Arboriculture & Urban Forestry 32(4): July 2006 145 sitoid–predator complex does not seem to prevent the scales from reaching high densities, even on minimally managed or wild hosts (Hubbard and Potter 2005). In Kentucky, adult parasitoids associated with calico scale are most active in April to early May, and from July through October, so short residual contact insecticides targeting recently hatched crawl- ers in late May or June will likely have relatively little impact on parasitism rates. Horticultural oil sprays are effective against many scale insect species (Johnson 1982; Baxendale and Johnson 1990), although often they provide only partial control (e.g., Turner and Buss 2005). In our trials, horticultural oil applied with optimal coverage gave just 38% to 50% corrected mortality of settled crawlers (Table 2). Unlike some soft scales that have only a thin dorsal layer of wax, calico scale nymphs produce heavy angular wax plates as they mature (Gill 1988, Figure 1). That wax may reduce vulnerability of crawlers to soaps and oils. Given the 2- to 3-week hatching window and location of settled crawlers, it is unlikely that soaps, oils, or other “soft” contact insecticides requiring full spray coverage will effectively control calico scale in large trees. Dormant oil sprays targeting late second instars and young adults also were ineffective in our trials. Trunk-injected or soil-applied systemic insecticides are suited for sites such as horse farms where aerial spray drift is a concern (Tattar et al. 1998; Potter et al. 2005). Trunk mi- croinjection with dicrotophos (bidrin) can be highly effective against some tree-feeding insects (e.g., McClure 1992; Potter et al. 2005). Although that tactic suppressed calico scale crawlers in three of our four trials, adjusted control was only 43% to 55% (Table 3). Within-tree control also was incon- sistent; crawler mortality often ranged from 0% to 100% on leaves from different branches or canopy zones (Hubbard 2004). Trunk- or soil-injected imidacloprid often provides good control of aphids (Sclar and Cranshaw 1996) adelgids (Steward and Horner 1994), psyllids (Young 2002), lace bugs (Gill et al. 1999), certain scales (Sclar and Cranshaw 1996), and other tree pests. In our trials, however, imidacloprid failed to control any life stage of calico scale regardless of whether applied by trunk injection in spring or summer or by soil injection in autumn, winter, or spring. Translocation of systemic insecticides is affected by tree vitality and health of the transport tissues and may be im- peded by old wounds, cankers, or scarring in trunk or branch tissues (Doccola et al. 2005). Species, size, and age of treated trees, environmental factors (e.g., drought, soil and air tem- perature), and time of year can affect translocation (Tattar et al. 1998), and efficacy may also depend on the specific tis- sues on which the target pest feeds (Rebek and Sadof 2003). Such interactions are poorly understood, so we cannot pin- point why dicrotophos was erratic and imidacloprid was in- effective against calico scale. Our study trees generally were in rural, favorable growing sites, the trials involved different tree species and were done over several years, and the appli- cations were made by an experienced, ISA-certified arborist. Trunk-injected imidacloprid controlled aphids and psyllids on large pin oak and eucalyptus trees, respectively, within 1 week after treatment (Tattar et al. 1998; Young 2002), and trunk-injected dicrotophos killed tent caterpillars in tall cherry trees within a few days (Potter et al. 2005). In contrast, several months may be required for soil-applied imidacloprid to be fully translocated within large trees (Tattar et al. 1998). Regardless, there should have been ample time for both our soil applications and trunk injections to reach their targets. Although specific feeding sites of calico scale life stages within plant tissues have not been studied, it is noteworthy that neither settled crawlers nor overwintering nymphs pro- duce noticeable honey. Rebek and Sadof (2003) suggested that lack of translocation beyond vascular tissue may account for imidacloprid’s poor efficacy against euonymus scale, Unaspis euonymi (Comstock), which feeds on subcuticular plant cell contents rather than phloem. Similar limitations may account for its failure to control calico scale crawlers. The spatial distribution of settled crawlers in relation to leaf veins differs markedly on different hosts (Hubbard and Potter 2005), which might also contribute to variable performance of systemic insecticides. Adult female calico scales produce copious honeydew, indicative of phloem-feeding, but if up- ward translocation of imidacloprid through xylem deposits the insecticide mainly in bud and leaf tissues, perhaps not enough moves back down the phloem to control scales feed- ing through the bark of branches and trunks. Additional re- search on how systemic insecticides move within trees may help clarify why some pests are more readily controlled than others, and why better control of calico scale was not ob- tained by that approach. IMPLICATIONS FOR ARBORICULTURE Calico scale, an invasive soft scale, is a severe pest of maples, sweetgum, honeylocust, crabapple, and other tree species in the eastern United States. Nymphs overwinter on bark, pro- ducing copious honeydew as they mature in spring. Eggs hatch in late May, approximately a week after adult color- ation fades from bright black and white to dull brown, and crawlers migrate to leaves where they feed until autumn. Hydraulic sprays targeting settled crawlers in summer likely will provide only partial control because they fail to reach all the tiny, waxy nymphs nestled along leaf veins. Treating at first egg hatch may be more effective by intercepting motile crawlers as they disperse over treated bark and leaves. Sev- eral foliar insecticides (bifenthrin, cyfluthrin, acephate, car- baryl, pyriproxyfen) are active against settled crawlers, but insecticidal soap and summer or dormant oils were ineffec- tive in our trials. Trunk-injected dicrotophos (bidrin) gave partial (40% to 50%) mortality of settled crawlers, but trunk- ©2006 International Society of Arboriculture
July 2006
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