Committee of the Whole - Middlesex County
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June 23, 2020 Page 1 of 8 C 11 - CW Info
Committee of the Whole
Meeting Date: June 23, 2020
Submitted by: Mark Brown, Woodlands Conservation Officer/Weed Inspector
SUBJECT: EUROPEAN GYPSY MOTHS
BACKGROUND:
The Gypsy Moth is a non-native insect from Europe that was introduced to North
America in the 1860s. The insect has become naturalized to Eastern North America,
including Ontario, Quebec and the Maritime provinces. It has spread widely in areas
where Oak is found, its preferred host tree species, in both rural and urban ecosystems.
This insects’ population dynamics can best be described as boom and bust, where often
every 8-12 years, the population within certain areas will reach epidemic proportions only
to collapse again and remain at endemic levels for another eight years or so. Gypsy moth
outbreaks may appear suddenly and may continue for two to five years in a location.
During a population outbreak, the caterpillars in very large numbers can be observed
feeding within trees, defecating, and dangling from silk threads.
As this insect is well established and widely spread within Eastern Canada, the Canadian
Food Inspection Agency (CFIA) only utilizes phytosanitary regulations for import and
export of materials that could inadvertently export the pest to other locations where it is
not yet established. The CFIA does not otherwise attempt to treat, control, or eradicate
the gypsy moth. The CFIA fact sheet European Gypsy Moth (Lymantria dispar) or
https://www.inspection.gc.ca/plant-health/plant-pests-invasive-species/insects/gypsy-
moth/fact-sheet/eng/1330355335187/1335975909100
The gypsy moth prefers approximately 150 primary hosts but feeds on more than 300
species of trees and shrubs including: Aspen, Birch, Cedar, Cottonwood, Fruit trees,
Larch, Oak (preferred host), Poplar, and Willow.
Over the past two years, forestry staff has been made aware of several localized
population outbreaks within Middlesex County, primarily in the Campbellville and Kilworth
areas, where residents have voiced concerns relating to the impact on their properties,
tree health, nuisance, etc.
June 23, 2020 Page 2 of 8 C 11 - CW Info
ANALYSIS:
History of outbreaks in Ontario
Figures 1, 2, and 3 provide some background from the Ontario Ministry of Natural
Resources and Forestry related to the Gypsy Moth outbreaks within the Province.
Figure 1 Forest Health Conditions in Ontario - Ontario Ministry of Natural Resources and
Forestry
June 23, 2020 Page 3 of 8 C 11 - CW Info
Figure 2 Gypsy Moth defoliation 2018 - Ontario Ministry of Natural Resources and
ForestryJune 23, 2020 Page 4 of 8 C 11 - CW Info
Figure 3 Gypsy Moth defoliation 2019 - Ontario Ministry of Natural Resources and
Forestry
The Ministry of Natural Resources and Forestry has noted that several caterpillar species
often feed alongside of each other and that they cumulatively have an impact on
woodlands. For example, the Forest Tent Caterpillar is often mistaken for the Gypsy
Moth and it causes more defoliation within Ontario than the Gypsy Moth.
Tree Damage Rural Woodlands / Forests
When the insect population dynamics are at their peak, Gypsy Moth caterpillars can
defoliate its host tree canopy severely and sometimes even completely. However, the
oak genus being a very long-lived tree, is relatively resilient and well adapted to threats
of defoliating insects and diseases. As a result, most often, healthy trees will replace the
leaf loss experienced in spring defoliation events later in the year with new leaf canopy in
order to perpetuate annual growth and development.
Natural rural forests growing in relatively undisturbed woodlands also tend to consist of
increased species diversity and support greater numbers of predators, thereby
possessing more resilience to disturbance events. Overall, within Middlesex CountyJune 23, 2020 Page 5 of 8 C 11 - CW Info
Gypsy Moth defoliation has not been observed as a significant issue to date within
undisturbed rural forests.
Tree Damage Urban Forests / Parkland Trees
In contrast to trees within a rural setting, trees in urban settings are typically exposed to
additional environmental stressors such as drought, altered hydrology, compacted soils,
fragmentation, pesticides, air pollutants, and fewer natural predators. These additional
stresses can expose urban forest, parkland trees, boulevard trees, and private yard trees
to an increased level of damage as they may have limited resources to draw from during
the recovery phase following defoliation.
The more severe local occurrences of infestation and defoliation appear to have been
associated with small urban or near urban interface trees and woodlands, particularly if
they have become fragmented and disturbed through development or other factors. As a
result, isolated groves of host trees either scattered throughout a development or
retained in a small urban park setting tend to become the concentrated focus for the
resident gypsy moth population. As the population increases, they cause an increased
level of defoliation in the infested area.
The adult females cannot fly so they are limited to drifting on air currents on silk threads.
Therefore unless they are situated in a large contiguous forest they cannot spread and
travel to find new food sources, and as a result, defoliation can become severe in a
concentrated area of host vegetation.
Gypsy Moth Life Cycle
The gypsy moth has four distinct life cycle stages:
1. egg – usually laid on or near the host tree
2. caterpillars – emerge in late spring (usually early May) and climb the tree to feed
on the foliage
3. cocoon – mid-July to early August caterpillars cocoon
4. moth – mid to late August, adult moths emerge from cocoons and reproduce
The gypsy moth has many preferred egg-laying sites:
• In bark cavities, under loose bark, and in bark crevices
• On branches, on the ground or on the underside of any type of ground litter
• On logs - including firewood
• On outdoor household articles such as birdhouses and picnic tables
• On stone walls and in the crevices of stone walls
• On the underside of rocks not tight to the ground
• On tree trunks in sheltered spots, such as under limbs
• Under the siding and eaves of buildings
• Egg masses may be found anywhere near trees in areas with preferred hosts.June 23, 2020 Page 6 of 8 C 11 - CW Info
Natural Predators
Despite this insect being introduced in North America it has three significant natural
enemies:
1. a fungus (Entomophaga maimaiga)
2. a virus (Nucelopolyhedrosis)
3. a small wasp (Encyrtidae family)
The fungus and virus can be very effective at naturally controlling populations; however,
they require a cool wet spring to be effective. The wasp can parasitize up to 30% of the
eggs that are near the surface of an egg mass, but cannot reach the eggs in the center of
the mass.
