Crawly collapse–Spongy moths laid low this summer

By Iola Price

During the summer of 2021, burlap bags were used to protect trees from the spongy moth caterpillar.
During the summer of 2021, big trees with burlap belts were a common sight in Manor Park and throughout eastern Ontario. Photo: Marie-Frédérique Caron

The expected (and dreaded) spongy moth emergence didn’t happen in 2022.

In all, 45 people in our four communities (Manor Park, Rockcliffe Park, Lindenlea and New Edinburgh) answered my call for information on the level of infestation in 2022. For the most part, our community experiences are comparable.

On a scale of 1 to 3 (1 being 1-9 caterpillars and 3 being, like 2021 and 2020, over 50 caterpillars picked and drowned per day), almost everyone reported that they saw very few, and sometimes no caterpillars at all in 2022.

The egg masses that were seen often sported pin-holes, indicating that a parasitic wasp had laid her eggs inside. After hatching, the wasp larvae would have feasted on the eggs and then emerged to repeat the cycle of mating and laying eggs in another spongy moth egg mass.

There were reports of virus-and fungus-infected caterpillars (limp and lifeless, hanging in an upside-down V or by the tail end).

Those who wrapped trees with burlap or sticky tape were delighted that few, if any, caterpillars were trapped under the skirt or on the tape. One person set out a pan trap in late July and considerably reduced the male moth reproductive potential.

In 2021, many people expressed their concern about egg masses on the RCMP and Police College grounds. Thanks to one enthusiastic on-site worker, a team of volunteers squished caterpillars over the 2021 season and scraped and “burlapped” in the winter and spring respectively; he reports that it definitely made a difference.

The leaves came back nicely in 2021 and there were not huge numbers of caterpillars all over the grounds or on the sidewalks in 2022. He and his team received many compliments and thanks for their work in deterring the caterpillars. He also noticed a lot of the egg masses never hatched or they had pin holes in them.

Five factors

With numbers so high in 2021, why were there so few caterpillars in 2022?

I think it was a combination of five factors:

  • our enthusiastic trapping of caterpillars using burlap and setting out bottle and pan traps in 2021 to lure unsuspecting male moths to their doom
  • those very cold days and nights in late January and early February in 2022 killing the eggs
  • over-winter and spring predation on the egg masses by small mammals and birds, especially chickadees and nuthatches
  • predation by the parasitic wasps
  • the cold, damp weather in the spring of 2022 that slowed down hatching and also led to an increased virus and fungal load that killed caterpillars

Insect and other animal populations often go through cycles and the spongy moth is no exception so sooner or later, the population would have crashed. But there are always a few survivors so we can expect them again in 6 to 10 years.

West of us in Carleton Place and beyond to Maberly, there were still enough caterpillars and moths to squish and trap.

In the meantime, remove the sticky tape and burlap from your trees, clean and save the burlap and use it to wrap shrubs and hedges this coming winter.