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Variation in Captures of Adult Winter Moths (Operophtera brumata) In Coastal Maine Over Two Years
Kaitlyn O’Donnell and Eleanor Groden

Northeastern Naturalist,Volume 24, Special Issue 7 (2017): B72–B780

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Northeastern Naturalist K. O’Donnell and E. Groden 2017 72 Vol. 24, Special Issue 7 Variation in Captures of Adult Winter Moths (Operophtera brumata) In Coastal Maine Over Two Years Kaitlyn O’Donnell1,* and Eleanor Groden2 Abstract - Operophtera brumata (Winter Moth) is an invasive insect defoliator named for its early winter activity in its native and introduced ranges. In this study, we examined the relative winter densities of adult female and male Winter Moths in Harpswell, ME, an area of recent outbreak. Additionally, we measured female densities to determine whether specific host-plant species are favored for egg laying. We found that Winter Moth densities during the second winter of this study were lower than in the first, possibly in response to extreme cold temperatures during January 2014. We also found that peaks in male flight coincided with temperatures rising above freezing and that female densities were highest on Quercus rubra (Red Oak), a known preferred host species. Introduction Operophtera brumata L. (Winter Moth), native to Europe, is an invasive insect in North America and causes severe defoliation in outbreak areas. The species was originally introduced into Nova Scotia in the 1930s but was not confirmed as Winter Moth until 1950 (Hawboldt and Cuming 1950). Following its initial establishment, the Winter Moth spread throughout the province causing widespread defoliation to forest hardwoods as well as orchard crops. More recently, defoliation of deciduous trees and shrubs by spring-feeding Lepidoptera was described in Massachusetts in the 1990s and initially attributed to outbreaks of the native Alsophila pometaria (Harris) (Fall Cankerworm) and Operophtera bruceata (Hulst) (Bruce Spanworm). However, in 2003, Elkinton et al. (2010) confirmed the primary defoliator to be the Winter Moth. This confusion resulted from the physical and phenological similarities between the invasive Winter Moth and these common native spring defoliators. In fact, all life stages of the closely related Bruce Spanworm are almost identical to those of the Winter Moth, and both species often co-occur in the Northeast where their ranges overlap (Childs et al. 2011). Since the identification of Winter Moth in Massachusetts in 2003, Elkinton et al. (2010, 2014) have identified Winter Moth populations in Connecticut, New Hampshire, New York, Rhode Island, and coastal Maine. Adults of this insect are cold hardy and are active throughout the winter months. Larval feeding occurs in the spring, with the caterpillars pupating in the early summer and remaining in this stage until late fall and early winter. Survival and emergence from the pupal stage relies on cold temperatures during the later period of 1Norfolk County Mosquito Control District, 61 Endicott St. Suite 66, Norwood, MA 02062. 2School of Biology and Ecology, University of Maine, 5722 Deering Hall, Orono, ME 04469. *Corresponding author - kaitm.odonnell@gmail.com. Manuscript Editor: Daniel Pavuk Winter Ecology: Insights from Biology and History 2017 Northeastern Naturalist 24(Special Issue 7):B72–B80 Northeastern Naturalist 73 K. O’Donnell and E. Groden 2017 Vol. 24, Special Issue 7 pupation (Holliday 1983). Adults appear in the late fall, with males emerging earlier than females. The females are flightless (vestigial wings) and after emerging from the ground, crawl towards a nearby host plant and into the canopy while emitting a sex pheromone to attract flying males. After mating, females lay eggs singly on the bark under lichen or in crags for protection. The eggs spend the rest of the winter on the bark of the host plant (Cuming 1961) and hatch in synchrony with swelling of leaf and flower buds in the early spring. Varley and Gradwell (1960, 1968) have described winter disappearance to encompass all mortality occurring from the time adults emerge from the pupal stage in late fall to the late instar larval population in May. They found this mortality to be the key factor determining Winter Moth population dynamics in England. Winter disappearance is a result of many different causes such as predators of adult, egg, and larval stages, asynchrony between egg hatch and bud burst, or