Rhododendrol and Susceptibility to the Bronze Birch Borer

Rhododendrol is produced in the inner bark of stressed and dying branches of most white-barked birch species by natural hydrolysis of the glucoside rhododendrin. Rhododendrol was identified by thin-layer chromatography and was synthesized by sodium borohydride reduction of the commercially available ketone. Mated female bronze birch borers (but not males or un-mated females) exhibited a short-range attraction to the synthetic rhododendrol. It is postulated that this compound may be the primary stimulus to oviposition in stressed trees with dying cambial tissue. Interspecific hybrids between rhododendrin-producing species (e.g. Betulapopulifolia) and the borer-resistant B. nigra, which does not contain rhododendrin, showed only traces of rhododendrol in hydrolyzed bark but produced another phenolic compound that was not found in either parent species. Studies of this unknown compound and further hybridizations with B. nigra to develop whiter-barked, borer-resistant birches are continuing.