Breeding strategies for the development of emerald ash borer - resistant North American ash

The emerald ash borer (Agrilus plannipennis; EAB) is a phloem-feeding beetle that is endemic to Asia. It was discovered in North America in 2002, found almost simultaneously near Detroit, Michigan and Windsor, Ontario, Canada. Adult beetles feed on ash (Fraxinus spp.) foliage, but larval feeding on phloem, cambium, and outer xylem is far more detrimental because it effectively girdles the tree, disrupting nutrient and water transport and ultimately killing the tree. As of 2007, it was estimated that 53 million ash trees had been killed by EAB in North America (Kovacs et al. 2010). Potentially, more than 8 billion ash trees could eventually be affected, or about 2.6 percent of the timber trees in the United States (Sydnor et al. 2007). Breeding for pest resistance in forest trees is a proven approach for managing both native and non-native insects and diseases (FAO UN 2011). We initiated a breeding program that employs two strategies to incorporate EAB resistance into North American ash species: hybrid and traditional breeding. In the hybrid breeding approach, we are looking for EAB resistance in Asian species of ash from EAB's region of origin (sympatric resistance). Asian ashes resistant to EAB will be crossed with native North American species to create hybrids. The hybrids subsequently will be subjected to rounds of testing, selection, and backcrossing to the native species that will be repeated until only the resistance genes from the exotic species are carried into the native population while all of the traits of the native species are retained, similar to the breeding program of the American Chestnut Foundation (Hebard 2006). For a more traditional breeding approach, we are searching for rare native individuals with resistance or tolerance to EAB (allopatric resistance). The following is an overview of the current status of our program.