The Douglas fir needle midge (Contarinia pseudotsugae): a potential threat to Douglas fir in the United Kingdom and Ireland?

Abstract

The recent identification of Douglas fir needle midge (Contarinia pseudotsugae Condrashoff) (Diptera: Cecidomyiidae; Genus Contarinia) in Western Europe represents a new and previously unrecognised threat to the Douglas fir resource in the United Kingdom and Ireland. This article provides an overview of Douglas fir needle midge, its morphology, ecology and impact on the health of Douglas fir. The objectives are to raise awareness of this potential threat to Douglas fir and to outline possible actions that might be introduced to limit its impact.

Full text

244 www.rfs.org.uk Quarterly Journal of Forestry
Douglas fir (Pseudotsuga menziesii) is one of the
most important and valuable introduced conifers in
Europe. Currently it covers over 0.8 million ha and
is the second most cultivated non-native tree species after
Sitka spruce (Picea sitchensis) (1.2 million ha) (Spiecker et
al., 2019). In the United Kingdom and Ireland there are
46,000ha and 10,380ha of Douglas fir, respectively (Forestry
Commission, 2019; Forest Service, 2018). As our climate
changes Douglas fir is likely to be planted more widely,
provided sites are not too exposed and have adequate soil
moisture (Ray et al., 2010; Forest Research, 2020). Being
moderately shade tolerant, it is also one of the most
suitable species for continuous cover forestry (CCF)
(Wilson, 2013). At present there are relatively few pests and
diseases of Douglas fir that have a significant impact on its
growth and performance (Savill, 2019).
The recent identification of Douglas fir needle midge
(Contarinia pseudotsugae Condrashoff) (Diptera:
Cecidomyiidae; Genus Contarinia) in Western Europe
represents a new and previously unrecognised threat to the
Douglas fir resource in the United Kingdom and Ireland.
This small fly is one of three species of needle midges (the
others being C. constricta Condrashoff and C. cuniculator
Condrashoff) known to cause damage to Douglas fir
throughout its native range in western North America
(Condrashoff, 1961; Roques et al., 2019). Occurrence of the
Douglas fir needle midge in other regions was first reported
in Michigan in 2003, where it is classed as a ‘native invader’
transported by human activity to a new habitat (EPPO,
2019). It is now also present in Pennsylvania (Rajotte, 2017).
In Western Europe the needle midge was initially reported at
several locations in Belgium and the Netherlands in 2015,
and then France and Germany in 2016 (Leroy et al., 2015;
EPPO, 2019).
Douglas fir needle midge is not currently known to be
present in the United Kingdom (Defra, 2020). The UK Plant
Health Risk Register states that the overall relative risk from
the insect is moderate, but the likelihood of it surviving and
perpetuating after it has entered the UK is high (5 on 1-5
scale) (Defra, 2020). A targeted survey is being planned to
provide a more accurate assessment of the needle midge’s
status in the UK; no other mitigation measures are in place
Douglas Fir Needle Midge
(Contarinia pseudotsugae)
A potential threat to Douglas fir in the
United Kingdom and Ireland?
Edward Wilson, Gilles San Martin and Gauthier Ligot highlight a
previously unrecognised threat to one of our key forestry species.
Figure 1. Adult female of Contarinia pseudotsugae with extruded
ovipositor, in the process of laying eggs within the expanding bud
of a Douglas fir shoot.
FEATURE ARTICLES
© Royal Forestry Society 2020 (www.rfs.org.uk)

