterpillars cannot survive on it.

Kieffer recommends replacing the plant in gardens with native alternatives such as hemp agrimony, purple loosestrife, hawthorn, blackthorn, wild roses or ivy. Ivy flowers late in the season and attracts large numbers of insects.

Anyone unwilling to remove the shrub should cut off its flowers before the seeds mature and dispose of them with household waste. Autumn is the right time to do this, Kieffer said. About 40 percent of native butterfly species are on the Red List.

The Kriens volunteer group plans to continue removing butterfly bush from the drained pond next year. Huser said the work was demanding but rewarding, and that plants could be eliminated after repeated removal.