Tree Maintenance

European Gypsy Moth

European Gypsy Moth caterpillar on a leafThe European Gypsy Moth is an introduced defoliating insect that is considered to be a major pest in North America. This invasive insect pest has been present in Toronto for more than twenty years.

The caterpillar, or larva stage of the insect, eats the leaves of trees making them more susceptible to disease and damage from other insects.  The extent of damage can range from light to almost complete defoliation depending upon a number of factors including; the degree of the infestation, past defoliation, the current health of the tree and other environmental factors. If the tree has been weakened or stressed by other conditions, and attacked repeatedly in recent years, the defoliation can result in the death of the tree.

To protect Toronto's urban forest from gypsy moth, Urban Forestry implements an integrated pest management program in affected areas of the city.

European gypsy moth (Lymantria dispar) is a defoliating insect that can severely weaken or kill trees. It is a major introduced pest to North America.

There are four stages in the development of this insect: egg, caterpillar, pupa and moth.

European Gypsy Moth egg masses and pupal cases on a tree branchEgg masses and pupal cases
European Gypsy Moth Mature LarvaMature larva
Male and female European Gypsy Moth on tree trunkFemale (white) and male (brown) moths

Caterpillars hatch from overwintering eggs in mid-spring. They feed on tree leaves at night for about 7 weeks. In mid-summer the caterpillars pupate in sheltered areas. Adult moths emerge about two weeks later in early August. Soon after mating, females lay oval shaped egg masses on tree limbs, rocks, buildings, vehicles, and other sheltered areas.

Hosts and Damage

Preferred hosts are oak, birch, poplar, willow, beech, cherry, maple, apple, basswood, elm, and others. Rare hosts are ash, catalpa, horse chestnut, locust, London plane, coniferous trees.

Young caterpillars chew small holes on the upper surface of leaves. Older larvae may eat entire leaves, except the major veins. Caterpillars disperse on silk threads to be carried by the wind to other trees. Most deciduous trees can withstand only one or two consecutive years of defoliation. Repeated leaf loss stresses trees and can lead to their death. During outbreaks the caterpillars are an extreme nuisance; trees lose their foliage, caterpillars crawl everywhere, and their droppings rain from trees.

Damage from young European Gypsy Moth catipillar Damage from young caterpillar
European Gypsy Moth damage to Oak trees Damage on oak trees in July

Specific Management Practices for Control of the Gypsy Moth

  • European Gypsy Moth catipillars under Burlap Caterpillars under the burlap
    Remove and destroy egg masses wherever you find them. This helps reduce the number of caterpillars for the coming season.
  • Wrap a piece of burlap cloth around tree stems folding the band to provide caterpillars with a hiding place from the heat of the day. Collect and destroy caterpillars from this shelter each afternoon.
  • Trap male moths by hanging pheromone (sex hor mone) traps on the trees. These traps act as decoys and prevent male moths from mating with female moths.
  • Attract to your yard birds and other benefic ial organisms by plant ing appropriate plants (herbs, flowers and shrubs). Birds eat caterpillars and moths.
  • In severe infestations apply the biological insecticide containing Bacillus thurigie nsis kurtsaki (Btk). This bacterial insecticide affects the caterpillars of moths and butterflies. It is only effective when caterpillars are in an early stage of development. Apply treatment as soon as feeding begins. A good indicator of the correct time to apply Btk is when Bridal wreath Spirea is in bloom.

The City implemented an Integrated Pest Management (IPM) program in the areas affected by European Gypsy Moth (Lymantria dispar) to protect vulnerable trees from defoliation. An aerial pesticide spray application was planned for wards 3, 4, 5 and 13.

Maps of 2017 aerial spray zones:

(For more information on our aerial spray zone interactive map please contact 311)

The aerial spraying was completed on May 23 and 27, 2017, between 5:00 a.m. and 7:30 a.m.

Urban Forestry staff use an Integrated Pest Management approach to address forest pest issues in Toronto. With respect to European Gypsy Moth, staff conduct annual surveys for egg mass numbers in high risk areas. These surveys help determine outbreak threshold levels of the pest. The primary objective of the control program is to protect vulnerable trees from moderate to severe defoliation. The aerial spray of Bacillus thuringiensis (Btk) is the last resort used in the Integrated Pest Management approach. This control method is used when the number of egg masses reach threshold levels for moderate to severe defoliation to continuous stands of mature oak trees. This threshold level is defined as over 1236 egg masses/ha or 10-15 egg masses per mature tree.