October 2020 Vol 114 No.4 www.rfs.org.uk 245
at present (Defra, 2020). Comparable information is not
available for Ireland. This article provides an overview of
Douglas fir needle midge, its morphology, ecology and
impact on the health of Douglas fir. The objectives are to
raise awareness of this potential threat to Douglas fir and to
outline possible actions that might be introduced to limit its
impact.
Identification of the Douglas fir needle midge
The adult Douglas fir needle midge is orange in colour and
approximately 3-5mm long (Fischer, 2015; EPPO, 2019)
(Figure 1). At the time of emergence in spring the adults
can be seen flying around the trees. However, many other
Cecidomyiidae have the same morphological
characteristics making the Douglas fir needle midge rather
difficult to identify. The female is
identified by a long ovipositor –
typical of the genus Contarinia –
that is used to probe between
bud scales and into partially
opened vegetative buds. With
this adaptation the female is able
to lay her eggs in protected
areas. The eggs are long, narrow
and orange-coloured. Later in
the growing season, the
presence of galls on the needles
of new shoots is the best way to
confirm the presence of the
Douglas fir needle midge.
Life cycle of the Douglas fir needle midge
The life cycle of the Douglas fir needle midge is described
in detail by Fischer (2015). The needle midge produces one
generation each year. Larvae overwinter in the soil under
infested trees. The following spring they pupate and adults
emerge from the soil to mate. Adult life span is short: 1-2
days for the male; 2-4 days for the female. The female lays
her fertilised eggs in groups on the needles of expanding
Douglas fir shoots or directly within
opening buds. The eggs hatch a
few days later and the larvae
burrow into their own individual
needle where they induce the
formation of a gall (Figure 2).
They feed on tissues inside the
gall throughout the growing
season. From mid-October to
December they exit the needle
leaving a small triangular hole,
drop to the ground and
overwinter in the soil (Bulaon,
2005).
Damage to Douglas fir trees
There is a distinct pattern to the damage caused to
Douglas fir trees by the needle midge, comprising initial
infection, seasonal progression and eventual defoliation.
Damage occurs on new needles in the current season of
shoot extension (Schmitz et al., 2016). The appearance of
“The recent
identification of Douglas fir
needle midge in Western
Europe represents a new and
previously unrecognised threat
to the Douglas fir resource in
the United Kingdom and
Ireland.”
Figure 2. Larva of Contarinia pseudotsugae (ventral face with
spatula visible) in a Douglas fir needle. The larvae can also be
orange or yellow. They can use their spatula to jump by bending
their body, jamming the spatula at the posterior end of their body
and suddenly releasing the grip. In stands with heavy infestation,
the forest floor can literally be covered with these tiny jumping
and coloured larvae just before winter.

246 www.rfs.org.uk Quarterly Journal of Forestry
FEATURE ARTICLES
damage to new foliage is more immediate than with some
other pests or pathogens of Douglas fir. The initial sign of
infestation is the development of pale patches on needles
that form into galls (Figure 3). The needles become swollen
and sometimes bent out of shape, which are key diagnostic
features. As the season progresses, infested needles
gradually darken and turn a reddish-brown colour; there
can be significant variation in the colour of infected needles
on the current shoot (Figure 4). Once a needle has been
attacked it falls prematurely from the shoot during the
following winter (Figure 5). In severe and sustained attacks,
over several years, the level of defoliation can become
significant; especially where other pests or pathogens are
also present (Figure 6).
Douglas fir is generally considered to be well-adapted
for growth in many areas of Britain and Ireland, and
currently has relatively few major pests and diseases (Savill,
2019). Three pests or pathogens that cause damage to
needles, and might be confused with Douglas fir needle
midge, include the Douglas fir (or woolly) adelgid (Adelges
cooleyi), Swiss needle cast (Nothophaeocryptopus
gaeumannii) and Phytophthora ramorum (Savill, 2019; Forest
Research, 2020).
The Douglas fir adelgid has a complex lifecycle that
requires two migration phases between spruce and
Douglas fir trees (Wood, 1977). On spruce it forms galls on
expanding shoots. On Douglas fir the white woolly aphids
can be seen on the underside of needles. Feeding activity
causes needle yellowing, bending and twisting that can
result in needle drop. Savill (2019) reports that damage can
be severe enough to arrest growth in some provenances
when trees are young.
Swiss needle cast is an endophytic fungus that causes
damage to Douglas fir needles. Symptoms are slow to
Figure 3. Needles of Douglas fir with the galls caused by larvae of
Contarinia pseudotsugae. The tissues of the needle are always
slightly swollen and the needle can be bent or not. The gall is
almost always tainted with brownish, reddish or purple colour,
but it can also sometimes become black or remain light green.
The presence of larvae inside the gall is discriminant, but the
larvae are very small and difficult to see before September.
Figure 4. Symptoms of Contarinia pseudotsugae infestation on a shoot of Douglas fir showing the variation of colour.
Photos taken in Belgium (Paliseul) in October and November 2015.

