Arboriculture & Urban Forestry 38(2): March 2012 Arboriculture & Urban Forestry 2012. 38(2): 37–40 37 Effects of Imidacloprid on Spider Mite (Acari: Tetranychidae) Abundance and Associated Injury to Boxwood (Buxus spp.) Adrianna Szczepaniec and Michael J. Raupp Abstract. Boxwoods are one of the most widely used woody shrubs in managed landscapes, but they suffer frequent attack by the boxwood leafminer (Monarthropalpus flavus). The neonicotinoid insecticide imidacloprid is highly efficacious in reducing the abundance of M. flavus when applied as a foliar spray or a soil drench. Recent reports of elevated populations of spider mites following applications of imidacloprid to other species of woody plants prompted an investigation to determine the effects of imidacloprid on abundance of a specialist spider mite, Eurytetranychus buxi, and the resultant damage it causes. Boxwoods treated with imidacloprid housed significantly more E. buxi and sustained more discoloration than untreated boxwoods. Moreover, there was a direct relationship between the abundance of E. buxi and the amount of associated injury. Arborists and landscape managers should be aware of the potential for elevated abundance of spider mites on boxwoods and greater levels of discoloration following applications of imidacloprid. Key Words. Buxus spp.; Eurytetranychus buxi; Imidacloprid; Injury; Monarthropalpus flavus; Secondary Pest Outbreak. The boxwood leafminer (Monarthropalpus flavus) is one of the most important insect pests of boxwoods in nurseries and land- scapes owing to the widespread use of Buxus spp. in planting designs and the propensity of boxwoods to incur infestations of this pest (Holmes and Davidson 1984; Gagne 1989; Batdorf 1994; d’Eustachio and Raupp 2001). In the early 2000s, appli- cations of conventional insecticides, such as carbamates and or- ganophosphates to control boxwood leafminer (Schread 1970) gave way to a new class of insecticides, neonicotinoids. Imida- cloprid in particular proved to be highly efficacious in controlling boxwood leafminer when applied as a foliar spray (d’Eustachio and Raupp 2001) or as a soil injection (Gill et al. 2002). Soon after the appearance of imidacloprid in the horticultural arena, reports surfaced regarding the outbreak of secondary pests, par- ticularly spider mites, on plants treated with imidacloprid. El- evated populations of spider mites following the use of imida- cloprid occurred on honeylocust (Gleditsia triacanthos) (Sclar et al. 1998), eastern hemlock (Tsuga canadensis) (Raupp et al. 2004), American elms, (Ulmus americana) (Raupp et al 2008; Szczepaniec et al. 2011), and rose (Rosa sp.) (Gupta and Kris- chik 2007). Hypothetical mechanisms underlying elevated populations of spider mites include disruption of the abundance and activities of natural enemies, enhanced quality of plant foli- age, and elevated fecundity of mites (Sclar et al. 1998; Gupta and Krischik 2007; Raupp et al. 2010; Szczepaniec et al. 2011). Although several previous accounts documented elevated populations of mites following imidacloprid use, none sought to examine its effect on abundance of spider mites on boxwoods or the relationship between spider mite populations and associ- ated injury to plant foliage. The objectives of this study were threefold. First, the researchers determined if a standard ap- plication of imidacloprid as a soil drench increased abundance of spider mites associated with boxwoods. Next, the research- ers determined if plants treated with imidacloprid sustained more injury in the form of mite-induced discoloration com- pared to untreated plants. Finally, using regression analysis, the study authors sought evidence of a relationship between spider mite abundance and injury expressed as foliar discoloration. MATERIALS AND METHODS Boxwoods (B. sempervirens) used in this study were an ele- ment of a managed landscape on the campus of the University of Maryland (College Park, Maryland, U.S.). Boxwoods were components of trimmed hedges in mulched beds that received no supplemental water other than rain. In a completely random- ized design, the study authors selected 20 boxwoods that were approximately 0.6 m tall and grew in rows separated by approxi- mately 0.3 m between individual plants. At least two boxwoods in the hedge, separated treated plants from untreated ones, serve as buffers between treatments. Ten plants received imidacloprid applications and 10 other designated as untreated controls did not. On July 3, 2003, imidacloprid was applied as a soil drench formulation (Merit® water soluble powder, 75% imidacloprid, Bayer, Research Triangle Park, North Carolina, U.S.) at the la- beled rate of 2 g/0.3 m of height dissolved in 200 ml of water. Abundance of spider mites was evaluated at intervals of approxi- mately one month from July through September by removing one shoot from each shrub, examining the terminal microscopically, and counting all eggs, nymphs, and adult mites on each of the five most distal leaves. Abundance of mites was expressed as the number per terminal. Each of the monthly estimates of mite abun- dance for each plant was summed over the three month interval of the study to calculate a cumulative estimate of mite abundance ©2012 International Society of Arboriculture
March 2012
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