Reduced infestation by Xylosandrus germanus (Coleoptera: Curculionidae: Scolytinae) in apple trees treated with host plant defense compounds

The ambrosia beetle Xylosandrus germanus (Blandford) is an invasive pest that has caused tree death and decline in NY dwarf apple orchards during the past ten years, despite efforts to control them using trunk sprays of chlorpyrifos or pyrethroids, alone or combined with the repellent verbenone.  From 2017 to 2019, we tested trunk applications of different repellents and plant defense compounds for protection against X. germanus in potted apple trees that were stressed by flooding, in semi-field trials adjacent to infested orchards.  Treatments included topical formulations of verbenone and methyl salicylate (MeSa), a host defense and signaling compound, alone and in combination, at different rates and timings.  Additional treatments evaluated included acibenzolar-S-methyl, Reynoutria sachalinensis, and salicylic acid.  The combination verbenone+ MeSa treatments had the lowest incidences of attack sites and galleries containing adults or brood, although results varied among years.  In a separate trial, we found no significant difference in numbers of adults caught in ethanol-baited traps placed 5–20 m from an apple bolt treated with the verbenone+ MeSa repellent, suggesting that the repellent's effect did not extend to those distances from the treated target.  Cross-sectional discs of tree trunk tissue sampled in late August were analyzed for levels of phytohormones; ergosterol, abscissic acid, salicylic acid, jasmonic acid, methyl salicylate, methyl jasmonate, trans-cinnamic acid, or indole-3-cinnamic acid did not significantly vary across treatments; however, trees having higher amounts of beetle damage contained higher levels of jasmonic and salicylic acid, which are key molecules in plant defense pathways.