The European Gypsy Moth (Lymantria dispar dispar) is an invasive forest pest that was introduced accidentally in the United States in 1869. Since then, the European Gypsy Moth has expanded its range over much of the eastern United States and Canada. It was first detected in Ontario in 1969 and has quickly spread across southern Ontario. It is now well established throughout southern Ontario and as far north as Sault Ste. Marie.
European Gypsy Moth larvae or caterpillars will feed on tree leaves. If the larvae population is high, they can defoliate whole trees and forests in a short amount of time. European Gypsy Moths prefer oak trees, but will feed on a variety of hardwood tree species. Under normal circumstances, defoliation caused by European Gypsy Moths won’t kill a tree. However, trees can decline to the point of death in some cases when defoliation is coupled with dry hot summers or impacted by other forest pests like Spring or Fall Cankerworm.
Figure 1: Late instar European Gypsy Moth larva
Figure 2: Female European Gypsy Moth laying eggs
European Gypsy Moth sspend the winter as partially developed larvae in eggs. The eggs hatch in the spring and the young larvae begin feeding by cutting small holes in the surface of leaves. As the larvae develop, they feed on the edge of leaves. Larvae have five pairs of blue and six pairs of red spots along their backs. Feeding is normally completed by early to mid-July.
Pupation occurs in a cocoon which can be found on a variety of surfaces including trees, rocks, houses, boats, trailers, fences, picnic tables, and firewood. In 13 to 17 days, the moths emerge. Both sexes have wings, but only the males can fly. The male moth is dark brown to beige, medium-sized, and is a very erratic flyer. The female is mostly white and has a wingspan between 60 to 70 mm. The female lays eggs in masses of 100 to 1000 on tree trunks, branches, houses, fences, etc. as well as under rocks and forest floor debris. The eggs are covered with fine brown hairs from the female’s abdomen. The egg masses will remain all winter and caterpillars will hatch in the spring, from late April to mid-May.
How The City is controlling European Gypsy Moths
Surveying has been underway since the winter of 2016 with the completion of egg mass surveys. In late 2020 / early 2021 The City had surveys completed to determine egg mass counts in areas that had shown high numbers in previous surveys but were not treated and / or areas where residents and staff noticed increased activities in 2020. Following a review of this information The City will be considering ground spraying using Bacillus thuringiensis ‘kurstaki’ (Btk), as well as banding of City trees in these areas.
Bacillus thuringiensis ‘kurstaki’ (Btk) is a soil-borne bacteria that is applied to the leaves of affected trees while caterpillars are in their early instar stage (immature). Once ingested, the bacterium disrupts the caterpillars’ digestive system with cessation of eating within 24-48 hours. Within days, caterpillars that have ingested Btk will succumb to its effects. Btk has very low residual qualities in the natural environment. Sunlight and fungi deteriorate the bio-pesticide within 1 to 4 days. Because Btk requires an alkaline environment in the gut in order to be effective, it does not have any detrimental effects to humans, birds, or bees. Btk will affect other caterpillar species (known as non-target species). Due to its low residual and the narrow spray window due to larval development, the non-target impact is expected to be low. Conservative and measured application will be undertaken with the goal of reducing the population below the 2,500 egg masses per hectare. This program is not intended to eradicate the pest entirely.
Members of the public are unlikely to experience any symptoms if inadvertently exposed to Btk spray, and no special precautions are necessary or required. However, individuals who have concerns should take reasonable precautions to avoid exposure during a spray program in the same way they would avoid pollen or other airborne materials during days when air quality advisories are issued. They can also reduce exposure by staying indoors with windows and doors shut during the spray period if spraying is taking place in their area, although this is not required by health officials. (Health Canada, 2009)
What homeowners can do to control European Gypsy Moths
Homeowners are encouraged to monitor their hardwood trees like oaks, maples, beech and walnut for the presence of larvae. You can help manage European Gypsy Moths by follow these techniques:
June to July - Hand Pick Caterpillars
Handpicking caterpillars is still one of the most effective ways to help control European Gypsy Moths on small newly planted trees, shrubs, and plants. If possible, you can also gently shake the tree, so caterpillars fall from the 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.
What to look for - The caterpillars of the gypsy moth are dark and hairy. They have five blue dot pairs and six red dot pairs on their back. They go through 4-5 "molting" events where they shed their skin and each time, they get bigger.