extreme cold temperatures. There has been extensive work on larval populations and early instar mortality; however, little work has focused on the adult populations. This study aims to assess the winter densities of adult male and female Winter Moths in coastal Maine. Field-Site Description We conducted this study over 2 years within the Winter Moth infestation area in Harpswell, ME, along the southernmost section of Maine State Route 123. Two closely located sites were utilized throughout the course of this study (43°45'7.5"N, 70°0'24.5"W and 43°45'15.3"N, 70°0'37.2"W). Both sites consisted of residential homes surrounded by mixed deciduous stands mostly composed of Quercus rubra (L.) (Red Oak), Malus pumila Miller (Apple) and Malus sp. (crab apple), Acer rubrum (L.) (Red Maple), Prunus pensylvanica (L.f.) (Pin Cherry), and Betula papyrifera (Marshall) (White Birch). Harpswell is in the coastal climatic region of Maine characterized by cooler summers and warmer winters than the rest of the state (Briggs and Lemin 1992). The historical climate data for the area, reported by the National Centers for Environmental Information under the National Oceanic and Atmospheric Administration as 30-year averages of climatological variables from 1981 to 2010 was taken from the Naval Air Station in Brunswick, ME, 12 miles inland from the field site. The monthly average historical temperatures and precipitation, respectively, for the months of November to February ranged from -6.4 to 3.6 °C, and 8.7 to 14.3 cm (measured as rainfall and liquid equivalent). Methods Male relative density During the winters of 2012–2013 and 2013–2014, two white Multi-Pher 1 Pheromone Traps with a green cover (designed by Jobin [1985], manufactured by Bio-Contrôle Services, Sainte-Foy, QC, Canada), provided by the Maine Forest Service, were hung on 2 Red Oak trees at the same study site each year. Each trap contained a Vaportape II insecticidal strip (Hercon Environmental, Emigsville, PA) Northeastern Naturalist K. O’Donnell and E. Groden 2017 74 Vol. 24, Special Issue 7 and was baited with the Winter Moth sex pheromone, provided by Dr. Joseph Elkinton. Baits consisted of small rubber stoppers impregnated with a 1000-μg mixture of the pheromone (90% [Z,Z,Z]-1,3,6,9-nonadecatetraene) and attached to traps with a metal clip. In field tests, the synthetic Winter Moth pheromone lure has been used successfully for the entire Winter Moth flight period (Elkinton et al. 2010, Roelofs et al. 1982). Traps were monitored daily by volunteer residents of Harpswell, and all moths were removed and counted by the researchers and volunteers. We dissected a subsample of adult males collected in the traps during November and early December 2012 and examined the shape of the uncus to determine whether moths caught were O. brumata or O. bruceata according to methods described in Elkinton et al. (2010). We set a ThermochronTM iButton temperature probe (Embedded Data Systems, Lawrenceburg, KY) on each trap to record hourly and collected them at the end of the male flight period. We placed an additional 2 iButtons in the soil 8 cm deep below the trees supporting uni-traps to monitor temperatures pupae are exposed to in the soil throughout the emergence period. Precipitation data was obtained from the Wiscasset Airport in Wiscasset, ME, through the National Centers for Environmental Information under the National Oceanic and Atmospheric Administration (NOAA 2015). We analyzed the relationships between the average number of males caught per day, average daily minimum and maximum air temperatures, and precipitation throughout the flight period using a multivariate regression analysis. We used a 1-way ANOVA to test for differences in the average number of males caught per day between years. All analyses were done in JMP®, Version 11 (SAS Institute, Inc. 1989–2007). Female relative density During the winters of 2012–2013 and 2013–2014, we selected 5 known Winter Moth host-tree species at both sites and wrapped them with sticky traps (described below). The selected host-tree species were: Red Oak, Apple, Red Maple, Pin Cherry, and White Birch. Traps consisted of a 3-cm deep strip of cotton batting covered with an outer plastic strip coated with Tanglefoot™ adhesive and placed with the sticky side facing the tree trunk. When emerging females crawled up the tree, they encountered the cotton batting and were directed towards the adhesive-coated plastic where they become stuck. During the winter of 2012 