The spongy moth (Lymantria dispar), is an invasive and highly destructive pest. When their numbers are high the larvae can completely defoliate trees, even forests. Formerly known as the gypsy moth, it was renamed by The Entomological Society of America in Feb. 2022. This species has a few sub-species including: Lymantria dispar dispar, Lymantria dispar asiatica, Lymantria dispar japonica, Lymantria umbrosa, Lymantria postalba and Lymantria albescens. They are often placed into two groups though, the European spongy moth and the Asian spongy moth. The European spongy moth LDD (Lymantria dispar dispar), was introduced to North America in1869 and has become a widespread and pervasive pest. The Asian spongy moth consists of the remaining sub-species and was accidently introduced to B.C. Canada in 1991. Although that introduction was eradicated (Gov. of Canada (Rev. 2022-02-15) it has since “been found on several occasions in North America, but successful programs have eradicated local outbreaks” (Pinkerton, Morgan and Amanda Hodges, (2016)). The Asian spongy moth is greatly feared, and authorities continue working hard to control its spread. The reason for the great concern is that the Asian spongy moth has the potential for much greater spread and destruction of our tree canopies, due to the female’s ability to fly and lay her eggs in a broader area (the European spongy moth females are not able to fly). The Asian spongy moths also prefer coniferous trees and have a wider range of hosts that they feed on, they are also better adapted to cold climates.
Identification and Life Cycle
Spongy moth caterpillars are the larvae stage of this pest and the one that causes all the damage. They have voracious appetites and on mass they can strip a tree bare. They prefer to feed on broad-leaved trees such as red oak, white oak, poplar and white birch. However, they are known to feed on approximately 500 species of trees including maple, willow, basswood, apple, tamarack, mountain ash, hawthorn, alder, eastern white pine, white spruce, beech and eastern hemlock.
After egg hatch the larvae are quite tiny, about 1/2 cm in length and blackish in colour. As they mature through their 6 instar stages, they grow in length to about 6 cm (2.5″) (although LDA’s may be larger up to 9 cm). Their colouring changes to a charcoal grey or light grey and they develop 2 rows of wort like bumps that run down their backs. These bumps typically have the colour pattern of 5 pair of blue dots followed by 6 pair of red dots, although there can be exceptions. These bumps are filled with long bristly hairs that are gray or gold in colour. These colourful spots paired with the bristly hairs serves as a deterrent to predators.
The caterpillars begin hatching from over wintered eggs in the spring, just as the tree buds are beginning to open. They feed voraciously at night for about 7 weeks, then enter the pupal stage. During the day the caterpillars seek refuge from the sun and predators and drop to the forest floor, seeking shelter and protection. The young larvae are able to move from one tree to another by spinning long silk threads and catching a ride on the breeze, up to one kilometer away.
Appear brown in colour with a tear-drop shape. They measure about 2.5–5 cm (with the males being smaller than the females) and are covered in a protective shell. This stage lasts about 2 weeks.
Female moths are whitish with dark zigzag markings, heavy bodies and hairy heads, with a wingspan of about 5-7 cm. They have comb like antennae (pectinate antennae), but the branches are much shorter and less obvious than the males. The LDD females are not able to fly and lay their eggs on or near the host tree she pupated from. She dies shortly after laying her eggs. LDA moths are able to fly and are a bit larger in size; they can lay their eggs a good distance from where they emerged from pupae and greatly widen their territory.
Male moths are greyish brown or brown with dark markings, slender bodies and a wing span of about 3.5 -4 cm. They have unique looking antennae that looks like a comb, (pectinate antennae). Both LDD males and LDA males can fly. This stage lasts for about a week during which time the males mate with several different females.
Adult moths do not eat and unlike many moths who are nocturnal, spongy moths are diurnal (come out during the day).
Spongy moths were named after the spongy, eggs masses that the females lay. These egg masses are buff in colour and covered in hairs, that the female pulls out of her abdomen, giving them a felt like appearance. They range in size from about 11mm (the size of a dime) to 26mm (the size of a Loonie). According to the government of Canada’s website the size of the egg mass is an indication of the size of their population; smaller population, smaller egg masses (Gov. of Canada (n.d. rev. 2013-06-04)).
