Populations of Spongy moth (a.k.a. Gypsy moth, LDD moth, and Lymantria dispar dispar) are expected to be low this spring in many areas in an around southern Ontario.According to the Ministry of Natural Resources and Forestry, “Defoliation caused by spongy moth in Ontario decreased to 2,529 hectares in 2023, down from 58,031 hectares in 2022. This area of moderate to severe defoliation occurred in the Aylmer-Guelph district in the Southern Region.” For us at Beautiful Trees Inc., this is a huge relief. The defoliation statistics from the last couple of years are extremely low, and do not even compare to the whopping 1.8 million hectares of defoliation that was observed in 2021.
All of us can remember the severe outbreak of Spongy moth larvae in 2021, they were literally everywhere, hanging off trees and crawling up the sides of our homes. We’ve had Spongy moth in southern Ontario since 1969, what happened to cause such an unusual spike in the population? Spongy moth, like other insects, are cyclic in their population with outbreak levels occurring every 7-10 years. The moth was already headed for an outbreak, but this population spike was amplified because of the perfect weather conditions during the larval feeding period in the spring of 2020.
In an average spring, we receive enough rainfall to create the humidity and leaf wetness conditions that support the growth of disease-causing fungi and bacteria. It is the spring leaf wetness periods that support the infection of most of the tree diseases that we see in the Ontario landscape. There are fungal and viral diseases that are pathogenic to Spongy moth caterpillars that normally kill off part of the population before they pupate into adults. These larval pathogens rely on high humidity and leaf wetness periods in order to colonize and infect the Spongy moth caterpillars. But in the spring of 2020, we had an unprecedented run of continuous weeks of dry weather, resulting in negligible larval mortality due to natural disease. Since the insect population was already cycling upwards, the dry spring of 2020 boosted a very successful population of larvae all the way into pupation and adulthood. The prolific egg-laying of the 2020 adult spongy moths resulted in the record-breaking levels of larvae feeding on our beautiful trees in the spring of 2021.
Aerial surveys and reported observations in 2023 showed that widespread spongy moth defoliation is not likely in 2024. Because of this, the Ontario Ministry of Natural Resources and Forestry decided that egg mass surveys to forecast spongy moth populations in 2024 were not needed (no egg mass surveys were conducted in 2023).
Predictions of low populations of Spongy moth for 2024 is wonderful news for us in southern Ontario and is cause for a collective sigh of relief. Because our trees were so heavily defoliated in 2021 from Spongy moth, we need to remember that it can take trees 3-5 years for trees to recover the lost carbohydrates and stored nutrients from consumed leaves. 2024 marks the 3rd year after that massive defoliation and its important for us to care for our trees, especially those that sustained significant feeding damage.
In order to support plant growth and photosynthesis this year, consider watering the root zone of trees once per week if there is less than 25mm of natural rain, especially when we experience >27C. Use low volume irrigation to minimize runoff and allow infiltration of water into the soil of the root zone (functional sprinkler system, trickle hose, soaker hose, Orbit sprinkler). Commit to irrigating 25 mm of water (subtract any natural rainfall accumulation from this number) from May to October. Remember, the tree’s root zone extends well beyond the drip line of the canopy. Irrigate in a solid circle from the trunk to 2 or 3 meters outside the canopy drip line if possible.
Another great practice is to fertilize trees with slow-release, low phosphorus formulations that provide the nutrients typically found to be deficient in Ontario landscape soils. Our Beautiful Trees Complete Nutrition for Woody and Herbaceous Plants fertilizer has been specially formulated by our scientist to provide nutrients and minerals that our landscape plants need most. We use specialized Deep Root Fertilizer equipment to inject our water-suspended fertilizer directly into the root zone – where it can be used for plant growth. Our fertilizer is blended to provide season-long nutrition. We also use our DRF system to inject much needed organics to feed the soil microbiome, which supports all plant life in our landscape. Also, by injecting a water suspension into the soil, the plants benefit from reduced compaction and increased pore space, perfect conditions to support the growth of more roots! Call or Email Us for one of our ISA Certified Arborists to conduct an onsite quote for your beautiful trees.
Although over 300 species of trees have been identified as hosts of the Spongy moth, caterpillars are most often found feeding on the foliage of maple (Acer, except red maple), birch (Betula), beech (Fagus), oak (Quercus), basswood (Tilia), poplar (Populus), cherry (Prunus), willow (Salix), and the evergreen such as spruce (Picea), pine (Pinus), and fir (Abies).
