Norway maple trees, Acer platanoides L. (Sapindales: Sapindaceae), that were trunk-injected with imidacloprid as part of an Asian longhorned beetle eradication program, were used to study the temporal and between-tree distribution of imidacloprid in twigs from June through September. The effect of injection time during spring on imidacloprid residues across the summer season and the distribution of imidacloprid in twig bark versus twig xylem were also investigated. Overall, we observed a significant decline in imidacloprid concentrations within each plant part sampled across the study period, although the 19 trees used in the study varied greatly in the pattern of imidacloprid residues over time. The concentration of imidacloprid in twig bark per dry mass was approximately two times higher than that of the twig xylem (means ± SD of 1.21 ± 2.16 ppm vs. 0.63 ± 1.08 ppm imidacloprid, respectively). The majority (>50%) of whole twig, twig bark and twig xylem samples from injected trees contained <1 ppm imidacloprid and 37% of twig samples contained 0 ppm. Maximum values were 9 ppm for whole twigs from trunk-injected trees, 12 ppm imidacloprid for twig bark, and 5 ppm for twig xylem. Leaves, sampled only in September, had much higher imidacloprid residues than twigs collected at the same time; the majority (53%) of leaf samples contained >5 ppm imidacloprid, with a maximum of 49 ppm. The concentrations of imidacloprid in whole twigs, twig bark, and twig xylem were highly correlated, and levels in leaves were correlated with imidacloprid levels in whole twigs.
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Vol. 106 • No. 2