Over the last few summers, woodlot owners across the province may have noticed hairy caterpillars eating their way through their deciduous trees. These are spongy moths, invasive insects that have gone through several outbreaks since their arrival to North America in the 1860s. The current outbreak, however, is the largest on record.
Identification
Eggs
The most recent year’s eggs are in oval, tan, fuzzy, irregular masses 4-6cm long. They can hold up to 1000 eggs. Old egg masses from previous years are white and faded, often with black pinholes indicating where larvae exited. Photos: J. Liu (fresh egg masses, left), J. Llewellyn (old egg mass, right)
Larvae (Caterpillar)
Hairy with 11 pairs of red and blue dots down their backs. 3mm long upon hatching in April/May, over 2 inches by pupation in late June.
Pupa (chrysalis)
Dark brown, noticeably hairy, approximately 1 inch long, often found in clusters with other pupae and webbing.
Female adult (moth)
White, 5cm wingspan, flightless, rarely moves from where she emerged from chrysalis. Does not feed.
Male adult (moth)
Brown, 2.5cm wingspan, clumsy and erratic flier. Does not feed.
Current Status
The spongy moth outbreak is entering its 5th season in Ontario. In 2021, 1.8 million ha of forest were moderately/severely defoliated. This is a large increase from 41,600 ha in 2019 and 569,000 ha in 2020.
The Ministry of Northern Development, Mines, Natural Resources and Forestry (MNDMNRF) are currently forecasting 2022 population numbers. This forecast should be available shortly and will be reported on MNDMNRF’s dedicated spongy moth page as well as on this page.
Management
Natural Enemies
Remember – the primary factor ending all previous spongy moth outbreaks is the buildup of its natural enemies, a fungus and a virus. These enemies proliferated last year, especially toward the end of the summer, and killed many spongy. They may end the outbreak in the next few years. You can read more about them in Michigan State University’s excellent article.
Integrated Pest Management Options
There are some management options for smaller/younger woodlots or maple orchards, including:
- Scraping egg masses off trees from August-April into soapy water, making sure none fall on the ground
- In summer, wrapping duct tape or a burlap sack around each tree and killing trapped caterpillars daily
See the resources below for more detail on integrated pest management techniques for small/young woodlots:
Government of Ontario: Lymantria dispar dispar (LDD) moth page – Control measures
Government of Ontario: Lymantria dispar dispar (LDD) moth and other defoliators – Protect your trees
Should I Spray My Woodlot?
If you have a large, mature woodlot, aerial sprays are usually the only realistic management option. To determine if you should spray, consider the following questions:
How healthy was my sugarbush this year?
Healthy hardwood trees can usually withstand defoliation (losing over half of its leaves) 2-3 years in a row before dying. Factors affecting this “buffer time” include:
- The severity of defoliation
- The time of year that defoliation occurs; usually spongy moth defoliate relatively early in the season, allowing the tree to push a new flush of leaves
- Any abiotic or biotic stressors affecting tree health (e.g. drought, extreme weather, other pests, etc.)
How many egg masses are present in my bush?
For a more quantitative approach, conduct a winter egg mass survey in your bush. The methods summarized below are adapted from MNDMNRF’s Modified Kaladar Plot protocols. Request the full protocols by emailing info.mnrfscience@ontario.ca.
- Lay out a 10x10m plot in an area of your woods that had an average amount of spongy moth defoliation
- Using binoculars, count ALL fresh egg masses on trees in the plot; do not count the faded old egg masses
- Multiply this count by 100
- Lay out 10 1x1m plots within the 10x10m plot
- Count all egg masses on the ground in the 1x1m plots
- Multiply this count by 1000
- Add tree and ground egg mass numbers from Steps 3 and 6 to get the number of egg masses/ha, look up in the table below
| Egg masses/ha | Defoliation Forecast Range | Management impacts |
| 0 | 0-5% | None |
| 1 to 1,235 | 6-40% | Noticeable defoliation |
| 1,236 to 6,175 | 41-75% | Growth loss |
| 6,175+ | 75-100% | Tree mortality |
8. Repeat 2-4 times in different areas of woodlot
Remember that young spongy moth larvae are very mobile; soon after hatching, a number of them will spin silk threads that carry them like hot-air balloons to new locations. Sometimes they can travel very long distances. Keep a watchful eye on your woodlot even if you have low egg mass numbers, especially if you know the moth has been active nearby.
If you decide that you need to engage aerial spray services, get in touch with the Ontario Centre for Forest Defoliator Control at 226 996 9702 or email info@ocfdc.com. OCFDC is facilitating Zimmer Air Services Inc.’s spongy moth spray program. Contact them as soon as you decide you will require aerial services. If you are unfamiliar with Zimmer Air Services, see their website here or watch their latest status update from December 1, 2021 here.
Why is the name being changed from “gypsy moth”?
“Gypsy” was a historic ethnic slur used to describe the Romani, a people originating in northwestern India and who were considered nomadic. The name was likely given to reflect the caterpillar’s wandering nature. Some Romani have protested this use; it is deeply uncomfortable when people globally are using a slur of your people to describe a hated pest!