Gypsy moth egg masses overwinter on the bark of trees, on leaves, on wooden furniture, on just about any stationary object but most commonly on tree trunks and the underside of branches. Although they are fuzzy brown when first laid, the egg masses turn a light tan after the winter.
This spongy moth egg mass was found on paper birch, note how egg masses lose their brown pigment after the winter, making them more difficult to detect. (Photo: Jen Llewellyn)
Spongy moths will lay their eggs on any available surface, even exfoliating bark. Adult females are unable to fly and are so heavy with eggs they cannot travel very far from where they pupated. (Photo: Jen Llewellyn)
Among the pathogens and predators of spongy moth is this egg parasitoid wasp, Ooencyrtus kuvanae, that is known to lay its eggs inside spongy moth eggs. The above photo is of the adult female parasitic wasps laying their eggs into the spongy moth egg mass. (Photo: Jen Llewellyn)
Parasitic wasp larvae consume the contents of the moth eggs and emerge out of the eggs as adult wasps. The photo above illustrates an overwintered spongy moth egg mass with empty egg cases where parasitoid wasps have emerged after feeding on embryonic moth larva. (photo: Jen Llewellyn)
Spongy moth larvae begin to hatch around the time that the trilliums are blooming here in southern Ontario. They produce a webby thread and use it to propel themselves out onto the winds, hoping to disperse away from their siblings and land on a plant that they find tasty. (Photo: Jen Llewellyn)
Young larvae feed by chewing holes in leaves. They feed during the day and shed their skin (molt) in order to grow larger in size. This stage of spongy moth larvae is very susceptible to biological insecticides and can be treated with professional application systems. Call or Email us for a Free Quote if you are finding spongy moth larvae feeding on your beautiful trees or shrubs. (Photo: Jen Llewellyn)
Many spongy moth larvae are killed during mid-late development from a fungal disease or NPV virus (like this one in the photo above). Although the NPV virus is a registered biological pesticide in Canada, it is not available for application in the industry. (Photo: Jen Llewellyn)
Mature spongy moth larvae feed mainly at night and are less susceptible to insecticide applications. Their nocturnal behaviour can make it tricky to detect them. (Photo: Jen Llewellyn)
Homeowners will install burlap skirts around trees for mature larvae to hide in during the day, making it much easier to collect and destroy the hairy caterpillars. Note that many people are sensitive to the hairs of spongy moth and exposure may result in an allergic skin reaction. (Photo: Jen Llewellyn)
Adult female spongy moths emerge in July. Once mated, they lay eggs on tree bark and cover the eggs with little brown hairs from their body. (Photo: Jen Llewellyn)
The Trouvelot home in Medfor, MA where in the 1860’s, Etienne Leopold imported spongy moth to breed in his backyard (photo courtesy of “The Gypsy Moth,” by E.H. Forbush and C.H. Fernald, 1896.)
Spongy moth is an invasive species, it was moved outside of its native habitat (Asia) and threatens a new environment, economy or society by disrupting local ecosystems. Spongy moth (formerly known as Gypsy moth), was intentionally imported in the 1860’s by an amateur insect enthusiast living near Boston, MA. Etienne Leopold Trouvelot was breeding silkworms in his backyard just outside of Boston and imported the spongy moths in hopes that they would breed with his American silkworms to make them less palatable to predation by birds. The spongy moths escaped, the larvae defoliated all the oak trees in town and the Trouvelots moved.
Unfortunately, it was several years before the Massachusetts peoples tried to eradicate the moths, and as you can probably guess, they were unsuccessful. The first detection of spongy moth in Ontario occurred in 1969; however, widespread defoliation did not occur until 1981. Outbreaks of this pest are cyclical, typically occurring about every seven to ten years. In Ontario, major outbreaks have peaked in 1985, 1991, 2002 and 2008. As discussed earlier, the most recent outbreak that peaked in 2021 was the largest on record in Ontario. (Source: Ontario Ministry of Natural Resources and Forestry).
It is introductions like these that have led to the creation of regulatory agencies that monitor the trade of live goods around the world. Spongy moth is a regulated pest by the Canadian Food Inspection Agency (CFIA). For more information on other introduced species that may impact the health of your Beautiful Trees, Click Here.
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