The City applied a pesticide, containing Bacillus thuringiensis subspecies kurstaki (Btk)-Foray® 48B Biological Insecticide Aqueous Suspension-Pest Control Products (PCP) Act Registration Number: 24977.

If you have any further questions in regards to European Gypsy moth, please contact 311.

European Gypsy Moth 2014 Control Program Summary

Below is an overview of the City's 2014 Gypsy Moth Control Program:

In the east affected area:

  • Approximately 200 City-owned trees underwent egg mass removal this past winter
  • 14 severely infested City-owned trees underwent ground spray applications of Btk in the spring prior to egg hatch
  • 5 City-owned trees were injected with a systemic insecticide (TreeAzinTM) in the spring prior to egg hatch

In the west affected area:

  • Approximately 40 City-owned trees underwent egg mass removal this past winter
  • 34 severely infested City-owned trees underwent ground spray applications of Btk in the spring prior to egg hatch
  • 15 severely infested City-owned trees were injected with a systemic insecticide (TreeAzinTM) in the spring prior to egg hatch
  • Staff demonstrated proper banding/trapping techniques on City-owned trees, as well as monitoring and maintaining these traps throughout the growing season.

Monitoring of the affected areas took place throughout the growing season.  Additional surveying in the fall of 2014 was implemented, depending on monitoring results. 

In combination with the above control methods, Urban Forestry staff worked with home owners throughout the year to help them implement appropriate control methods on their privately owned trees to help protect against gypsy moth damage.   

June 2014 Status Update

The joint efforts of Urban Forestry staff and dedicated residents have resulted in a successful 2014 European Gypsy Moth Control Program.   Monitoring of the two control areas has confirmed there is no significant gypsy moth population remaining that can present any risk to the trees in either control area.  The timing of both the TreeAzin stem injection and the Btk spray treatments was optimal.

 The caterpillars were caught early in their life cycles, which prevented further development and feeding on tree foliage.   The second round of the Btk treatment of the 48 selected city-owned trees will be completed by mid-June.  It will target any remaining caterpillars that may have hatched after the initial Btk treatment. The TreeAzin should persist in the foliage of the 29 city-owned injected trees and protect them from gypsy moth damage for the remainder of the growing season. The remaining untreated trees in the control areas have had very minor gypsy moth presence which should not pose any risk to them or the surrounding environment.    Urban Forestry's Forest Health Care staff will continue to monitor for the presence of the gypsy moth populations city-wide over the summer and fall of 2014.  Results of the monitoring will determine if any additional treatments are required.  

Residents also played a significant role in controlling gypsy moth populations in privately owned trees in 2014.    Under the guidance of Urban Forestry staff and Ward 3 Councillor Peter Leon, the community of Princess Gardens formed a committee to protect the mature oak population on private property in their neighbourhood.   This committee organized a competitive bid process to hire a local contractor to provide TreeAzin injections in privately-owned trees in order to protect their trees from gypsy moth damage.  Through their actions, this committee was successful at protecting 221 mature oak trees throughout the local neighbourhood from gypsy moth damage.  This community-level initiative sets a great example for other residents throughout the city to join together to protect private trees.

Click here for more information on this initiative.

February 2014 Status Update

Urban Forestry has monitored the spray area to assess the effectiveness of the 2013 aerial spray for the control of the European Gypsy Moth. Results show that no significant damage to tree canopies was experienced in the 2013 growing season. 

During fall 2013, staff conducted multiple surveys to assess the population of surviving insects in different areas of the City.  The results of these surveys indicate that a large remnant population still persists at some locations in the City.  Locations in Ward 3, and Wards 39, 40 and 41 have a significant presence new gypsy moth egg masses.  This is likely due to the extremely high numbers that were present prior to the spray in spring 2013.  

