66 Eirich: Action Thresholds for Control of Cankerworms Arboriculture & Urban Forestry 2008. 34(2):66–73. Establishing Action Thresholds for Control of Cankerworms in Regina, Saskatchewan, Canada Russell Eirich Abstract. An action threshold (AT) is often established to determine if and when control treatments are required for pest populations. In municipal settings, control treatments for insects defoliating the urban forest are typically determined in response to customer requests, maintenance demands of highly visible municipal public spaces, economic requirements, and actual insect damage. In 2006, the City of Regina Integrated Pest Management section created a new method of establishing an AT for cankerworm control by determining if monitoring counts for female cankerworms (Paleacrita vernata Peck and Alsophila pometria L.) were greater than 95% of an identified “normal” population range when based on historical records. Key Words. Acer; action threshold; Alsophila pometria; defoliation; empiric rule; Fraxinus spp.; Geometridae; Lepidoptera; Paleacrita vernata; Populus spp.; Ulmus americana. Regina, Saskatchewan, Canada, is a midsized Canadian city that was founded on a treeless prairie. Nearly all trees within Regi- na’s city limits were hand-planted and are highly valued by residents. As of 2006, Regina’s urban forest contained approxi- mately 45,000 municipally owned American Elms (Ulmus americana) and 32,000 privately owned American Elms. The city is divided into 77 distinct service areas (sectors) for the purpose of urban forestry and Integrated Pest Management (IPM) operations (Figure 1). The IPM section monitors and con- trols pest populations within the urban forest on a sector by sector basis (Regina Urban Forest Management Strategy 2000). The life cycle and habits of cankerworms have been known for over two centuries. In 1795, William D. Peck published The Description and History of Cankerworms (Pfadt 1985). This species, commonly known as spring cankerworm, was identified by Peck as Paleacrita vernata. A fall cankerworm (Alsophila pometria L.) would later be identified. Cankerworms belong to the family of Geometridae in the order of Lepidoptera. Both species are widely distributed across North America. Canker- worm larval feeding occurs primarily on elm (Ulmus spp.) and also occurs on maple (Acer spp.) and other ornamental trees (La France and Westwood 2006). The common name of spring and fall cankerworms reflects the period of adult emergence. In Regina, spring cankerworm adults emerge in early spring as soon as the snow cover recedes from the base of the trees. The gravid wingless female climbs into the crown of the tree and deposits egg masses of up to 400 eggs in bark crevasses and under bark scales (Philip and Mengersen 1989). Fall cankerworm adults emerge from the soil in late Sep- tember and October. The gravid wingless female of this species climbs up the tree and deposits egg masses of up to 100 eggs in bark crevasses, twigs, and other branches. The eggs of the fall cankerworm are the overwintering stage (Philip and Mengersen 1989). The eggs of both species hatch in spring as soon as the buds begin to open. Larvae can move between trees and escape preda- tors by descending from long, thin silk lines. This silk allows the larvae to be able to swing between differing trees (Johnson and ©2008 International Society of Arboriculture Lyon 1991). On maturity, the larvae “drop” on the silk lines and begin to pupate just beneath the soil surface (La France and Westwood 2006). Two methods of control are common. The first is tree banding. A sticking agent applied to the trunk traps wingless gravid fe- male cankerworms as they move along the trunk to deposit their eggs (Johnson and Lyon 1991). The second method is tree spray- ing using a registered insecticide often containing Bacillus thur- ingiensis kurstaki (Btk). Btk specifically targets feeding canker- worm larvae (La France and Westwood 2006). Bacillus thuring- iensis is a naturally occurring bacterium found in soils and grain dust (Baum et al. 1999). Historically, cankerworms have been a viewed by Regina’s residents as a chronic nuisance to elm, maple, and ornamental tree populations in the urban forest. In reality, this pest has the potential to defoliate a significant portion of the city’s urban forest in a short period of time. During the most extreme can- kerworm defoliation events, ash (Fraxinus spp.) and poplar (Populus spp.) have also been entirely defoliated. The IPM sec- tion has been tasked with monitoring and controlling canker- worm populations when necessary. For over 20 years, the IPM section monitored cankerworm populations by installing tree bands on randomly selected city- owned elm trees. Tree bands have been placed on an approxi- mate band to tree ratio of 1:110. Sectors with higher populations of elms had a greater number of bands. As a general rule, sectors rated as requiring the greatest priority of control were those with the highest numbers of female cankerworms. If female counts in two sectors were equal, the number of male cankerworms was used as a second determining factor. High-priority sectors were also evaluated for defoliation by cankerworm larvae. Sectors would be treated with a single application of Btk in an effort to treat as many sectors as possible. On average, control operations would continue for a 4-week time period. Treatments would stop when cankerworm larvae were no longer present and had stopped feeding. This method of setting sector priority for treatment had a number of drawbacks. First, sectors with greater numbers of
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