Breeding for Leafminer Resistance within the Genus Betula

Abstract

The birch leafminer (Fenusa pusilla) is an important insect pest of white-barked ornamental birches (Betula spp.). Although genetic resistance to birch leafminer exists within this genus, it has not been identified in the desirable white-barked species. Twenty-three interspecific crosses were performed in an attempt to combine ornamental white bark and leafminer resistance. The parental species and resulting hybrid populations were assayed for resistance. All assays utilized adult birch leafminers reared in our laboratory using a method we developed to provide a year-round supply of fresh leafminer adults. Previous studies viewed birch leafminer resistance as the unwillingness or inability of the female leafminer to oviposit eggs into the leaves of resistant Betula species. Our work has shown that female birch leafminers are unable to distinguish between suitable and nonsuitable host species. This work also has discovered two separate mechanisms of inherent resistance to the birch leafminer: leaf chemistry and a hypersensitive response. Two native species, B. alleghaniensis and B. lenta, kill the leafminer larvae shortly after hatching. Analyses suggest that a specific compound or class of compounds is responsible for leafminer mortality in these species and that this resistance is reduced in F1 hybrids that include a susceptible white-barked parent. The second resistance mechanism involves a hypersensitive response to the deposition of the leafminer egg and results in failure of the egg to hatch. Interestingly, this response is seen in one of our breeding lines (B. populifolia × B. maximowicziana), but has not been observed in either parental species. These hybrid progeny are currently in field test plots for further evaluation of leafminer resistance and ornamental characteristics.