Arboriculture & Urban Forestry 43(6): November 2017 tant factor in regulating ALM populations. While no infested mined leaf tips were held to check for parasitism, a visual examination of mined leaf tips and larvae during two IBT assessments in autumn–winter 2000–2001, and late-summer 2001, did not reveal evidence of parasitism, and no parasitoids were found associated with ALM larval mines over two successive ALM generations. Van Driesche et al. (1996) noted two spe- cies of parasitoids, Pentacnemus bucculatricis and Apanteles dedelliae, associated with ALM in Connecticut, U.S. (Britton and Zappe 1922). In Quebec, Canada, Juillet (1972) recorded 26 spe- cies of parasitoids collectively destroying 50% of several species of arborvitae leafminers. In addition, a study by Bazinet and Sears (1979) found seven parasitoids species caused 9%–18% mortality of ALM, but their impact was reduced due to 63% winter-related mortality in 1977. It is highly probable that other morphologi- cal and/or chemical plant characteristics may be responsible for making these very dense plants less favorable or desirable, but their inves- tigation was beyond the scope of this study. Results from the current study suggest some important guidelines for ALM management use- ful to arborists, urban foresters, landscape profes- sionals, and nursery growers. First, while ALM can be a serious defoliator of arborvitae, and early detection and management is critical for overall plant health, some varieties appear to be more affected than others, and there may be different tolerances for damage depending upon a given situation (e.g., plants grown in a nursery for sale versus those in established landscapes). A study by Raupp et al. (1988) on bagworm defoliation on American arborvitae to determine aesthetic injury levels (AIL) and economic injury levels found 50% of observers perceived damage at 8% defoliation and discoloration. Further, Koehler and Moore (1983) examined the relationship between density of cypress tip miner leafminer as an indicator of aesthetic injury using an “unsight- liness rating” as one of the first attempts at quan- tifying the relationship between pest abundance and aesthetic quality of a landscape plant. Weston (2008) calculated woody plant pest indexes incor- porating detectability, damage potential, and ease of control, with leafminers falling in the moder- 213 ate range with suggested treatment thresholds of 50% of leaves infested. He based his leafminer threshold on the notion that 25%–50% defo- liation can be tolerated by a woody plant before damage is done. However, he did not distinguish between leafminers feeding on deciduous and evergreen plants. The authors of the current study did not conduct a formal analysis of “unsightli- ness” or conduct a formal damage assessment; the authors did receive verbal communication from The Morton Arboretum’s Manager of Plant Col- lections conveying his concern over the level of plant damage on the arborvitae study plants (K. Bachtell, personal communication). In the current study, this suggests an AIL had been approached, if not exceeded; two cultivars (‘Hetz Midget’ and ‘Hoopesii’) were below the 8% defoliation– discoloration level identified by Raupp et al. (1988). According to Raupp et al. (1988), evergreen plants (i.e., arborvitae) can tolerate up to 20% defoliation, and entirely re-foliate in one to two years. Using a suggested AIL of 20% defoliation, an additional five species/cultivars would probably not require plant protection treatments. The remaining nine species/cultivars with IBTs >20% would require some type of appropriate control measures. Second, when selecting arborvitae for the landscape and/or for managing existing plants, selecting and incorporating short, dense, and globe-shaped arborvitae species/cultivars may be less prone to ALM feeding damage compared to taller, columnar, and pyramidal-shaped trees. Third, when monitoring for ALM, visually assessing and/or sampling from the upper can- opy on shorter plants, and the lower and middle canopy areas of larger trees, will provide an accu- rate assessment of ALM population levels and potential plant damage. Scouting for IBTs would best be performed in spring, prior to adult emer- gence in preparation for adult foliar sprays, if warranted, or in late-summer to early-autumn, when most leafmining activity has peaked. Fourth, examining any aspect (N, S, E, W) of the tree canopy should provide an accu- rate assessment of existing feeding damage. Fiſth, biological control may not always be a reliable management tactic as seen in this study. Finally, because of ALMs univoltine life cycle, and late-summer and early-spring larval feeding, ©2017 International Society of Arboriculture
November 2017
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