Step by Step Story and Photos by Stacie Hernandez Over the course of Todd Johnson’s three-year study, different methods were used to collect data necessary for his research. In the first two years of his study, Johnson researched the defensive chemistry of ash trees in four size classes. By studying trees treated with emerald ash borer eggs, methyl jasmonate, or no treatment at all, Johnson tracked how tree defensive chemistry changes across these three conditions, the mortality of EAB across the different size classes, as well as the effectiveness of the introduced parasitic wasps. In the third year, Johnson focused on two size classes and studied how changing the timing of methyl jasmonate application could increase or decrease mortality to EAB. In all three years, the trees were introduced to parasitic wasps that are known to target EAB, and the trees were removed for study afterward.
Each year prior to the initiation of the study in early June, Johnson and Casey Coupe, a lab technician at UNH, searched for ash trees that had not already been impacted by EAB in the greater Durham area. Across three years, approximately 80 locations were scouted for the presence of green and white ash trees. When appropriate sites were found, trees were randomly selected in one of four size classes. Some of the trees that were used were located on Forest Society land, including Jennings Forest in New Durham in 2020, and Powder Major Forest in Madbury in 2021, as well as other state- or town-owned forests. The steps Johnson followed in his research are outlined below.
1) A baseline sample of the tree’s phloem tissue is collected to detect any chemicals present in the tree prior to treatment. 2) The first treatment is the placement of EAB eggs on the bark. The eggs, which are the size of the tip of a pencil, are placed in the beginning of June when they would naturally be laid. The process includes scraping the top layer of bark, sticking eggs in the bark, and protecting the treated area from the elements with Tyvek wrap.
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3) When the eggs hatch after a couple of weeks, Johnson and Coupe apply a naturally occurring plant hormone, methyl jasmonate, as the second treatment to some of the trees. Methyl jasmonate is a hormone trees produce upon the detection of herbivores. For the first two years of the study, methyl jasmonate was applied to trees without EAB, to understand how the plant hormone alone changes plant defensive chemistry. In the third year of the study, methyl jasmonate was applied at different points in time to trees with and without EAB. This was done to see if trees could be given an early warning prior to attack and to determine if early application of methyl jasmonate would affect the survival of EAB. 4) After treatments are finished, a second phloem sample is collected to detect the chemical compounds the plant activated as a defensive reaction. 5) Parasitic wasps (T. planipennisi and S. galinae) are then released to measure biocontrol success in EAB larvae.
16 | FOREST NOTES Winter 2022
