The very hungry caterpillars crawling all over Mississauga

There once was a tiny, hungry caterpillar, who found its way to Mississauga.

This one furry caterpillar turned into hundreds, then thousands.

Their ravenous appetite posed a major problem for the city, munching through thousands of trees with green, luscious leaves. 

Residents despaired over the dying trees, as much of the protective urban canopy started to fade away.

For a small insect, the Lymantria dispar dispar (LDD) moths, more commonly known as gypsy moths, are causing quite an uproar across Ontario this year. The Pointer will be using the insects scientific name as the word “gypsy” is considered offensive to some. 

At Wednesday’s Mississauga City Council meeting, Councillor Karen Ras, chair of the Credit Valley Conservation Authority (CVC) moved a motion to spray the urban canopy in 2022 to help control the growing LDD moth problem. The City last sprayed for the invasive species in 2018 to prevent the spreading of cankerworms (another type of invasive caterpillar). City staff will bring forward more information on where to spray and the budget for the project in September.

The spray, known as BTK, is a naturally occurring bacteria found in soil. It is only harmful to caterpillars in larvae form and has no known impact on humans or other wildlife. Canada has been using the pesticide for over 30 years to successfully kill off LDD moths. When they ingest the spray, it takes two to five days for the insect to die. 

The CVC, which covers the majority of Mississauga, and is responsible in part for maintaining its remaining green spaces, has been keeping a close eye on the rapidly multiplying insects. Freyja Whitten, a program manager of terrestrial restoration at CVC, told The Pointer even though residents would like CVC to spray every year, the pesticides can have an effect on other caterpillars and overuse could diminish a food source for other animals like birds.

The 2021 infestation is particularly bad in areas that missed out on the 2018 spraying, Jodi Robillos, director of parks and forestry for the City of Mississauga told council. Areas such Applewood Hills have not been sprayed for the moths since 2007, the 11-year gap, compared to other parts of the city, has allowed the infestation to become sizable in certain neighbourhoods, particularly those with a more mature tree cover. 

The destruction caused by these insects is not their only impact. Councillor Matt Mahony brought up rashes some people are getting if they are in contact with the furry creatures. Whitten told The Pointer, rashes are common if people touch both the caterpillar as well as their eggs. 

Aside from a rash that is hard to ignore, the presence of the caterpillars in some urban areas can be easy to spot if there are a small number of trees, which causes the LDD moths to stand out. 

“People are more attached to those trees because there's less, so if you have one big oak tree in your front yard and it gets completely defoliated, you're going to be really upset,” Whitten said.

City staff are using this to their advantage, putting together an interactive map to highlight which trees in the municipality have caterpillars. The tool is updated when a constituent reports a tree in distress due to the invasive species. The information will be used to help identify areas in need of spraying next year.

Areas in the city hit hardest by the moths are more tree dense neighbourhoods, such as Port Credit, Mineola, Clarkson and Erin Mills. (Map from City of Mississauga)

LDD moths are a, “cyclical population,” with large infestations in certain years, the population will eventually decrease due to natural causes. 

In 1868 a French scientist in Massachusetts tried to cross breed the LDD moth (native to Europe) with a North American silkworm in an effort to try and create a silk producer that could be more productive and cheaper to feed. The crossbreeding failed, and the invasive moth made its way to Ontario by 1969. By 1981 there was a severe impact on tree cover in many cities, by 1991 the caterpillars had eaten their way through 350,000 hectares of trees across the province.

“The population would build, and then it would crash, and then it would drop down. So it was every five to 10 years. Usually these outbreaks would only last for about three years,” Whitten said. “So we've been sort of seeing highs and lows and after 1991, the peaks were much smaller, like in 2002, it was only 150,000 hectares defoliated.” 

LDD moths, unlike some other invasive species, have natural predators, like birds, that assist in keeping their population at bay. Whitten said CVC knew 2021 was going to be a bad year for the moths since 2020 saw 569,000 hectares of trees covered with them across Ontario. She is hopeful this will be the peak of the cycle and next year will see a large decrease.

Despite the annoyance for people, birds have been thoroughly enjoying the amount of these moths this year. Other predators of the pests include viruses and types of fungus. 

The fungus can only occur over cool, wet winters in the soil. In order to successfully kill off large populations of the eggs, the spring needs to also be wet and cold, which was not the case in 2021. 

The nuclear polyhedrosis virus (NPV) needs to see large infestations of caterpillars in order to successfully diminish the population. As the birds eat large amounts of the insects, the virus is spread through their feces. 

“We're really, really hoping because our population is so high of (the) moths this year, that we will actually start to see that virus. And so it's expected sort of in the next month that there may be signs of that virus killing them,” Whitten said. 

Climate change has been a key cause of the rapidly growing caterpillar populations. The eggs are molded into white or yellow balls that hang onto buildings, trees, under decks and parked cars over the winter. Since the changing climate has been making winters warmer, more of the eggs survive until the spring. Lots of snow can also keep them isolated. To kill off large populations of the caterpillars there needs to be at least two consecutive years of an average of minus-20 or colder winters. 

Egg masses such as these, need to be scraped away and soaked in soapy water or burned to make sure the larvae can’t survive. (Image from Ryan Hodnett - Wikimedia Commons)

There is beauty in how quickly nature can adapt and learn to fight back. Trees that had many of their leaves decimated one year will grow their leaves back tougher and less tasty for the bugs. However, with climate change on the side of the invasive species, many trees are not able to adapt fast enough, especially when surrounded by other trees of the same type that allow the caterpillars to proliferate.

As there is power in genetic diversity among the human population, the same goes for trees and forests.

The more biodiverse an area is, the stronger the foliage and canopy. Trees in cities are under constant stress, of heat, floods and other human impacts, making them weaker, and the fight against the LDD moth harder. 

Biodiversity is important in forests because it creates stronger, more resilient trees. (Photo from Natasha O’Neill/ The Pointer)
 

To prevent the invasive caterpillars from chowing down on trees in the summer, residents are encouraged in the spring to wrap the trunks of their trees in burlap. During the day the caterpillars will come down from the leaves and find shelter from the heat inside the folds of the burlap. Removing the wrap and placing the whole sack in soapy water and leaving it covered for 48 hours will kill them. In the fall and winter months, any egg masses around the property should be scraped into soapy water and soaked for 48 hours.

Whitten says the eggs are “hardy” and will survive and hatch if scraped and left on the ground. If permitted by bylaw or firefighters, adding the eggs to a bonfire will also kill them quickly.

All the work has a broader consequence.

Trees in urban areas are extremely important. They improve air quality, provide shade for buildings (which reduces energy consumption), prevent flooding and reduce stormwater runoff and they are natural stress relievers. 

Without spraying every so often, patches of destruction in the urban canopy are sure to be seen, quickly reducing the quality of life for all city dwellers. 

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