The European Gypsy Moth is a non-native defoliating insect that feeds on a variety of tree species found in the Greater Toronto Area (GTA) and throughout North America. The City of Toronto’s management program focuses on controlling outbreak levels of gypsy moth in areas with trees that are at risk of defoliation, typically oak dominant communities. Since gypsy moth is firmly established in Toronto and North America, the complete removal of these insects is no longer a possibility.
The gypsy moth is an invasive insect from Europe that was accidentally introduced in North America in the 1860s during an attempt to rear an alternative silk-producing insect. Gypsy moth caterpillars are 5 to 60 millimetres in length, dark and hairy, with five pairs of blue dots and six pairs of red dots on the back. They feed on a wide range of hardwood trees, as well as evergreen trees, but show a preference to oak trees.
Gypsy moth is established in the northeastern U.S.A. and in regions of the eastern Canadian provinces. The spread of gypsy moth across North America is attributed to the movement of the insect via infested wood or hitchhiking on vehicles. Public awareness could significantly reduce and limit the spread of gypsy moth to new areas.
The Early Detection and Distribution Mapping System (EDDMapS) of Ontario will display the current range of the European Gypsy Moth.
Gypsy moth has four distinct developmental stages:
Egg masses remain secured to surfaces such as tree bark, outdoor furniture, landscape stones and vehicles and hatch to coincide with the warm weather and the opening of tree buds. Egg masses range in size from 2-8 cm long and can contain between 100-1000 eggs.
Newly hatched caterpillars are about half a centimetre long and dark in colour. The tiny caterpillars begin the climb up to the tree canopy, though if it is rainy they may stay put for several days. The caterpillar will either begin to feed on the newly flushed leaves or travel to a nearby tree though a method called “ballooning”, where they dangle from long silken threads at the end of branches and are carried away by the breeze. While their light bodies can travel several kilometres, they will typically be carried to the tree right next to them.
Gypsy moth caterpillars grow and change in appearance over the span of one to two months. As they grow, they moult or shed their skin. By the fourth moult, the pairs of characteristic blue and red dots are visible on their back.
A single caterpillar can eat an average of one square meter of foliage. They continue to feed, moult, and feed until they are about six to seven centimetres long. Once they’ve finished feeding, they seek shelter to cocoon.
After the adult moth emerges, it leaves the empty cocoon behind. The female cocoon is larger than the male cocoon.
An adult gypsy moth’s only function is to reproduce. Unlike other species of butterflies and moths, adult gypsy moths do not eat anything. The female is larger than the male and is cream coloured. The female moths cannot fly. Instead, she uses pheromones to attract male moths. Male moths are smaller and brown in colour.
The gypsy moth caterpillar is known to feed on most species hardwood trees with the exception of ash, tulip-tree American sycamore, London plane, catalpa, fir and cedar. Most healthy trees can withstand one to several years of severe defoliation by gypsy moth. Under normal circumstances, defoliated trees should regrow their leaves in two to three weeks or by early July depending on the year.
Oak species, particularly white oaks, and Colorado spruce are very vulnerable to gypsy moth feeding and can be killed in one season when a significant proportion of the foliage is consumed. Severe loss of leaves can also stress trees and make them more susceptible to further harm from other insects, diseases and weather fluctuations.
Gypsy moths have three significant natural enemies:
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.
Help manage gypsy moth by following these Integrated Pest Management Strategy techniques.
Handpicking caterpillars is most effective on small newly planted trees, shrubs, and plants infested with gypsy moth. If possible, gently shake the tree so caterpillars fall from leaves. Thoroughly inspect the remaining foliage, branches, and trunk for caterpillars and using gloves, pick them off your tree. Fallen and collected caterpillars should be placed and left to soak in soapy water to destroy them.
For severe infestations, apply a product that contains Btk to foliage at the early stage of caterpillar development when caterpillars just begin feeding. This is usually around mid-May. The pesticide must be ingested by the caterpillar to be effective.
For small trees and shrubs that you can reach, products like Safer® Brand Caterpillar Killer can be purchased at local garden supply stores.
For larger trees, it is recommended that you consult with a private arborist.
Once European Gypsy Moth caterpillars grow to about an inch (2.5 cm) in length by mid-June, they will move down the trunk to seek shelter from predators and heat. Reduce the number of larvae on the trees in your yard by trapping them.
Survey your property for egg masses and scrape them off surfaces into soapy water to destroy them.
Egg masses can be located high up in trees. Care needs to be taken if trying to access
anything aloft, especially if using ladders. Some private tree care companies can be hired to
provide this service at heights.
Consult an arborist for treatment options or contact your local garden supply store for the availability of materials.
Gypsy moth outbreaks may appear suddenly and may continue for two to five years in any one location. Natural control factors such as disease, parasites and predators eventually combine to cause a collapse of these outbreaks. Consecutive years of gypsy moth at outbreak levels can cause severe defoliation which can lead to tree mortality and intervention may be required.
The gypsy moth management program aims to control outbreak levels of gypsy moth in areas with trees that are potentially at risk of severe defoliation or mortality if no action is taken. Eradication of gypsy moth is not a realistic management objective since it is well established throughout North America.
To help manage infestations, the City will continue to use the Integrated Pest Management (IPM) Strategy. IPM is a sustainable approach to managing pests by combining biological, cultural, physical and chemical controls in a way that minimizes economic, health and environmental risks.
IPM aims to suppress pest populations to an acceptable level or threshold.