to 2013, we deployed 5 sticky bands for 1 week during the estimated peak of Winter Moth activity (7 December 2012 to 14 December 2012) based on Maine Forest Service observations from the previous year (C. Donahue, Maine Forest Service, Augusts, ME, pers. comm.). Bands were placed on 2 Red Oaks, 1 White Birch, and 2 Red Maples. After 1 week, bands were taken down and the number of females were counted on each band. During the winter of 2013 to 2014, we deployed 7 bands for the entire Winter Moth flight season from 12 November 2013 to 23 January 2014 on the following trees: 2 Red Oaks, 2 Apples, 1 Pin Cherry, 1 White Birch, and 1 Red Maple. Each week, we replaced old sticky bands with a new band and counted the numbers of females caught on each band. For each sampled tree, we measured the diameter at breast height (DBH). Using a 3-way ANOVA with JMP®, we assessed the number Northeastern Naturalist 75 K. O’Donnell and E. Groden 2017 Vol. 24, Special Issue 7 of total females caught on each host plant species over one week in 2012 to 2013 and the average number of females caught per week in 2013 to 2014, with host plant and DBH as factors and site as a blocking variable. Results Male relative density The flight period for male Winter Moths occurred from 6 November 2012 to 12 January 2013 in the first winter and from 2 November 2013 to 21 January 2014 in the following winter (Fig. 1). The average daily minimum temperature and male flight activity was positively correlated, with the mean moth trap catch increasing as minimum temperatures rose above freezing (2012–2013: Spearman’s ρ = 0.63, P < 0.0001; 2013–2014: Spearman’s ρ = 0.58, P < 0.0001). Peaks in trap catch coincided with days when the minimum temperature was above 0 °C. In January 2014, when the average high temperatures rose above 0 °C consistently Figure 1. Mean number of adult male moths trapped in 2 pheromone traps at 1 site from (A) November 2012 to January 2013 and (B) November 2013 to January 2014 plotted with the mean high and low daily temperatures. (C) Mean number of adult male moths trapped in pheromone traps compared between different years. Northeastern Naturalist K. O’Donnell and E. Groden 2017 76 Vol. 24, Special Issue 7 for several weeks, there was a small increase in trap catch from 15 January until 21 January 2014 after a long period of inactivity. Additionally, no correlation was found between precipitation and male flight activity in either year of the study (2012/2013: Spearman’s ρ = 0.08, P = 0.55; 2013/2014: Spearman’s ρ = 0.02, P = 0.87). Male densities were lower in the second winter of this study than in the first (1-way ANOVA: F1,151= 9.11, P = 0.003; Fig. 1). We dissected a total of 150 adult male moths for identification. Only 13 out of this subsample were identified as Bruce Spanworms, all other moths were confirmed to be Winter Moths. The average maximum and minimum air temperatures for the activity period during the winter of 2012– 2013 were 4.25 °C and -6.14 °C, respectively. The average maximum and minimum air temperatures for the winter of 2013–2014 were 3.63 °C and -4.70 °C, respectively. Air temperatures fluctuated throughout the day, with the extremes reaching 16.11 °C and -18.89 °C in 2012–2013, and 12.5 °C and -20.5 °C in 2013–2014. Soil temperatures were more consistent, remaining between 0 and 5 °C throughout the Winter Moth flight period. Female relative density The number of females trapped per tree did not differ significantly with the host plant species or tree size (3-way ANOVA; host plant: F(4,5)=0.88, P = 0.54; DBH: F(4,5)=0.43, P = 0.54; limited degrees of freedom did not allow testing the interaction). Observed densities were highly variable depending on the individual tree. Though 1 Red Oak hosted the highest number of females overall, another Red Oak included in the study trapped fewer females than other host plants. Similarly, we observed 1 Apple tree hosting a high number of females and 2 separate Apple trees at a different site with a much lower quantity of females (Table 1). Table 1. Number of adult female Winter Moth trapped on different tree species (A) totaled over one week from 7 December 2012 to 14 December 2012, and (B) counted weekly from 12 November 2013 to 23 January 2014. Data in the first column are presented as average number of females per week per centimeters DBH and as the cumulative number of females per tree throughout the 10-week sampling period. Host plant Site Females/week/m DBH Total cumulative females/tree