Voracious caterpillars threatening Ontario's trees

They're only about four to six centimetres long, but gypsy moth caterpillars are a massive threat to Ontario's forest cover.

Scientists are warning that 2021 is shaping up to be a bumper year for the invasive species, brought to North America around 1860 by a French entomologist who hoped to cross-breed them with silkworms. 

The good news is that landowners can help battle the bugs using burlap and soapy water.

"A caterpillar can eat about one square metre of leaves as it goes from a little tiny new caterpillar to a great big adult one that's ready to pupate. That's a fair amount of foliage for just one caterpillar," noted Chris MacQuarrie, a research scientist with the Great Lakes Forestry Centre in Sault Ste. Marie, Ont., one of five research centres that form part of the Canadian Forest Service. 

The gypsy moth caterpillar (Lymantria dispar) isn't fussy about what it eats, either.

"It really likes oak and birch and aspen," said MacQuarrie. "It also eats maple and beech, and it'll even eat some of the softwood species such as white pine and balsam fir and … Colorado blue spruce.

"Once it's done with the trees, it'll even drop down and feed on understory plants and bushes."

When larger caterpillars are in a feeding frenzy, their "frass," or excrement, falls from above. The sound it makes hitting the ground has been compared to the pitter-patter of rain drops. 

Their voracious appetite can have a real impact. In 2019, Ontario's Ministry of Natural Resources and Forestry used aerial photography to determine gypsy moths had defoliated 47,203 hectares. By 2020, the damage had spread to 586,385 hectares.

"[That's a] 1,200 per cent increase, which is a lot," MacQuarrie told CBC Radio's Ottawa Morning. 

The population of gypsy moths is cyclical, with large outbreaks every seven to 10 years. The last scourge hit Ontario in the early 2000s. Disease and predators help control the population some years, but "when the weather's right, those diseases don't do such a good job of controlling the caterpillar and they can outbreak," said MacQuarrie. 

In May, gypsy moth caterpillars are only about one centimetre in length, clumping together on a tree near where their mother laid the egg mass the previous year. Once they start to grow, they develop a distinct pattern of blue and red dots down their back and grow "really big tufty hairs all over their body," said MacQuarrie.

"They don't look like something you'd really want to touch, but they are quite striking to look at."

There are ways to combat the invasive species and protect trees. BTK Caterpillar Killer is a biological insecticide that is available for home use against very young gypsy moth caterpillars, but is less effective when they grow bigger.

MacQuarrie recommends a burlap banding technique to trap larger caterpillars. "Wrap a band of burlap around the tree. As the caterpillars come down from the canopy of the tree during the day, they'll take shelter underneath the burlap. Folks can come along at night … and pull them out from underneath the burlap and throw them in a bucket of soapy water," said MacQuarrie.

There's little to be done about the insects once they've pupated, usually in July, but in late summer and fall, it is possible to spot clumps of eggs left by females on trees. "Those you can scrape off your trees or squish with a credit card," said MacQuarrie. "That'll help control the population for next year."

Some Ontario municipalities such as London and Hamilton spray BTK to control outbreaks, but that's not something Ottawa is prepared to do yet.

In 2020, the city set up a monitoring program to track the population of gypsy moths in 2021. "Although the populations of gypsy moth are rising, insecticide spray is not being considered by the City of Ottawa at this time," said Jason Pollard, a forestry manager with the city, in a emailed statement.

There is information on the city's website about what residents can do. Natural Resources Canada also has more information on the caterpillars. For more on the outbreak, you can refer to the Ministry of Natural Resources and Forestry website.