Defoliation by Spongy Moth expected again in 2022

PEMBROKE – In Renfrew County and across Ontario, 2021 was a record year for spongy moth, a non-native, invasive pest (previously known as LDD moth and Gypsy moth). Spongy moth has more than 300 known host plants, but prefers poplar, oak, maple, birch, willow, white pine and white spruce. The visual impacts during a severe defoliation year can be quite startling – a single spongy moth caterpillar can eat one square metre of leaves in a season. Although trees are stressed by defoliation, most healthy deciduous trees will produce a second crop of leaves shortly after, enabling them to continue to grow and survive two to three years of defoliation. Conifers are unable to reflush and are more likely to suffer branch dieback or tree death.

In Ontario, spongy moth defoliation increased from 586,385 hectares in 2020 to almost 1.8 million in 2021* . Defoliation in Renfrew County increased more than tenfold between 2020 and 2021, with more than 140,000 hectares mapped last year** . The mild winter and dry spring in 2021 provided perfect conditions for spongy moth to thrive.

It is likely that spongy moth will impact some areas of Renfrew County in 2022, again or for the first time in this outbreak cycle. Based on the egg mass surveys conducted by The Ministry of Northern Development, Mines, Natural Resources and Forestry (NDMNRF), defoliation in Pembroke district (Renfrew County) has the potential to be severe again in 2022.

Several factors can reduce the outbreak: including cold winter temperatures, low snow load, parasitism by wasps, predation by birds and small mammals and high spring moisture levels. Most importantly, a natural fungus (Entomophaga maimaiga) and viral infection (nuclear polyhedrosis virus or NPV) can cause the spongy moth population to crash and signs were observed in some areas in 2021.

There are measures that home or landowners can take to reduce the impact until the local population crashes. If you observe egg masses on your property, now is the time to scrape them from surfaces and dispose of them in a soap and water mixture or burn them. Residents are advised to wear gloves while completing this work as the small hairs covering egg masses can cause a reaction in some people. Once eggs hatch in May, landowners can consider purchasing Btk (Bacillus thuringiensis kurstaki) from local hardware stores to use on feeding caterpillars. This biopesticide should be used according to the label instructions or applied by a registered pesticide applicator on a larger scale.

* Ministry of Northern Development, Mines, Natural Resources and Forestry (MNDMNRF), 2021. Published Online.
** Personal Communication, Forest Health Technician, MNDMNRF, 2021.

Another control measure during the caterpillar stage is trapping in burlap. Start by wrapping a 45 centimetre (18 inch) wide strip around the tree at chest height. Tie a string around the centre and fold the top portion down to form a skirt, with the string acting as a belt. Pick off the caterpillars daily and dispose of them as described above. Pupae can also be picked off in late June/early July and disposed of before moths emerge.

Much more information on identification and control measures can be found online from the Invasive Species Centre and MNDMNRF.

As spring approaches, we can all get scraping and cross our fingers for a cool, wet spring to encourage the success of natural control measures.

For more information, contact:

Lacey Rose, RPF, County Forester

Jason Davis, Forestry and GIS Manager