Elm trees at risk - new harmful invasive species found in Finland

The Elm Zigzag Sawfly is a new and harmful invasive species in Finland. The insect larvae feed on the leaves of elm trees and eat easily distinguishable and recognisable zigzag patterns on them. Abundant occurrence exposes the tree to other insects and diseases, which can eventually destroy the tree. The spotted sawfly is not a threat to animals or humans.
The first Elm Zigzag Sawfly found in Finland. The specimen was found in Espoo, southern Finland. Photo by Marko Mutanen

Professor Marko Mutanen from the University of Oulu discovered the Elm Zigzag Sawfly in Espoo, southern Finland in early June. "I have been following the spread of the insect in Europe, which has been advancing at a rate of about 100 kilometres per year. Since it was found in Estonia a couple of years ago, I thought it might already occur in Finland. There are a lot of elm trees in Espoo, and my guess was right, as I found an adult specimen of the species by net-sweeping the undergrowth of an elm forest."

The new sawfly is one of the many invasive species that often arrive in Finland. "The Elm Zigzag Sawfly is an unfortunate summation of a now fairly common story, where a newcomer has no natural enemies in a new area of distribution, so the species proliferates so quickly that it causes damage, in this case to elms," says Mutanen. With climate change, different types of newcomers may adapt to northern conditions. "However, the main problem is that humans are intentionally or unintentionally spreading species to areas where they do not naturally occur."

The Elm Zigzag Sawfly is therefore a typical representative of the phenomenon whereby the international movement of people allows species to spread to entirely new areas. "I think it's likely that it has flown across the Gulf of Finland on its own wings, but it's also possible that it has travelled across the sea with humans," Mutanen says.

The sawfly reproduces asexually, forming several generations a year, and therefore multiplies rapidly, making it difficult to control. In Europe, it has destroyed entire forests of elm trees. The species is likely to be contributing to the spread of the Dutch elm disease. This fungal disease has killed elm trees on a large scale in Europe. Dutch elm disease strikes more easily on trees that have been weakened by other insects. It also poses a threat to many species that depend on elm trees in Europe by eating a large proportion of the leaves of elm trees.

In Finland, the various types of elm trees are mainly planted in parks and gardens, and ornamental elm trees are grown for up to hundreds of years. In southern and central Finland, there are also wild elm forests.

It is expected that the Elm Zigzag Sawfly will continue to spread in Finland's elm groves and natural forests.

The scientific name of the newcomer is Aproceros leucopoda. The larvae are a couple of centimetres long and green, while the 6-7 millimetre long adult is mostly black. It is originally from China and Japan.

Professor Marko Mutanen of the University of Oulu examines elm leaves. He has discovered new invasive species in Finland, this time the Elm Zigzag Sawfly, which is harmful to elms and whose larvae eat a distinct zigzag pattern on the leaves of elm trees. Photo: Erkka Laine

Learn more about Ecology and Genetics Research at the University of Oulu

Last updated: 14.6.2024