Dec 2012 Red Oak 1 7.43 214 Red Oak 1 0.81 33 White Birch 1 1.86 76 Red Maple 2 2.25 54 Red Maple 2 0.46 11 Nov 2013–Jan 2014 Red Oak 1 6.90 1242 Red Oak 2 2.37 625 Apple 1 2.93 352 Apple 2 1.09 98 Pin Cherry 2 0.29 14 White Birch 1 0.89 255 Red Maple 2 0.63 120 Northeastern Naturalist 77 K. O’Donnell and E. Groden 2017 Vol. 24, Special Issue 7 Discussion Although the Winter Moth, as its name implies, has a high level of cold tolerance enabling adult emergence, mating, and egg laying to occur during the winter months in northern temperate zones in Europe and North America, our study suggests that their activity is limited by low winter temperatures. We found that adult male activity is correlated with temperature, with peaks in activity occurring when air temperatures are above freezing. Our results support findings in Nova Scotia, where peaks in adult male flights coincided with temperatures above 0 °C (Cuming 1961). In January of 2014, after temperatures increased above freezing following a prolonged period of cold, we observed an increase in males caught from 15 January to 21 January. Because adult males live for about 1 week, and there is continual emergence throughout the activity period (Van Dongen et al. 1999), this increase was likely a small, late emergence of males due to the warmer temperatures. Male flight activity is possibly driven by not only the challenge of being active in extreme cold, but also the ability of males to detect the volatile female pheromones. A study that examined the Winter Moth sex pheromone found that males were responsive to female pheromones between 4 and 15 °C; this temperature range is on the lower end of response ranges for other moth species exposed to sex pheromones (Roelofs et al. 1982). However, throughout our study the actual temperature range we observed during the Winter Moth flight period, with temperatures dropping well below 10° C for much of December, was on the low end of this reported ideal pheromone temperature range. We observed a decrease in adult Winter Moth populations during the second winter of this study, coinciding with extreme cold temperatures when average daily temperatures remained below 0 °C for more than 2 weeks in late December and early January, with a low of -20.5 °C reached on 2 and 4 January 2014. During this cold period, adult females were observed dead on the snow pack at the base of host trees. This decrease in adult population preceded lower spring larval populations observed in May and June of 2014 (K. O’Donnell and E. Groden, unpubl. data). These findings have implications for spring defoliation levels and for the control of this insect. The host-specific parasitic fly of the Winter Moth, Cyzenis albicans (Fallén), has recently been released in Maine with the hope of establishment and eventual control of Winter Moth. The life cycle of this insect is such that it pupates underground throughout the entire adult Winter Moth flight period, and emerges in the spring as an adult. Adult emergence occurs after Winter Moth egg hatch, allowing it to parasitize the Winter Moth larvae in its third instar or later by laying eggs on host-plant leaves that will ultimately be consumed by the Winter Moth caterpillars. Thus, with enough insulating snow, this insect is not likely as vulnerable to extreme cold temperatures as the adult stage of the Winter Moth. The soil temperature data we collected at our study sites demonstrated that the temperatures experienced in the soil were more stable and remained at or above freezing throughout the winter, while air temperatures were variable and often dropped below freezing. Additionally, many studies have shown that once the parasitoid is established and reduces Winter Moth densities, other causes of mortality, such as pupal predation, become Northeastern Naturalist K. O’Donnell and E. Groden 2017 78 Vol. 24, Special Issue 7 important for controlling outbreaks of Winter Moth populations because the parasitoid is most effective when Winter Moth populations are high (Frank 1967, Horgan and Myers 2004, Varley and Gradwell 1960). Populations of Winter Moth have been detected as far north as Machias, ME (Elkinton et al. 2010, 2015); however, outbreaks remain in localized pockets in the southern to mid-coast area. Though the Winter Moth has rapidly expanded its range throughout southern New England since its introduction, extreme cold temperatures may currently be a limiting factor for Winter Moth expansion and outbreak in northern coastal