340 a test of which will require additional experimenta- tion because we did not inject at intermediate dis- tances. However, the prediction provides a starting point to guide future experiments. In the first experi- ment, injection sites were spaced an average (± 1 SD) of 32.2 ± 4.7 cm apart using the Tree I.V. In the sec- ond experiment, the interval between injections was decreased to 8.9 ± 0.5 cm. The analysis suggests that decreasing the distance between injection sites to approximately 10 cm around the basal circumference may be most effective for the lateral distribution of the injected chemistry. Interpolation was based on the regression equation y = 1.14 – 0.016x (Figure 2). We applied this closer spacing for tree injection in exper- iment 3. Micro-injection applications (in this experi- ment, using the air hydraulic) delivered product in less time compared to the TREE I.V., with realized savings in both materials and labor. MPB Success and Blue-Stain In spite of the increased concentration of residues and better coverage in the 2009 treatments (experiment 2), 7 of 10 treated trees died. Our sampling procedures and whole-tree autopsies showed that treated tree boles were not successfully attacked by MPB. We observed no adult gallery construction beyond the area of attack and no reproduction (Table 3). Trees were, however, colonized by blue-staining fungi. Injection procedures used in experiment 2 were apparently effective for delivering lethal concentrations of EB to MPB adults across more phloem area than in our first experiment, a result that suggests something other than girdling from adult beetle gallery construction caused tree mortality. The number of MPB attacks was not reduced, yet crowns of our injected trees faded more slowly (Table 3), further suggesting that mortality was not due to insect attack per se. Initially, we had not identified the agent of mortal- ity in experiment 2. However, many associates are introduced by Dendroctonus beetles (e.g., Hofstetter 2011; Klepzig and Hofstetter 2011). Fungal symbi- onts have been shown to cause tree mortality by themselves (Mathre 1964; Basham 1970; Shrimpton 1973; Strobel and Sugawara 1986; Yamaoka et al. 1995). Blue-staining fungi effectively colonized the xylem, cambium, and phloem tissues of experimental trees attacked by MPB. In our lab, we identified two isolates morphologically from experiment 2 as Gros- mannia clavigera (Rob.-Jeffr. & R.W. Davidson) ©2020 International Society of Arboriculture Doccola et al: Emamectin Benzoate Efficacy Against Bark Beetles Zipfel, Z.W. de Beer, & M.J. Wingf. (Ophiostoma- tales, Ascomycota) and Leptographium longiclavatum (S.W. Lee, J.J. Kim, & C. Breuil)(Ophiostomatales, Ascomycota). These isolates were subsequently con- firmed by DNA sequencing conducted by Drs. Wing- field and de Beer, now maintained in the culture collection (CMW) of FABI under Grosmannia clav- igera #38988 and Leptographium longiclavatum #38989. In a greenhouse study, these isolates, when inoculated into western white pine seedlings, resulted in plant mortality independent of the presence of MPB (Wyka et al. 2016), demonstrating their potential for pathogenicity. Fungal Infection and Tree Interaction In transverse section, the colonization of the ophiostomatoid fungi appears blue; however, when viewed in tangential aspect (bark removed) the sap- wood appears streaked and dark (brownish-purple or Figure 3. Brownish necrotic lesions in sapwood (bark excised in tangential section) associated with ophiostomatoid infection above and below the mountain pine beetle attack points. Illustrated are two individual attacks by MPB in western white pine. Mountain pine beetles introduce fungal spores when they mine into the tree’s vascular tissues.
September 2020
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