In addition, birds, bats and several mammals predate the species at various stages of
their life cycle.
Control Methods
Natural population control factors such as disease, parasites and predators most often
combine to cause a collapse of gypsy moth outbreaks. However, consecutive years of
the gypsy moth outbreak can cause defoliation which can lead to tree mortality and
intervention may be considered. Control of populations should ideally take place as part
of an integrated pest management plan.
Any gypsy moth program should aim to control outbreak levels of gypsy moths in areas
where trees are potentially at risk of severe defoliation or mortality if no action is taken. It
should be reitereated that the eradication of gypsy moths is not a realistic management
objective since it is well established throughout North America.
The following control measures are commonly utilized:
• Egg mass scraping in the fall and winter can significantly reduce the population in the
following years.
• Placing burlap wraps around trees to trap and dispose of the caterpillars climbing up
the trees to feed.
• Placing sticky banding around the stems of trees to trap caterpillars climbing up the
trees to feed.
• TreeAzin – high value trees can be injected with this pesticide formula, which, when
ingested by the caterpillar, halts its growth and leads to mortality.
• Ground based spraying - high value trees in accessible locations can be sprayed with
Bacillus thuringiensis subspecies kurstaki (Btk) which when injested leads to
mortality. This must be applied to the foliage prior to the caterpillars migrating into the
tree canopy.June 23, 2020 Page 7 of 8 C 11 - CW Info
• Aerial Spray Program - for high-risk areas, an aerial spray program may be
considered when gypsy moth outbreaks are severe and widespread across large
areas of dense high-risk tree canopies, usually oak-dominated.
• Pheromone Traps/Lures - The female adult moth does not fly; therefore, they cannot
be trapped, but the male adult moths can be trapped with the use of female sex
pheromone traps. Although the traps are commercially advertised as a control
method, research (including the City of Toronto) suggests that traps are more
relevant to monitoring for the presence of the insect rather than to be considered for
use as a control method.
It is noted that the spray control methods do put other non-target leaf eating insect
species at risk.
It is also noted that the gypsy moth most often travels by hitching a ride on human modes
of transport, vehicles, travel trailers, household items, firewood lumber etc. It is therefore
extremely important that homeowners do not inadvertently vector or relocate this insect
when they travel from place to place, move homes, or travel to recreation destinations.
Items being transported should be inspected and cleaned prior to transport.
Tracking and Recording of Outbreaks
The Woodlands Conservation Officer records all reported incidents of gypsy moth
occurrence, and if the occurrence is reported or suspected to affect public property field
investigation is conducted to confirm the species and assess the severity of the impact
on local forest conditions. All records are maintained in the GIS forest service’s
database. Local municipal staff are advised if occurrences affect woodlands under their
jurisdiction.
Select Jurisdiction Review in Ontario
Municipality of Middlesex Centre - has contracted a private company to perform ground
based spraying of trees in Westbrook Park (Kilworth) and has offered surrounding private
property owners the opportunity to contract the same company to spray trees on their
private property for a fixed fee.
https://www.middlesexcentre.on.ca/articles/dealing-gypsy-moths
Town of Pelham – has completed an aerial spray program to control gypsy moth in
selected neighborhoods.
https://www.pelham.ca/en/living-here/gypsy-moth.aspx
City of Toronto – managed as part of an integrated pest management program and
implement control measures on as needed basis, this year a limited aerial spray program
was implemented in Etobicoke.June 23, 2020 Page 8 of 8 C 11 - CW Info
https://www.toronto.ca/news/city-of-toronto-to-aerial-spray-for-tree-damaging-european-
gypsy-moth-in-parts-of-etobicoke/
City of Hamilton – performed aerial spray program in 2018 and 2019 however they have
stopped treatment this year.
https://www.hamilton.ca/home-property-and-development/property-gardens-trees/gypsy-
moth
City of London – performed aerial spraying in 2009 and since that time they have
reduced activities to education outreach and monitoring. Some defoliation occurred in the
Byron area and as a result the City performed egg mass survey during the winter
2019/20 and have removed egg masses early in 2020 in identified hotspot areas.
http://www.london.ca/residents/Environment/Trees-Forests/Pages/Gypsy-Moth.aspx
City of Sarnia – performed aerial spraying this spring within select areas on both public
and private property to control gypsy moth population, a contractor has been retained to
perform monitoring and to coordinate the spray program.
https://www.sarnia.ca/gypsy-moth-control-program/
Conclusion
It has been the position of the County that the management of pests (animals, insects,
weeds, etc) are the responsilbity of the private land-owner. The Woodlands Conservation
Officer frequently consults with the public on a variety of pests and offers his experienced
advice on dealing with such pests. The public are also directed to authoritative
information sources (OMNRF, CFIA, etc) and made aware of known private companies
that offer pest management services.
Local municipalities may wish at their discretion to undertake control activities for
parkland and street trees within areas experiencing an outbreak. This should ideally take
place as part of an integrated pest management plan. The Woodlands Conservation
Officer is available to assist municipalities with background to such activities.
Finally, the County’s website will be upated with additional information concerning pests
on private land including identifying gypsy moth egg masses and safe methods for their
destruction utilizing website and video demonstration.You can also read
