New thesis on how to protect birch from pests and disease

Improved silver birch (Betula pendula) is attracting increasing interest as Swedish forestry looks towards a more diverse range of tree species. In her doctoral thesis Dr. Sezer Olívia Kaya at the Swedish University of Agricultural Sciences (SLU) and within Trees For Me, investigated three potential challenges: weevil damage to young birch seedlings, root rot caused by Heterobasidion root rot fungus, and the potential threat from the invasive bronze birch borer (Agrilus anxius).

Low levels of root-rot infection

While the problem of Heterobasidion root rot is well known in conifers and causes serious root disease in northern European forests, considerably less is known about the risk in managed birch stands.

Her research indicates that primary infection by Heterobasidion on birch stumps following thinning was infrequent in the stands studied in southern Sweden.

”I was surprised by how little Heterobasidion incidence I found on the untreated birch stumps after thinning”, says Sezer Olívia Kaya.

The researchers also studied the effect of the two biological control agents Phlebiopsis gigantea (Rotstop) and Pseudomonas protegens (Basinox). No Heterobasidion infections were detected on treated stumps, while laboratory experiments showed that both treatments could restrict the establishment and growth of the pathogen on birch wood.

The field experiments focused on root rot were based in southern Sweden while pine weevil damage was assessed on sites also in the central and northern parts.

Less weevil damage on birch than on conifers

In the first study of the thesis, conducted in southern Sweden, Sezer Olivia Kaya demonstrated that several weevil species can damage newly planted birch trees and that mechanical protection against pine weevils reduced attacks during the establishment phase. Despite high damage in untreated seedlings, weevil-associated mortality was low.

Results from her second and larger scale weevil study confirm that pine weevils can attack silver birch seedlings; however, the damage and mortality rates were generally lower than those observed in Scots pine, Norway spruce, and larch even though the conifers had, in most cases, been treated with mechanical protection against weevil attacks, whereas the silver birch had not. These findings suggest that mechanical protection against pine weevils may be unnecessary for silver birch planted following site preparation.

"The damage was more extensive in southern and central Sweden and less so in the north," says Sezer Olívia Kaya. This geographical pattern suggests that a changing climate could influence the risk of damage in the future.

Preparing for potential insect threats

The last part of the thesis concerns the bronze birch borer, an insect native to, and widespread in, North America that has not been detected in Europe yet but could pose a severe threat to European birches if introduced.

”To our knowledge, bronze birch borer has not arrived to Europe. Monitoring of potential invasive pests is carried out close to ports of entry across Sweden and in different parts of Europe, handled by the relevant plant protection organisations. Such surveillance is essential. During our field visit to Canada, it was quite striking to see how much more severely bronze birch borer attacks introduced silver birch than native paper birch ”, Sezer Olívia Kaya concludes.

The research has resulted in molecular methods that can detect the insect both in the laboratory and rapidly in the field, which could enable faster action in case of a potential introduction of the insect.