Last year, the forests of southern Quebec and Ontario and much of New England became eerily leafless. The air buzzed with the nibbling of mandibles and the tree trunks were covered in a writhing carpet of caterpillars, while rains of caterpillar droppings fell softly on the heads of unsuspecting hikers and campers.
The population of the European spongy butterfly, which had been steadily increasing since 2019, peaked dramatically in 2021 and disappeared completely this year.
In 2020, the hungry caterpillar damaged 583,157 hectares of forest in Ontario and that number is set to increase when the 2021 numbers are revealed.
Insect infestations are one of the most significant natural disturbances in Canadian forests. As a biologist who has worked on plant-insect interactions for over 20 years, I see that the frequency, intensity and scope of insect outbreaks keep changing. To protect the trees in our forests and cities, we need the diversity of trees.
Insect infestations
An infestation of insects can be scary. In deserts around the world, vast swarms of locusts can block out the sun for hours as they fly overhead. In the Rockies, the hillsides are covered in dead trees, killed by the mountain pine beetle.
![Trees whose leaves are eaten](https://oponame.com/wp-content/uploads/2022/10/Why-the-gypsy-moth-epidemic-has-disappeared-in-Quebec.0&q=45&auto=format&w=754&fit=clip.jpeg)
THE CANADIAN PRESS/Paul Chiasson
However, insect infestations are not a new phenomenon. Chinese historical records document locust infestations for nearly 2,000 years, while paleoecological studies show that Quebec’s boreal forests have experienced spruce budworm infestations for at least 8,000 years.
These insect outbreaks are part of the workings of temperate and boreal forests, as well as grasslands and semi-arid deserts. Insect infestations stimulate nutrient cycling, accelerate forest succession and can renew forests.
![A butterfly laying eggs](https://oponame.com/wp-content/uploads/2022/10/Why-the-gypsy-moth-epidemic-has-disappeared-in-Quebec.0&q=45&auto=format&w=237&fit=clip.jpeg)
(AP Photo/Bob Child, File)
Female insects can produce hundreds of offspring and for the population to remain stable, only two of them need to survive. A small increase in survival, due to factors such as favorable weather conditions, can lead to a population explosion and an epidemic.
In the case of the mountain pine beetle and the desert locust, warming temperatures, increased cyclone activity and other similar effects of climate change are bringing these favorable conditions more frequently to new areas, greatly increasing the extent epidemics.
However, these outbreaks always end due to what ecologists call staggered density-dependent population dynamics. Here, density dependent means that the mortality rate of insects depends on its population density. As the population increases, the mortality also increases and the survival rate decreases. Meanwhile, lagged means there is a delay in this process – insect mortality increases more slowly than population growth, causing an outbreak.
The outbreak collapses when insect mortality eventually catches up to its population size. This usually happens due to a combination of factors, including a low food supply and an increase in predators, parasitoids – insects that lay their eggs inside other insects – and disease.
Where did the squishy butterfly go?
Students in my lab have been raising moth moth caterpillars for three years and have found that the mortality of these caterpillars increases gradually as the population increases.
In 2019, a student, Pamela Yataco Marquez, bred over 300 caterpillars and observed an 80% survival rate. However, this year, despite an extensive search, Marie-Eve Jarry, Geovana Demarchi and Victoria Yip were only able to find and breed 97 caterpillars and only six survived to adulthood.
![A freshly emerged laboratory butterfly.](https://oponame.com/wp-content/uploads/2022/10/1666532945_410_Why-the-gypsy-moth-epidemic-has-disappeared-in-Quebec.0&q=45&auto=format&w=754&fit=clip.jpeg)
(Victoria Yip), Author provided
Several agents of death including a virus Lymantria dispar multiple nucleopolyhedrovirusfungal disease Entomophaga maimaiga and two tiny parasitoid wasps called Cotesia melanocele and Ooencyrtus kuvanae finally caught up with the insect population.
When the eggs of parasitoids – laid inside the eggs or bodies of other insects – hatch, they devour their host from within and eventually emerge from the dead host, ready to begin the life cycle again.
They are more like predators than parasites because they kill their host and are effective biological control agents that reduce pest insect populations.
Crawling across borders
Although the spongy moth is native to Europe, it has been present in eastern North America since the 1860s and is now part of our fauna.
It has not yet reached the western part of the continent and the best way to stop this is to inspect outdoor equipment for caterpillars or egg masses before traveling and not to move firewood. heater.
The Asian spongy moth population has yet to spread to North America, and entomologists are working hard to keep it out.
![A butterfly caterpillar on a partially eaten leaf.](https://oponame.com/wp-content/uploads/2022/10/1666532946_166_Why-the-gypsy-moth-epidemic-has-disappeared-in-Quebec.0&q=45&auto=format&w=754&fit=clip.jpeg)
(Geovana Demarchi), Author provided
Over the past 150 years, many natural enemies of the European gypsy moth, including the fungal disease mentioned above and several parasitoids, have also been introduced, either inadvertently or deliberately. Our findings show that these natural enemies are well established in our region and have been effective in stemming the epidemic.
The current range of the spongy moth in North America extends to southern Canada. Here, eggs that overwinter on tree trunks experience high cold mortality, which reduces the survival rate regardless of population size.
Forest managers in Quebec and Ontario are watching for an increase in gypsy moth outbreaks – including more severe, longer-lasting outbreaks similar to those seen in the United States – and a possible northward shift in range.
Miscellaneous forests
While a leafless tree in July may appear dead, many trees can survive a few years of defoliation, drawing on stored reserves to flush out new leaves.
The gypsy moth epidemic in the Montreal area in the late 1970s slowed tree growth, but did not cause widespread death of forest trees. However, tree mortality occurs further south in the United States and depends on the diversity of trees in the forest zone. Death of preferred tree species by the caterpillar is lower in diverse forests that mix with less vulnerable species.
Diversified forests are more resistant to various stresses than more homogeneous forests. We must create and preserve such diverse forests to help prepare for new types of insect outbreaks in our changing world.
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