and inland areas of Maine. However, if winter temperatures continue to warm, Winter Moth may expand inland in Maine, as it has in Massachusetts (Elkinton et al. 2015). Additionally, the composition of Maine forests is expected to change with the warming climate. The Picea (spruce) and Abies (fir) forests that characterize coastal Maine are predicted to recede, being gradually replaced by deciduous hardwoods that are more susceptible to Winter Moth damage, such as Red Maple (Jacobson et al. 2009). Winter Moth populations may also have the potential to spread by way of genetic changes. Recent studies have described hybridization between Winter Moth and Bruce Spanworm, which may promote inland range expansion as the Bruce Spanworm is more cold tolerant and has a natural, widespread inland range (Elkinton et al. 2010, 2014; Gwiazdowski and Elkinton 2013). The Bruce Spanworm is a native North American pest of many different tree species and often has periods of outbreak, causing occasional defoliation in the northern United States and Canada (Brown 1962, Elkinton et al. 2010). Winter Moth and the closely related Bruce Spanworm are equally attracted to the pheromone mixture used in this study (Elkinton et al. 2011, Roelofs et al. 1982). Out of 150 dissected males, we identified only 13 as Bruce Spanworm. These dissections were done only for late November and early December trap catches, as this is the period of time during which Bruce Spanworm activity and Winter Moth activity overlap in Maine (J. Elkinton, University of Massachusetts, Amherst, MA, and C. Donahue, pers. comm.). It is possible that through hybridization with the Bruce Spanworm the Winter Moth will no longer be limited to coastal habitats in Maine. This potential range shift may put new host plant species at risk of defoliation by Winter Moth in inland habitats. We observed the highest number of females utilizing Red Oak trees for egg laying throughout both years of this study. Similarly, spring larval densities are higher on oak trees than on other host plant species (K. O’Donnell and E. Groden, unpubl. data). However, during December 2012, there were fewer females found on 1 of the sampled Red Oak trees than on other host plants. These conflicting results may be a result of the limited sample size. Although, the numbers of females were highly variable between individual host plants, these differences remained consistent throughout the 10-week sampling period in 2013–2014. Because females are flightless, natural dispersal of this insect is dependent on ballooning during the larval stages (Cuming 1961). As such, though eggs may be predominantly laid on one host plant species, the larvae are able to freely disperse to new host plant types when faced with competition from other larvae, inferior plant quality, or unopened Northeastern Naturalist 79 K. O’Donnell and E. Groden 2017 Vol. 24, Special Issue 7 plant buds (Feeny 1970, Travis et al. 1999, Varley and Gradwell 1960). This is of interest when developing control methods that target protection of the primary hostplant species of the Winter Moth because, depending on the timing, such efforts could foster spread to alternative hosts. In light of a changing climate and the broad host-plant potential of this species, further monitoring and research is needed to evaluate the spread of Winter Moth in Maine and its population dynamics in relation to extreme cold winter temperatures and host susceptibility. Acknowledgments We would like to thank Charlene Donahue and the Maine Forest Service for support in experimental design, assistance in the field and for supplying trapping materials. We also thank Dr. Joseph Elkinton, University of Massachusetts Amherst, for supplying the pheromone baits and sharing knowledge of trapping methods as well as the design of the sticky band traps. We are grateful for the volunteer citizen scientists, Sharon Whitney and Marlene Ward, who checked traps each day and helped with counting and observations and were very welcoming and generous in allowing us access to their properties. We also appreciate all of the support from the technicians who assisted in winter field work and are grateful to our funding sources, the University of Maine Graduate Student Government and the Maine Agricultural Experiment Station. Literature Cited Briggs, R.D., and R.C. Lemin Jr. 1992. Delineation of climatic regions in Maine. Canadian Journal of Forest Research 22:801–811. 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