City staff conduct egg mass surveys in residential neighbourhoods, parks and natural areas to identify areas infested with gypsy moth. Surveys are typically conducted in parts of the city with historic gypsy moth infestations and in new areas where residents have reported gypsy moth sightings. These surveys allow City staff to delineate the infested areas and to forecast gypsy moth population levels for the following spring. City tree(s) with high numbers of egg masses that are at risk of being defoliated are candidates for treatment.
The City will prescribe different types of treatments depending on the infestation level, tree species, size of the tree, and its location. Each treatment has limitations, so a combination of control measures is essential for a successful control program. Treatments are to reduce the gypsy moth’s population levels and to prevent severe canopy damage from occurring and tree mortality.
The physical removal of egg masses is an effective method of control for small to medium-sized, low-risk tree species, such as Norway maple, linden and honey locust, located on residential streets or City parks. City staff are equipped with vacuums attached to extension poles that are used to remove and collect the egg masses off of the tree. Egg masses collected this way are destroyed off-site to ensure they no longer pose a threat.
TreeAzin is a botanical injectable pesticide formulated with an extract of neem tree seeds that provides a treated tree with protection from gypsy moth feeding for one season. When a caterpillar eats the leaves of a treated tree, the pesticide kills the insect by preventing it from growing any larger. This limits the amount of damage to the tree. High-risk trees – trees that are predicted to be severely defoliated – are generally selected for this treatment. The timing of treatments is essential for successful control.
The City uses products that contain the active ingredient Bacillus thuringiensis subspecies kurstaki (Btk). This is a naturally occurring bacteria found in soil that can be sprayed onto the foliage of individual trees for gypsy moth control. Ground spraying for gypsy moth is a control often used in City parks to treat spruce trees since other methods have proven to be unsuccessful. The timing of these applications is extremely time sensitive as there is a very narrow treatment window for Btk applications. Because of this, the number of trees selected for this method of treatment is limited.
The City plans aerial spray programs when gypsy moth outbreaks are severe and widespread across large areas of dense high-risk tree canopies, usually oak-dominated. Aerial sprays are only conducted when there is no other Integrated Pest Management control option available that would reduce the populations significantly enough to meet acceptable thresholds.
During the months following the treatments, City staff conduct post-treatment monitoring surveys to assess the level of feeding on treated trees/areas. Treated trees are inspected for signs of feeding and caterpillar abundance and are later re-inspected in the fall/winter for egg masses. The goal of the treatment program is to protect individual or groups of trees from damaging levels of defoliation and to reduce gypsy moth population levels. Low to moderate levels of feeding – small holes in leaves – and the presence of caterpillars can still be expected following treatments as it is impossible to completely eliminate the insect.
The City deploys gypsy moth traps across parts of the City where gypsy moth is present. Though they are commercially marketed as a control method for this insect, these traps are a population monitoring tool, not a control measure. The traps are baited with a sex pheromone that is effective in attracting male moths. Traps are hung in the lower canopy of trees in the summer, before the expected flight period in late July through August. Once the moths have died off for the season, the traps are collected and the number of moths counted. The data collected from these traps is used to establish further monitoring and treatment efforts.
Parasitism is one of the greatest factors that contribute to the natural collapse of gypsy moth populations. Since gypsy moth has natural enemies that affect the insect at each stage of development, City staff monitor the level of parasitism throughout its life cycle. City staff monitor the location, prevalence and severity of parasitism in areas of the city to help predict gypsy moth population levels the following spring.
The City sprays for gypsy moth when populations rise to levels that could have devastating effects on Toronto’s tree canopy. Aerial spraying has proven in the past to be very effective in lowering gypsy moth populations over large areas.
Depending on the levels of gypsy moth, an annual aerial spray may or may not take place.
The need for an aerial spray is determined after City staff finish extensive egg mass surveying.
The area(s) scheduled for spraying are those where there is no other Integrated Pest Management (IPM) Strategy control option available that would reduce gypsy moth populations to meet acceptable levels. Areas predicted for severe defoliation, typically with a high density of oak species, are considered priority areas for aerial spraying. Spray zones are refined using extensive field data.
The City of Toronto uses a product that contains Bacillus thuringiensis subspecies kurstaki (Btk). The product is registered under the trade name Foray® 48B. Btk is a naturally occurring bacteria found in soil and is not a chemical. The same product has been successfully used by the City of Toronto in previous years to control gypsy moth populations. The Cities of Mississauga, Oakville, Burlington and Hamilton have completed similar spray programs in the past.
Btk is very specific and only activates in insect stomachs with low/no acid (alkaline environments). It produces a toxic protein that breaks down stomach walls. The insect usually dies within two to five days. Btk:
Btk has been used in many countries around the world, without health impacts on individuals on medications or vulnerable populations. It has been extensively studied by Health Canada and the US Environmental Protection Agency (EPA). Research shows that Btk poses minimal risk to human health when used as directed and has been approved by Health Canada for aerial use over urban areas.
Residents are unlikely to experience any health effects, and no special precautions are required. People who have concerns can take reasonable precautions to avoid exposure during an aerial spray.
In the months following the spray, City staff conduct post-treatment monitoring surveys to assess the level of feeding in the treated area(s). A spray is considered a success when tree canopies appear lush and green and the majority of leaves are intact. Small signs of feeding, like holes in leaves, and the presence of caterpillars can still be expected following a spray since Btk may not reach small trees and shrubs located directly underneath large trees.
Egg mass surveys are also conducted within the spray area(s) in the fall/winter months to help determine if there may be a residual population in the following spring that would require further control.