Late May to early June - BTK Application
You can apply a product that contains BTK (Bacillus Thuringiensis “kurstaki”) 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. Residents should have their trees sprayed with BTK by a qualified pesticide applicator if choosing this method.
For small trees and shrubs that you can reach, products like Safer® Brand Caterpillar Killer can be purchased at local garden supply stores.
April to August - Burlap Banding
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.
- Wrap and secure a piece of burlap cloth around the stem/trunk of your tree
- Tie twine or rope around the center or slightly below the center of the burlap
- Drape the burlap cloth over the twine or rope so there is an overhang where the caterpillars can crawl underneath to seek shelter during the day
- Check the trap by lifting the overhanging burlap cloth every afternoon and collect any hiding caterpillars
- Put them into a bucket of soapy water for a few days to destroy them
July to August - European Gypsy Moth trap
- You can purchase the traps at various retail stores or online throughout your area
- Put the traps out when the moths are active after coming out of their pupa stage of growth. Only male moths are attracted to the traps.
- Homemade traps can be created with various designs found on websites. The traps have a bait inside their lid that smells like female European Gypsy Moth pheromone for attracting males.
- Once captured, put the moths in a container of soapy water and leave them there for a few days, then dispose of the contents.
August to April - Egg Mass Removal
Survey your property for egg masses and scrape them off surfaces into soapy water to destroy them.
- Place your catchment container below the egg mass
- Use your scraper tool to remove the egg mass from the surface. Ensure that all eggs
- are scraped. Try not to leave any residual eggs in bark ridges or crevices.
- Empty the contents of your catchment container or bag into a bucket of soapy water
- Leave the eggs sitting in the bucket for a day or two, then dispose of the contents
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.
Frequently Asked Questions
What kinds of trees are most affected by the European Gypsy Moth caterpillar?
European Gypsy Moths prefer the leaves of deciduous hardwood trees like maple, elm and oak. It will also feed on apple, alder, birch, poplar and willow trees. As the caterpillar matures, and population levels increase, it will also begin to attack evergreens such as pine and spruce. European Gypsy Moths don't appear to like ashes, sycamores, butternuts, black walnuts and dogwoods.
How much damage can European Gypsy Moth cause to trees?
Tree damage depends on the degree of infestation, past defoliations, the tree's vulnerability and the environment and can range from light to almost complete defoliation. 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.
Does the European Gypsy Moth have any natural enemies?
Yes. Predators include other insects like wasps, flies, beetles, ants and spiders as well as birds such as chickadees, blue jays, robins and nuthatches. Animals such as chipmunks, squirrels and raccoons will also prey on the caterpillar.
The wasp that targets the European Gypsy Moth is a parasite of the egg. It is now commonly found wherever European Gypsy Moths are and has become an important natural control.
Also, the European Gypsy Moth is susceptible to several naturally occurring diseases caused by bacteria, fungi and a virus. The virus and bacteria escalate when populations peak. The European Gypsy Moth virus disease is often referred to as “wilt” because dead caterpillars hang in an inverted “V” from tree trunks or foliage.
These natural biological controls contribute the most to keeping levels within a normal range and tend to follow 2-3 years after the gypsy moth populations peak.
What can residents do?
Property owners are responsible for managing trees (and pests) on their property. You are encouraged to monitor trees on your property - look for egg masses in winter, caterpillars in spring, and moths in July and August - and take action to remove European Gypsy Moths as often as possible.
Should property owners consider a commercial insecticide to help control the European Gypsy Moth population?
During severe infestation an insecticide may be considered a viable option. Homeowners can consider consulting with, and hiring a licensed contractor to apply pesticide sprays or tree injections. Timing of the application and the treatment of the entire canopy is essential to the success of control. You should also be aware that pesticide applications do not produce an instant defense and will not completely eradicate the problem, but can be very effective in reducing the insect population when used appropriately.
Why are there still some egg masses on boulevard trees, after the City has been by?
Our arborists have worked to remove the egg masses from City trees to reduce the population. It is not possible to eliminate this pest completely as it is well established in our region. Our overall objective is to reduce numbers. We continue to monitor numbers and will take further action if deemed necessary.
My family has been experiencing rashes that we think come from European Gypsy Moth caterpillars. What should we do about this?
The hairs of the European Gypsy Moth contain histamine which some people are allergic to. Not everyone will have a reaction if coming in contact with the caterpillar, but it is possible and is a known adverse effect. If you are experiencing any sort of reaction, please contact your family care physician for medical advice.
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