October 2020 Vol 114 No.4 www.rfs.org.uk 247
develop and may not be apparent until 2-3 years after
infection (Rajotte 2017). In contrast with the needle midge,
new shoots in the current growing year remain healthy in
appearance. Yellowing (chlorosis) of the needles occurs in
the second and third year after infection, leading to necrosis
and premature needle loss (needle cast).
Phytophthora ramorum is a fungus-like pathogen best
known for its impact on larch. It causes widespread needle
loss and tree mortality. Douglas fir is considered
susceptible when grown in close proximity to other infected
plants, which are a major source of spores (Forest
Research, 2020). Abiotic factors that cause needle damage
include winter desiccation and late spring frosts.
It has been reported that populations of Douglas fir
needle midge can fluctuate widely from one year to the next
depending on environmental factors (EPPO, 2019). Heavy
infestations can lead to severe defoliation (Figure 6). In
Washington State infestation has been reported to be as
high as 100 percent of the needles during a severe
outbreak of the Douglas fir needle midge (Fischer, 2015).
Recovery can take several years, due to the extent of
needle loss. Douglas fir needle midge is not considered a
lethal pest of its host, but it could reduce tree growth and,
where Douglas fir is planted for Christmas trees, it can have
a significant negative impact on the aesthetics and market
value of the crop (DeAngelis, 1994).
Spread of Douglas fir needle midge
in Europe
Adult Douglas fir needle midges are able to fly. However,
there is no available information on the species’ potential for
natural spread. It is also uncertain how the Douglas fir
needle midge was introduced to Europe. The most likely
cause was trade in Douglas fir plant materials, but this has
not been confirmed (EPPO, 2019). The main pathways for
human spread of the Douglas fir needle midge include:
Douglas fir planting stock; cut branches (including
Christmas trees) of Douglas fir; and soil from countries
where the Douglas fir needle midge is present (EPPO,
2019; Defra, 2020).
Possible risks to Douglas fir
In North America the Douglas fir needle midge is
recognised as a pest in Christmas tree plantations and
seed tree orchards (DeAngelis, 1994; pers. comm. M.
Vallee, 2020). Pest control measures are sometimes
necessary, and focus on surveillance trapping and
insecticide application against the adults before they lay
their eggs (DeAngelis, 1994; Bulaon, 2005). Traps are
placed on the ground to determine the emergence date of
the adults, to ensure that the insecticide is applied to
maximum effect at the correct moment. In forest conditions
chemical control is not considered feasible. Several insect
parasitoids are thought to regulate pest populations within
the native range (Fischer, 2015).
A recent Continuous Cover Forestry (CCF) workshop
(January 2020) in the Ardennes region of Belgium provided
an opportunity to observe widespread infestation of
Douglas fir seedlings and saplings in regeneration gaps
within pure and mixed-species stands managed under CCF
principles. Although there was no clear sign of a rapid
increase in mortality, there was significant needle loss on
many plants (e.g. Figures 5 and 6). The potential impact of
this damage on the growth of Douglas fir saplings in stands
undergoing transformation to CCF is explored further by
Ligot et al. (2020). This study suggests that the growth of
Douglas fir saplings is now weaker than it was ten years
ago; many Douglas fir saplings are now struggling to out-
compete the other admixed species such as Norway
spruce (Picea abies). Nevertheless, EPPO (2019) states that
future impacts of the Douglas fir needle midge are difficult
to predict.
Figure 5. Shoot of Douglas fir sapling showing moderate needle
loss due to infestation by larvae of Contarinia pseudotsugae in
combination with terminal necrosis probably caused by
Sirococcus conigenus. The needles of the previous year are
still present and only the needles of the current year have been
attacked and have partially fallen. Photo taken in Belgium
(Gedinne) in November 2015.
Figure 6. More severe attack than figure 5. Almost 100% of the
needles in the current year are attacked by Contarinia
pseudotsugae and will fall during the winter. All the needles of
previous years have already fallen due to earlier attacks by
Contarinia pseudotsugae or Swiss needle cast. This level of
defoliation is now frequent in Belgium. Photo taken in Belgium
(Gedinne) in October 2018.