The egg masses, that can consist of up to 1000 eggs, are the main means by which spongy moths are transported to new locations. They have a reputation for laying their eggs on almost anything and commonly hitch a ride on vehicles, trailers, boats, camping equipment, firewood, etc. In and around the yard you will find their egg masses on tree trunks, the underside of branches, fences, outdoor furniture, under the eaves of buildings, under side of plant pots, outdoor play equipment, machinery and many more creative places.
Managing Spongy Moth Populations
- Sanitation is an important control measure of spongy moths. Eggs masses must be located and scrapped away with a knife and disposed of in a bucket of hot water and household bleach or squished or burned.
- Plant resistant varieties where possible.
- Monitor for the pest so you can act quickly when it is first detected.
- Keep trees well-watered and otherwise healthy, so they stand a better chance of surviving an attack of spongy moths.
- Banding involves tying a band of burlap around the tree trunks at about chest height. The caterpillars seek shade during the day (and feed at night) and will crawl under the band, where they can be collected and dropped into a bucket of warm soapy water.
In spite of their warning colours and stiff bristles, spongy moth do have some natural predators that help to manage their populations during non peak times. Some examples include small mammals such as mice, shrews, and voles and several bird species such as Chickadee, Blue jay, Towhee, Northern oriole, Robins, Crows, Grackle, Starling, Red-winged blackbird and others. Other insect predators include Ground beetles and ants, although not terribly effective. For a more complete list visit Predators of the Gypsy Moth.
- Bacillus thuringiensis kurstaki (Btk) is the most notable biological control. It is registered for use in organic gardening and is considered a safe and effective way to bring larger populations of spongy moth under control. Cities with large spongy moth populations and forested areas may conduct aerial sprays of Btk. In essence Btk is a naturally occurring bacteria found on dead or decaying matter in soil. When the caterpillar ingests Btk it turns into a toxic protein in their digestive system, causing them to soon stop feeding, and to die in about 5 days. It is selective and does not affect people, birds, mammals, insects or fish. It is safe for the environment and bio-degrades in sunlight in about 3-7 days. To apply, mix the crystals in your sprayer, according to the manufacturer’s instructions. Make 2 applications, the first one about 10 days after the eggs hatch and the second about two weeks later. Begin monitoring in about late April.
- Entomophaga maimaiga is a fungus that is native of Japan but is thought to have been introduced North America between 1910 and 1911, although it could not be found by researchers until the spring of 1989 (Butler L., (1998)). The fungal population increases dramatically, during peak out breaks, and kills many of the larvae, reducing their populations to manageable levels again. Spongy moth caterpillars killed by the fungus appear vertically on tree trunks dried out with their heads facing down (Butler L., (1998)).
- Nuclear polyhedrosis virus has similarly been implicated in infecting the spongy moth larvae during times of peak out breaks. Viral-killed larvae often hang in an inverted V, and the bodies are moist and ooze dark fluid (Butler L., (1998)). It is important to note that these spongy moth outbreaks are cyclical and occur every 10 years or so, when the weather conditions are right for them. They then decline due to either fungal and/or virus infections.
- There are a few parasitoids that will parasitize one of the life stages of the spongy moth. Ooencyrtus Kuvanae is a very tiny black wasp that parasitizes the eggs in the outer layer of the spongy moth egg masses, killing about 20-30%. Cotesia melanoscelus parasitize young caterpillars. Compsilura coccinnata a parasitic fly that attacks spongy moth caterpillars, as well as the caterpillars of more than 100 other moth and butterfly (Dr. McCullough D.G, et al. (1999)).
- Dormant oil combined with lime sulfur and sprayed on during dormancy, will suffocate overwintering eggs. The main limitation here being the number of trees that are sensitive to the spray. To see a list of sensitive trees, visit Dormant Oil for the Control of Over-wintering Insect Pests.
- Chemical Insecticides are available for the control of spongy moth. Check with your region to see what you are allowed to use. These insecticides must be sprayed directly on the young caterpillars (the younger the better).
Photo credits: all photos taken by the author.
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