Slovenia is facing an infestation by the oak lace bug, an invasive pest that attacks primarily oak trees. Experts say there is no major damage yet, but there could be extensive damage if the insect reproduces for several consecutive years. There are no effective measures against the bug.

This year, the number of oak lace bugs (Corythucha arcuata) in Slovenia is exceptionally high, and the leaves of oak trees, which the insect attacks, turned a yellowish brown in the summer across the country.

The impact of this plant-eating bug is particularly pronounced at the end of summer, when their number is the highest, the national Forestry Institute says.

Oak lace bug originates from North America and it first appeared in Europe in 2000, in Italy, and in 2002 it was found in Turkey. It was detected in Slovenia for the first time in 2016 in the eastern part of the country, close to a railway line.

Travelling like a stowaway

The invasive species most often spreads over long distances like a "stowaway" on road or rail vehicles, or on clothing. Adult oak lace bugs can also fly or they can be carried by the wind, but only over short distances.

Its main host is oak, but in case of large infestations it can also appear on some other deciduous trees, such as linden, elm, chestnut and hazel. Damage to these trees is never as intense as on oak, experts say.

Adult specimens and their larvae suck the sap on the underside of leaves, damaging the leaves and causing loss of chlorophyll.

As a result, photosynthetic activity decreases, and the leaves eventually fade, dry and fall off, and repeated damage may result in the tree getting weaker and growing at a slower pace.

According to the Forestry Institute, the oak lace bug has not caused major damage in Slovenia so far, but if the insect reproduces for several consecutive years, this may result in a lower increment of oak trees and decreased seed production.

The whole country infested

This invasive pest is present in the entire Slovenia, including in areas where there are no oak trees. The infestation is worst in central and eastern parts of the country, and is connected with the above-average temperatures this year.

"Due to its distribution and biology, the damage done by the oak lace bug cannot be prevented because there are no effective selective control measures. Oak lace bug control is thus not advised," the institute has told the Slovenian Press Agency.

Oak represents around 7% of the growing stock in Slovenian forests, with sessile oak being the dominant oak species.

In addition to the oak lace bug, also appearing in urban areas is the very similar plane lace bug (Corythucha ciliata). It attacks plane trees, which are frequent in parks.

Since both insects are widespread throughout most of Europe and no systematic measures against them are implemented anywhere, they can no longer be eradicated in Slovenia, the institute said.