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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
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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)
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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
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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.
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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
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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
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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
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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.
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