248 www.rfs.org.uk Quarterly Journal of Forestry
FEATURE ARTICLES
Monitoring and research
In Belgium the progress of the needle midge and the levels
of attack on host trees are being monitored by field
observations at approximately 150 sites. Since the
discovery of the species, the infestation level has increased
steadily: in 2015 most of the Douglas fir stands had 1-10%
of the current year needles attacked; in 2018 the majority of
the stands had 30-50% or more of the current year needles
attacked, but no massive dieback has been observed.
Research has been initiated at a number of laboratories in
Belgium and France, including the Walloon Agricultural
Research Centre (CRA-W), the Belgian and French Forest
Health Observatories (OSF, OWSF) and the French National
Research Institute for Agriculture, Food and Environment
(INRAE). Since 2015 research on Douglas fir needle midge
has included taxonomy, the impact on young trees, and the
combined effects with other pathogens. The needle midge
is known to combine its effect with the Swiss needle cast
and Sirocccus blight of conifers (Sirococcus conigenus),
leading to needle loss and necrosed shoots. Necrosis has
been seen mainly in the Ardennes region, which is cooler
and rainier than other areas in Belgium. In 2015 the
Douglas fir needle midge infestation seemed to be much
stronger in that region. Currently, high levels of attack have
spread also in other regions.
Discussion and recommendations
In June 2019 the UK legislated to set a target of net zero
greenhouse gas (GHG) emissions by 2050 (Natural Capital
Committee (NCC), 2020). To deliver this target the UK’s
Committee on Climate Change has called for both a rapid
reduction in greenhouse (GHG) emissions and land use
change, including an increase in the annual tree planting
rate to 30,000ha (i.e. 90-120 million trees/year).
Unfortunately, it is unlikely the domestic nursery sector has
capacity to supply all the necessary seedlings (NCC, 2020).
Importation of planting stock from European growers is
likely to continue, despite the known link with previous
introductions of tree pests and diseases (NCC, 2020).
Managing an invasive pest or pathogen after it is
established in a new area brings significant challenges to
forest management (Roberts et al., 2020); clearly the most
important strategy is to strengthen prevention, quarantine
and biosecurity measures.
The presence of Douglas fir needle midge in Western
Europe represents an emerging risk to the health of
Douglas fir in the United Kingdom and Ireland. The most
likely route of introduction will be via trade in tree seedlings,
other plant materials and soil from infected areas. The
likelihood of the insect surviving and perpetuating once it
has entered the UK is high (Defra, 2020). According to the
UK Plant Health Risk Register, the planned action for
Douglas fir needle midge is to complete a targeted survey,
but no further details have yet been published (Defra,
2020). Given the current spread in several European
countries, it would appear prudent to initiate the targeted
surveys as soon as possible, provide training for surveyors
and other staff, and consider collaboration with active
research groups in Belgium, France and elsewhere. Six
actions can be recommended in the short term for foresters
and woodland owners working with Douglas fir:
Identify – As with all pests and pathogens, it is
important to understand the lifecycle, identification features,
and the signs and symptoms of damage due to Douglas fir
needle midge. It is also necessary to differentiate other
causes of damage as part of early detection surveys.
Report – Report any suspicious lesions in Douglas fir

