Stacking the deck: Should tree growth regulators be added to the treatment arsenal for Fraxinus spp. trees infested with emerald ash borer?

Since the discovery of emerald ash borer (EAB, Agrilus planipennis Fairmaire) in 2002, a variety of treatments have become available to protect Fraxinus spp (ash).  Tree defense priming using paclobutrazol (PB), a gibberellin inhibitor, could be another option.  Studies suggest PB application can enhance vigor and defensive response of some tree species.  If ash trees respond similarly, PB applications might augment the defensive response of trees to EAB.  Our objective was to assess whether PB treatment alters ash tree physiology or structure or EAB host preference.  In a common garden, seven blocks (21 total blocks) containing one each of F. americana, F. mandshurica, F. nigra, F. quadrangulata and F. pennsylvanica, were treated with PB (2010), fertilizer (2010, 2011) or left as untreated controls.  We assessed radial growth, gas exchange, biomass partitioning and foliar nutrition among ash species and treatments.  Leaf area consumption and beetle mortality were assessed using EAB placed in cages with intact leaves (i.e. leaves were not removed from trees).  Trees were debarked in October 2011 to quantify density of EAB exit holes, woodpecker attacks and living, parasitized, and dead larvae.  Preliminary results suggest PB-treated ash trees have lower relative growth rates and above:below ground biomass than control or fertilized trees.  In addition, EAB leaf consumption and larval gallery density were affected by tree species and treatment.