Susceptibility of Betula pendula clones to Phytobia betulae, a dipteran miner of birch stems

Larvae of Phytobia betulae Kangas (Diptera: Agromyzidae) mine within the differentiating xylem tissue of birch trees from canopy to stem base, resulting in permanent brown tunnels in the wood. In birch products the tunnels are considered to be an aesthetic defect. The aim of the study was to determine whether European white birch (Betula pendula Roth) clones differed in their susceptibility to Phytobia and to ascertain the relationship between the size of the trees and their susceptibility. We counted the number of pith flecks (larval tunnels) in eight or nine micropropagated clones of B. pendula in three 4- to 6-year-old field trials. After adjusting for the positive effect of tree size on Phytobia there were significant two- and three-fold differences between clones in Phytobia resistance. A significant genotype × environment interaction in susceptibility of birch to Phytobia was observed. A slow-growing clone was the least susceptible to Phytobia.