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Long-dead caterpillars blamed for an acorn scarcity

Turns out, there's a good reason for black bears, turkeys, squirrels and blue jays to really hate spongy moths

The local conservation authority is bringing attention to what could be called a “moth effect." 

An invasive species with a taste for oak trees might have caused a food scarcity problem for bears and other wildlife that rely on the fat and carbohydrate-rich acorn for their fall chow downs. 

File this one under the list of things blamed on the spongy moth (formerly known as the gypsy moth) infestation and the voracious appetite of their adolescent larvae phase: the very hungry caterpillars. 

The spongy moth – aka Lymantria dispar dispar – is native to Europe and Asia. Its latest name comes from the spongy texture of the egg clusters deposited on tree trunks by the moths at the end of their lives. Considered an invasive species in North America, the blue-and-red spotted caterpillars have found a foliage smorgasbord and have done away with self-control. 

The year 2021 was an “outbreak” year, but not because of COVID-19. In 2020 and moreso in 2021, spongy moth caterpillars were so prolific, frass (their droppings) seemed to rain from leafless trees. But the caterpillars' glutton has been their demise and the population of the invasive species has dropped since past generations don't seem to have any instincts to leave some leaves for the next guys.

According to Kyra Howes, manager of lands and operations for the Nottawasaga Valley Conservation Authority, the infestation of spongy moth caterpillars was classified as “severe” in 2021, which means more than 75 per cent of the forests in the watershed region were impacted. 

In 2022, the infestation is classified as “light”, which means about 40 per cent or fewer trees are impacted. 

But the voracious appetites of the spongy moths in 2021 has left a shortage of one important staple for many wild animals. 

“I didn’t see one acorn this year,” said Howes. 

Acorns grow on oak trees, which happen to be the prime rib on the buffet for spongy moths. During the 2021 outbreak, spongy moths ate foliage indiscriminately, like locusts in a Biblical plague. But Howes said the moth larva does prefer hardwoods: firstly oaks, then birch and poplars. Last year, they ate the needles off the tamarack trees. 

“It’s just like if you and I were at a buffet, we’ll pick our preferred food first, but if we’re at the end of the line of the buffet, then we’ll go for the carrot sticks and things,” said Howes. 

Since the oak trees had to focus on growing several sets of leaves in a season, they rerouted what energy would have been used to make acorns, causing very few, maybe none, to grow this year. 

“Acorns are an important food source for a lot of species, including deer, ruffed grouse, turkeys, squirrels, and blue jays,” said Howes. 

Bears also love acorns, and often rely on them for a season each year. The NVCA notes acorns are a fall staple for bears looking to bulk up for hibernation because an acorn is high in fat and carbohydrates. 

“You will see, for example, deer will congregate whenever there’s an acorn proliferation in a spot,” said Howes. “A lot of species are able to diversify … survive without acorns. But it’s definitely beneficial when they do have a lot of acorns.” 

Without a ready supply of the fall nut, bears may have to travel farther for food, and that could include venturing into more residential areas. 

The Ministry of Natural Resources and Forestry (MNRF) hasn’t collected information on whether there is a decline in acorn production, but acknowledged oak trees are very susceptible to defoliation by spongy moth caterpillars. Defoliation can reduce the number of acorns produced by reducing the flowers that develop into acorns and reducing the number of flower buds formed.

“Oak tree acorn production depends on weather and tree age, size, and vigour,” stated the MNRF in an email to CollingwoodToday. “Typically, trees with larger crowns generally produce more acorns … heavy defoliation or mortality of trees that consistently produce large acorn crops could affect acorn availability and consequently natural oak regeneration in affected areas in the short term.” 

The ministry doesn’t collect data to correlate spongy moth infestations with bear sightings, but less food means more looking for food.

“When there are a lot of natural foods available, the number of reported bear sightings is lower,” stated the MNRF. “When natural food sources are scarce, reported sightings are higher because black bears will look for alternative food sources – sometimes in urban or developed areas.” 

While tree-attacking invasive species are not new– the emerald ash borer has made a rarity of ash trees – this is the first time the area has seen such an infestation of the spongy moth, and it’s the first one that’s targeted oak trees. 

Howes isn’t aware of anything that has caused an acorn shortage on such a “landscape level.” 

“Oaks were our stoic, solid presence in the forests,” said Howes. “There will still be things that impact them, drought for example, but not on such a landscape level.” 

Acorns grow on a two-year cycle, so the trees that didn’t work on them in 2021 because of the caterpillars, should be able to produce them for 2023, she explained. 

The bears and other animals will have to find alternative food sources, but don't be tempted to assist in their search. Setting food out for wild animals can do more harm than good, experts warn.

The NVCA and MNRF suggest taking steps to avoid potential conflicts with bears at any time, not just during acorn shortages.

The best way to avoid conflict with bears is to avoid attracting them. There are tips online to help residents be bear-wise. They include: 

  • Only put garbage out on the morning of garbage day, not the night before pick-up.
  • Store meat scraps in the freezer until garbage day. 
  • Frequently wash garbage cans, and store them with tight-fitting lids in a basement or garage where possible. 
  • Fill bird feeders only through the winter months and take them away in the spring. You can offer birds natural alternatives such as nesting boxes, flowers, and fresh water. 
  • Keep your barbecue clean by burning off food residue, washing the grill, and emptying the grease trap. 
  • Do not leave pet food outdoors, in screened enclosures, or on porches.

You can report a non-emergency bear sighting to the MNRF by calling 1-866-514-2327. Call 911 in an emergency.

Erika Engel

About the Author: Erika Engel

Erika regularly covers all things news in Collingwood as a reporter and editor. She has 15 years of experience as a local journalist
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