seedlings and saplings as soon as possible to Forest
Research, using the Tree Alert tool and protocol (see details
below).
Trusted suppliers – Specify planting stock from
approved growers and trusted suppliers, and ideally avoid
importing seedlings from areas where Douglas fir needle
midge is known to be present. Phytosanitary procedures
and regulations should be considered to minimise the
potential risk of transporting contaminated soil or plant
material from one region or site to another.
Traceability – As with all establishment projects,
consider traceability of planting stock. Detailed
documentation should be retained for seedlings from the
nursery to their eventual destination. It will be essential to
have these records if there is a pest outbreak and any
chance of mounting effective control measures.
Awareness – Raise awareness among colleagues and
throughout the wider community of the potential threat to
Douglas fir in the UK and Ireland. An extension of this would
be to consider the role of citizen scientists, although more
research is required to assess the risk severity, potential
impact and ease of identification of the Douglas fir needle
midge. The anticipated targeted survey in the UK will be an
important opportunity to learn and promote awareness of
this potential threat to Douglas fir.
Research – Monitor research and developments in
nearby European countries. Consider a systematic review of
pests and diseases of Douglas fir. This could follow a
similar structure to recent work on threats to Sitka spruce by
Tuffen and Grogan (2019). This would underpin a wider
strategy aimed at reducing the risk of other pests and
pathogens being introduced that could threaten the
resilience of Douglas fir in the United Kingdom and Ireland.
Conclusions
Douglas fir is one of the most important conifer species
cultivated in the United Kingdom and Ireland. Until now
Douglas fir has been relatively free from pests and diseases
in these countries. The Douglas fir needle midge was first
reported in Western Europe in 2015-2016, where an active
programme of monitoring and research has been
established. Measures to minimise the risk of Douglas fir
needle midge being introduced to the United Kingdom and
Ireland should be combined with advanced monitoring;
early detection of any introduced pest gives the greatest
opportunity for effective management and control. The
ability to recognise signs and symptoms of the Douglas fir
needle midge, and to distinguish these from other pests
and pathogens, is crucial. Finally, it is recommended that
research and increased awareness activities are
commenced at the earliest opportunity to protect the
Douglas fir resource in the United Kingdom and Ireland
from this new biotic threat.
Acknowledgements
This article was compiled following a Continuous Cover
Forestry Workshop hosted by Forêt.Nature at Marche-en-
Famenne, Belgium on 20 January 2020. We extend our
gratitude to Christine Sanchez and Maude Vandenabeele
(Forêt.Nature) for organising the field excursion and for
sharing their expertise in CCF management. Photographs
were taken by Gilles San Martin. We thank Michel Vallee
(Vancouver Island University, retired), Mike Osborne and
Karen Murray for helpful comments during the preparation
of the manuscript.
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Further reading from the QJF archive
These articles can be accessed online by logging into the
members’ area of the RFS website, then following links to
the Quarterly Journal of Forestry.
Blake, M. & Pérez-Sierra, A. (2020) Pests and Diseases of Conifers - What
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FEATURE ARTICLES
Edward (Ted) Wilson is a silviculturist with interests in
tree biology, silvicultural systems and forest
conservation. Current research focuses on
transformation of planted stands to continuous cover
forestry. He is Walsh Scholar in silviculture with
Teagasc/University College Dublin, Ireland and Adjunct
Professor of Silviculture at the Institute of Forestry and
Conservation, University of Toronto, Canada.
Teagasc Forestry Development Department, Ashtown
Research Centre, Dublin 15, Ireland.
Email: ted.wilson@teagasc.ie
Gilles San Martin is an entomologist and data
scientist working at a public research centre in Belgium
(CRA-W). He has a broad range of interests in applied
entomology (e.g. agronomy, forestry, invasive species)
and environmental sciences (e.g. ecotoxicology,
biodiversity conservation, wild bees and honeybee
health).
Walloon Agricultural Research Centre (CRA-W), 2 Rue
de Liroux, 5030 Gembloux, Belgium.
Email: g.sanmartin@cra.wallonie.be
Gauthier Ligot is a senior research assistant/junior
lecturer working at Gembloux Agro-Bio Tech, University
of Liège. His research focuses on monitoring and
modelling the dynamics of mixed and irregular forest
stands in both temperate and tropical forests.
Forest is life, TERRA Teaching and Research Centre,
Gembloux Agro-Bio Tech, University of Liege,
Gembloux, Belgium.
Email: gligot@uliege.be