232 Szczepaniec et al.: Effects of Dinotefuran and Imidacloprid on Elm Arthropods Due to dinotefuran’s mode of action being similar to that of imidacloprid as a nicotinic acetylcholine receptor agonist and its widespread use in the arboricultural industry as a preventa- tive and curative insecticide for controlling invasive coleopteran borers (Herms et al. 2009; Raupp et al. 2010; Szczepaniec et al. 2011; Raupp et al. 2012), the study authors wanted to know if applications of dinotefuran resulted in elevated populations of spider mites. Based on previous studies, which demonstrated the propensity of American elm to experience moderate to se- vere secondary outbreaks of spider mites following the appli- cation of imidacloprid (Szczepaniec et al. 2011), researchers selected this important landscape tree and its associated spider mite, T. schoenei, as test subjects. The objectives were twofold. First, researchers sought to determine if applications of dinotefu- ran resulted in elevated densities of spider mites and rust mites (Diptilomiopidae) on American elms. Second, having observed reductions in populations of sucking insects, such as European elm scale (E. spuria), following applications of imidacloprid to American elms (Sclar and Cranshaw 1996; Szczepaniec et al. 2011), the effects of dinotefuran and imidacloprid on the abundance of two sucking insects, European elm scale and elm cockscomb gall aphid (Colopha ulmicola), were examined. MATERIALS AND METHODS Ulmus americana used in this study were part of a common garden of 27 elms planted at the University of Maryland’s Paint Branch Turf Research Facility (College Park, Maryland, U.S.). The research plot was established in July 2005. Containerized elms approximately 2.5 cm DBH were planted in native soil (Keyport Silt Loam) on 4 m centers in three rows of nine trees. Trees received approximately 5 cm of shredded hardwood mulch after planting and supplemental irrigation on a weekly basis in the summer and autumn of 2005 on weeks with no rainfall. Thereaf- ter, rainfall was the only source of water except when insecti- cides were applied. At the time of installation, all trees received a single application of fertilizer (Osmocote 14-14-14) at the recom- mended rate of 0.32 kg per 9.3 sq m (Gillman and Rosen 2000). In June 2006, trees were assigned to one of three treat- ments in a Latin square design. Nine trees served as untreated controls. Nine trees received imidacloprid and nine trees re- ceived dinotefuran. Applications of insecticides were made on June 5, 2006; May 2, 2007; and May 13, 2008. Imidacloprid was applied as a single soil drench application (Merit® 75WP, Bayer Environmental Science, Research Triangle Park, North Carolina, U.S.) at the label rate of 1.96 g per 2.54 cm DBH of trunk in 7.6 L of water. Dinotefuran was applied as a single soil drench application (Safari 20 SG® , Valent USA Corpora- tion, Walnut Creek, California, U.S.) at the label rate of 12 g per 2.54 cm DBH of trunk in 7.6 L of water. Untreated con- trol trees received a basal drench of 7.6 L of water on the dates that insecticides were applied. Mulch was removed around the base of the tree to permit infiltration of water and insecticides. On June 14, 2006, two branches 15 to 30 cm long on opposite sides of each tree were removed. The branches and excised foli- age from each tree were collectively bagged, placed in a cooler, brought to the laboratory, and refrigerated until spider mites and other arthropods were counted using a dissecting microscope. All spider mites, rust mites, scales, and aphids on adaxial and abaxial surfaces of the two most terminal, fully expanded leaves were counted. To compute densities of arthropods, leaf area was mea- ©2013 International Society of Arboriculture sured using an area meter (Model LI-31100C, Li-Cor, Lincoln, Nebraska, U.S.) and abundances of all arthropods were expressed as the number of individuals per cm2 sampling was used previously to estimate densities of arthropods on Ulmus americana in landscapes (Szczepaniec et al. 2011). Researchers considered samples taken in June 2006 as estimates of arthropod abundance prior to the effects of insecticides (pre- counts). Soil applications of neonicotinoids generally require sev- eral weeks to reach the canopy of a tree (Herms et al. 2009) and these samples were taken nine days after insecticide applications. The effects of imidacloprid and dinotefuran on arthropod abundance were evaluated in the summer of 2008. On June 24, July 21, August 11, and September 11, two branches 15 to 30 cm long on opposite sides of the tree were removed. The branches and excised foliage from each tree were collectively bagged, placed in a cooler, brought to the laboratory, and refrigerated until spider mites and other arthropods were counted using a dissect- ing microscope. All spider mites, rust mites, and scales on adaxial and abaxial surfaces of the five most terminal, fully expanded leaves were counted. Leaf areas were measured and densities of arthropods were expressed as the number of individuals per cm2 of leaf area. Aphids were sampled on two dates, July 21 and August 11, and counted by dissecting the galls through a small incision on the abaxial surface of each sampled leaf. The mean seasonal abundance for each taxon was calculated by comput- ing the mean across all four sample dates for spider mites, rust mites, and scales, and for two sample dates for gall aphids. The mean seasonal abundance for each of the four taxa sampled were tested for differences among treatments by way of comparison. Estimates of mean seasonal abundances were not normal for any of the taxa studied, nor were their variances homogeneous due to a preponderance of samples with zeros. Therefore, com- parisons of arthropod abundance among treatments were con- ducted with Kruskal-Wallis one-way nonparametric analyses of variance (Zar 1999; Statistix® Analytical Software 2003, Tallahas- see, Florida, U.S.). Following each analysis, an all-pairwise com- parisons test was used to resolve differences among treatments (Statistix® Analytical Software 2003, Tallahassee, Florida, U.S.). RESULTS AND DISCUSSION Samples taken on June 14, 2006, did not reveal significant dif- ferences in the abundance of T. schoenei (Kruskal-Wallis Statistic = 0.2267, d.f. = 2, P = 0.8929), Diptilomiopidae mites (Kruskal-Wallis Statistic = 1.5979, d.f. = 2, P = 0.4498), E. spuria (Kruskal-Wallis Statistic = 2.7368, d.f. = 2, P = 0.2545), or Colopha ulmicola (test not possible, no aphids present). By contrast, applications of imidacloprid significantly increased the abundance of T. schoenei over the course of the 2008 study (Figure 1). The mean seasonal abundance of T. schoenei on elms treated with imidacloprid was approximately ten times greater than the abundance of mites on untreated elms, and five times greater than on elms treated with dinotefuran (Kruskal-Wallis Statistic = 15.8329, d.f. = 2, P = 0.0004). Dinotefuran showed a slight trend to elevate densities of T. schoenei; this differ- ence, however, did not differ significantly from densities of mites on untreated elms over the course of the study (Figure 1). The mean seasonal abundance of Diptilomiopidae mites did not differ among treatments (Kruskal-Wallis Statistic = 0.9982, d.f. = 2, P = 0.6071). Their abundances over the course of the study were 0.608 ± 0.33 (mean ± standard error) mites per cm2 on untreated of leaf area. This method of
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