Intraspecific variation in susceptibility to emerald ash borer: a restoration strategy for North American ash. Jennifer Koch, USDA Forest Service, Ohio, USA.

Additional Authors: David Carey, Kathleen Knight, Therese Poland, Daniel Herms, Mary Mason

The emerald ash borer (EAB), Agrilus planipennis, is native to far eastern Russia, northern China, Japan and Korea where it is normally problematic for indigenous ash species only in conjunction with other stressors. However, the situation in North America, where EAB was first detected in 2002 in the Detroit area in Michigan, USA and in Windsor, Ontario, Canada, is quite different and millions of ash trees in both urban and natural settings have been killed. EAB has since moved into 22 additional states and is currently found in over 20 municipalities in Ontario and Quebec. Mortality rates nearing 100 % are frequently reported for the most prevalent North American ash species that EAB has encountered to date, including green ash (Fraxinus pennsylvanica), white ash (Fraxinus americana) and black ash (Fraxinus nigra).

Permanent monitoring plots were established in forested areas throughout Ohio and southeast Michigan, USA, in 2004. Annual monitoring of ash trees in these plots identified a small number of green and white ash that have persisted when over 98 % of their ash counterparts have died from EAB. These surviving trees may have reduced susceptibility or increased tolerance to EAB, indicating that there is variation in EAB-response within these species. To further assess this possible tolerance to EAB, individual trees were selected for propagation and further experimentation based upon the following criteria: 1. DBH of 24 cm or greater, 2. stand mortality due to EAB was greater than 95 % at least two years prior to selection, and 3. healthy canopy. To date, over 50 genotypes across 21 sites have been identified and successfully propagated through grafting.

Subsets of these select, surviving trees have been used in bioassay experiments in an effort to confirm tolerance to EAB and identify specific phenotypes associated with tolerance or reduced susceptibility. Adult feeding preference studies identified some genotypes that were significantly less preferred than susceptible controls. Experiments performed by affixing eggs onto grafted ramets demonstrated that some of the surviving ash selections did not differ from the susceptible controls, but other selections were significantly different having lower larval weight, differences in overall larval development, and/or larger numbers of larvae killed by host tree defenses. One lingering ash genotype was significantly different from susceptible controls in both the adult feeding and egg bioassays. These results demonstrate measurable phenotypes that explain why some ash genotypes have tolerated EAB attack longer than the majority of their counterparts, and indicate that more than one mechanism may be responsible. Full-sib families have been generated to carry out genetic studies to evaluate heritability of EAB-tolerance and to understand the mode of inheritance so that a breeding program may be initiated. Research plantings for evaluation of field performance are currently being established.