Top-down and Bottom-up Controls of Adelges tsugae in the Pacific Northwest Inform Its Biological Control in Eastern North America Public Deposited

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  • The hemlock woolly adelgid (Hemiptera: Adelgidae: Adelges tsugae; HWA) is an invasive and damaging pest on hemlock in eastern North America. HWA is native to the Pacific Northwest, where it does not damage hemlock. In support of classical biological control of HWA in eastern North America, abundances of native predators on HWA in the Pacific Northwest were assessed in an observational study. HWA and its predators were counted and identified on branch samples from western hemlock (Pinaceae: Tsuga heterophylla) in the Puget Sound area. Species densities were calculated on a per-centimeter basis. Two species of cryptic congeners, silver flies (Diptera: Chamaemyiidae: Leucopis argenticollis and Le. piniperda), were innovatively identified using DNA sequencing. More individuals of the two Leucopis spp. (120 in total) combined were found than the next most abundant predator (52 in total), a beetle (Coleoptera: Derodontidae: Laricobius nigrinus). Laricobius nigrinus has already been introduced by the hundreds of thousands and established on hemlock as part of HWA biological control efforts in eastern North America. However, La. nigrinus has not succeeded in perceptibly reducing hemlock decline or mortality there. Leucopis argenticollis was more abundant than Le. piniperda, with 55 flies identified as Le. argenticollis and 25 identified as Le. piniperda. Since ~67% of the flies were successfully identified to species, it was inferred using relative proportions that the most abundant predator collected was Le. argenticollis, with ~83 individuals, followed by La. nigrinus, and then Le. piniperda, with ~38 individuals. The occurrence of the Leucopis spp. on both seasonal generations of HWA, while La. nigrinus was found only on the early generation, confirmed a natural history difference between the flies and the beetle; Leucopis occurs and feeds on both generations of HWA, while La. nigrinus pupates underground during the late HWA generation. This implies that Leucopis could complement La. nigrinus in control of HWA. Because beetle pupae were not collected (whereas fly puparia were) due to this natural history difference, puparia were excluded for additional comparison. Leucopis spp. larvae and adults totaled 69 compared to 52 La. nigrinus. Since site was found to be non-significant as a factor in explaining mean cumulative species densities in three one-way ANOVAs (HWA: F-value = 1.88; d.f. = 3, 21, p-values = 0.17; Leucopis: F-value = 1.95; d.f. = 3, 21, p-values = 0.15; La. nigrinus: F-value = 0.4; d.f. = 3, 21, p-values = 0.75), densities were related across sites in further analysis. Pearson’s product-moment correlations of HWA and La. nigrinus densities were non-significant, while HWA and Leucopis spp. densities were positively correlated (e.g., for larval Leucopis densities, t-value = 2.64; d.f. = 23; p-value = < 0.01; 95% CI= 0.17 to 1; estimated correlation = 0.48). Combined larval and adult Leucopis spp. and La. nigrinus densities were moderately negatively correlated (t-value = -1.81; d.f. = 23; p-value = 0.04; 95% CI = -1 to 0.02; estimated correlation = -0.35). Leucopis argenticollis and Le. piniperda densities were negatively, though non-significantly, correlated (Spearman’s rank correlation coefficient: rho = -0.28; d.f. = 23; p-value = 0.09). In a companion project, HWA abundance levels were assessed on western hemlock clones of known genetics at a commercial seed orchard, allowing for consideration of spatial versus host resistance explanations for the distribution of HWA there. The results were not strongly supportive of potential resistance to HWA by western hemlock. Hence, evidence for bottom-up control of HWA in the Pacific Northwest was weak from this limited dataset, while the data on predators contributes to knowledge of top-down control of HWA in the Pacific Northwest. These results recommend further study of the Leucopis spp. for potential introduction as biocontrols of HWA in eastern North America.
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