Trees have both new and old egg masses on their stems that are not easily distinguished from the ground. Only the new egg masses have potential to hatch into feeding caterpillars in the spring, therefore it was necessary for the survey to establish the ratio of new egg mass to old egg masses and to identify locations where gypsy moth is present in significant numbers (over 15 new egg masses per tree). When we also consider the impact of natural enemies of gypsy moth such as egg parasitic wasps, the Entomophaga mamaiga fungus and the Nucleopolychedrosis virus, we can conclude that only 5% of the egg masses contain viable gypsy moth eggs. 

Finally, a survey of every tree within the 2013 aerial spray blocks was also conducted to identify possible hot spots, where a high gypsy moth population may still exist. Species of tree, its diameter and a visual count of egg masses was recorded.  The results of this survey indicate that the size of the problematic area has decreased and many residents should not be significantly affected by this pest in 2014. Due to the extremely fragmented population levels in the City, Urban Forestry staff will treat individual trees in the spring of 2014, rather than conducting another aerial spray.  Through a combination of different control methods, Urban Forestry will provide treatment for selected City-owned trees that are at risk.  Staff will also work with home owners to help them to implement appropriate control methods on their privately-owned trees.

Urban Forestry staff have completed inspections of the spray areas and determined that the threat of significant defoliation of trees was averted and the population of gypsy moth reduced to tolerable levels.  As a result, a third aerial spray treatment of Btk is not required and will not be implemented. 

The fungus Entomaphaga mamaiga and the Nuclear Polyhedrosis Virus (NPV) were also detected in the caterpillar population.  These natural controls aid in reducing gypsy moth numbers and are often responsible for the cyclical collapse of this pest.

In addition to the effects of the spray and natural controls, residents in many of the spray areas played a significant role in reducing the number of gypsy moth caterpillars through egg mass removal and banding and should be commended for their efforts.

In addition to the aerial spray program, Urban Forestry also conducted egg mass removals on over 500 trees, sprayed over 60 individual trees with Btk and injected over 25 trees with TreeAzin® as part of our Integrated Pest Management (IPM) approach to this pest. 

Urban Forestry staff will continue to monitor the effects of the control program and the gypsy moth population over the next few months.

Affected Areas:


An aerial spray program of Bacillus thuringiensis ssp. kurstaki (Btk), was conducted in 11 areas of Toronto including an area on the Toronto Islands.

Maps of each of the aerial spray zones for 2013:

The timing of the aerial spray operation is dependent on development of the insect and trees and the weather.  The Gypsy Moth spray window is May 21 to June 11, 2013.  Spraying will take place between 5:00 a.m. and 7:30 a.m. Specific spray dates are weather dependent and will be selected 48 hours in advance and posted on our website.  A minimum of two sprays is required for each spray zone, with the sprays occurring approximately 3-5 days apart.

In addition to the aerial spray operations, ground spray with Btk, tree stem injection with TreeAzin® and manual egg mass removal will take place on individual trees in numerous locations throughout Toronto.

Aerial Spraying

European Gypsy Moth Aerial Helicopter SprayIn May 2013, the City of Toronto completed an aerial spray of Bacillus thuringiensis ssp. kurstaki (Btk) to control outbreak levels of European Gypsy Moth in selected areas of the city.   These areas were subdivided into eleven aerial spray zones, that each received two applications of Btk.  In addition to the aerial spray program, Urban Forestry also conducted egg mass removals on over 500 trees, sprayed over 60 individual trees with Btk and injected over 25 trees with TreeAzin® as part of our Integrated Pest Management (IPM) approach to this forest health threat. 

Post-treatment inspections conducted by Urban Forestry staff, determined that the threat of significant defoliation of trees was averted and the population of European Gypsy Moth was reduced to tolerable levels. 

The fungus Entomaphaga mamaiga and the Nuclear Polyhedrosis Virus (NPV) are two natural controls that aid in reducing the European Gypsy Moth population and are often responsible for the cyclical collapse of this pest.  Both of these natural control agents were detected in the caterpillars that were analyzed in 2013.

In addition to the effects of the spray and natural controls, residents in many of the spray areas played a significant role in reducing the number of European Gypsy Moth caterpillars through egg mass removal and banding of host trees.  Residents are commended for their efforts.

Urban Forestry staff will continue to monitor the effects of the control program and the European Gypsy Moth population within the city of Toronto.