Forest tent caterpillars on the move
As if dealing with the flood threat wasn’t bad enough, the forest tent caterpillar population is on the rise—although whether the Fort Frances area ends up seeing a full-on invasion remains to be seen.
The beginning of the outbreak here is on schedule with the 10- to 12-year periodicity of caterpillar outbreaks, noted Taylor Scarr, provincial forest entomologist with the Ministry of Natural Resources.
Similar outbreaks are being seen in Saskatchewan and Manitoba.
But exactly how numerous and widespread the critters will be in this part of the province is still unclear.
“It’s still early in some parts of the northwest to estimate the population levels,” said Scarr, adding the MNR’s Forest Health Monitoring program has technicians who are doing ground surveys to locate the infestations.
So far in this district, the forest tent caterpillar population seems to be much lower than elsewhere.
“We have noticed very low levels of [caterpillars] along Highway 11 east towards Mine Centre,” noted Scarr.
“We have not documented defoliation and have only noted live specimens in low numbers,” he added.
The overall status will be better confirmed once aerial observations have been conducted.
“Later in July, the technicians will carry out aerial surveys to map the exact areas being affected this year,” Scarr explained.
“But from the ground surveys so far this season, we know that the populations are increasing over 2013 and the area affected is much larger.
“This increase in extent and severity of the outbreak was expected based on the historical pattern for this insect,” he added.
“Outbreaks erupt quickly, and last about three years in a given area.”
Scarr clarified the Fort Frances are experienced defoliation by forest tent caterpillars last year, but it wasn’t as widespread as this year is projected to be.
“We expect this pattern to continue over the next few years in the northwest,” he noted. “And the outbreak spreads and the areas of defoliation coalesce into very large areas of forested affected over several million hectares [across Ontario].
“For example, in 2012 there were 48,000 hectares of [forest tent caterpillars] in Ontario,” Scarr said. “In 2013, this increased substantially to 320,000 hectares.
“But this insect has the potential to reach several million hectares across northwestern and northeastern Ontario, as well as in the hardwood forests of southern Ontario,” he stressed.
The last outbreak in the Fort Frances area ended around 2002, Scarr recalled, although few scattered pockets persisted for a year or two after that.
That outbreak reached 13.2 million hectares in Ontario.
The caterpillars emerge in the spring and start feeding about the same time as the buds start to flush on their host trees, Scarr said.
They feed for about six weeks, depending on temperatures (the warmer the weather, the faster they grow and the more they eat per day).
During low or moderate population cycles, the now full-sized insect will spin a cocoon by rolling up a leaf on the tree and then molting inside it from a caterpillar to a pupa.
But when populations are high, these leaves get eaten, so the caterpillars migrate from the tree and search for sheltered places to spin their cocoon and pupate.
The pupal period lasts about two weeks, when the moths emerge, lay their eggs, or fly to new areas.
Moth migrations can be large. And since the moths are attracted to lights, they often pile up outside areas such as truck stops or coffee shops where lights are on overnight.
The eggs are laid in bands around the twigs of their host trees. They are contained within a frothy mass that soon hardens, and is covered by a varnish-like coating (this protects them from cold weather and predators).
The insect then spends the winter as a tiny caterpillar inside the eggs—ready to emerge as soon as warm weather arrives the following spring.
“In the Fort Frances area, feeding by the caterpillars is usually completed in July, with egg-laying finished in August,” Scarr noted.
What to do?
Scarr said the MNR does not typically carry out tree protection programs for forest tent caterpillars as they’re seen as a normal part of the ecosystem.
“It will erupt about every 10-12 years, cause widespread defoliation, and then return to low, almost undetectable levels,” he explained.
“Tree mortality is usually low, although if other stressors are present (e.g., other defoliating insects or drought), tree mortality can be worse,” added Scarr.
There are exception, such as in the early 2000s, when northeastern Ontario experienced back-to-back outbreaks of forest tent caterpillars and tree mortality was severe, with more than 700,000 hectares of dead or declining aspen.
Private landowners sometimes conduct their own aerial spray programs at their own expense over cottage areas, woodlots, or maple sugar bushes. The product used is a very safe insecticide called Btk.
It is a bacterium that is eaten by the insect, perforates the stomach, and disrupts its ability to digest its food.
It affects only caterpillars that have an alkaline pH and lasts for only a few days—dying from exposure to ultraviolet light.
Individual trees can be protected with Btk, which can be purchased at garden centres and hardware stores.
Trees also can be injected by a professional applicator with a product called TreeAzin, which is a botanical insecticide derived from the neem tree of India.
As for individual homeowners, some people use high-pressure water sprays to try to dislodge the caterpillars. A band of tape then is placed around the trunk to stop the caterpillars from climbing back up or from invading from the neighbours.
The tape is covered with a sticky product (Tanglefoot or Pestick) that blocks the crawling caterpillars.
“It’s important to use a barrier between the tree and the sticky goo to prevent damage to the bark,” stressed Scarr.
“The same products and tape method can be used to keep the caterpillars off buildings or from crawling up posts onto decks,” he added.
During the fall and winter, after the leaves have dropped, the egg bands can be clipped from the twigs on smaller trees and destroyed.
“Otherwise, we can wait out the caterpillars, knowing that soon they will be pupating and after two-three years in an area, they will go back to low levels for about 10 or more years,” Scarr reasoned.
Forest tent caterpillar outbreaks last about six-10 years in the province, but it is unusual for any specific area to have defoliation for more than three years.
The collapse of the outbreak is caused by several factors, which include starvation and caterpillar mortality caused by the nucleopolyhedrosis virus (NPV).
The virus infects the caterpillar’s body and liquefies its internal tissues. The caterpillar’s body eventually bursts, spilling liquid and virus particles over the leaves and bark.
Other caterpillars crawl across the contaminated surface and pick up the virus, get infected, and eventually die, too. But it takes large numbers of caterpillars and a few years of an outbreak for this process to occur.
A parasitic fly, dubbed the “friendly fly,” also can kill large numbers of forest tent caterpillars. It lays a live larva that burrows inside and consumes the pupa.
The fly, known to cause up to 80 percent mortality among caterpillars, resembles the house fly, but is much hairier and has a white-and-black checkerboard pattern on its abdomen.
Despite rumours to the contrary, the friendly fly was never imported into our area. It is a natural part of the ecosystem, and mirrors the caterpillar outbreaks.
The trees also respond. Those that lose more than 50 percent of its leaves will produce a second flush of foliage.
But these leaves typically are smaller and tougher, and thus are more difficult to digest and lower in nutrition.