Tiny caterpillars turn Edmonton's leafy hardwoods into 'ghost trees'

Like an unseen, invading army set to devour every bit of foliage in their path leafroller caterpillars have arrived in Edmonton.

Each spring and early summer, the destructive insects feed on city trees, leaving their canopies bleached or barren.

The leafrollers' handiwork can be seen in neighbourhoods across Edmonton, said Mike Jenkinsthe City of Edmonton's pest co-ordinator.

"It's been going on for a few years now," Jenkins said in an interview with CBC Edmonton's Radio Active. "They strip all the trees and they leave a lot of silk behind.

"It creates these kind of spooky-looking ghost trees."

There are various species of the caterpillar. But Edmonton's most prevalent is the larger boxelder leafroller (Archips negundana), Jenkins said.

These bright green, tiny and well-camouflaged larvae live up to their name.

Protection from predators and parasites

The caterpillars spin silken nests inside the leaves of plants and trees and begin feeding, causing the leaves of their unwitting hosts to curl up and wither.

If disturbed, the larva quickly drops from the tree, suspended by a silky thread.

"They actually roll up the leaves basically to protect themselves from predators and parasites," Jenkins said. "It also makes them more difficult to hit with pesticides."

Manitoba maples, prevalent across the river valley, are a favourite target for the leafrollers.

An infestation may stunt the tree's growth slightly for the seasonbut the maple treesare incredibly hardy, Jenkins said.

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Thefoliage will usually bounce back after the caterpillars have grown their wings.

"It takes a lot to put those trees down," he said. "They're well-adapted for Prairie conditions."

They go bungee jumping on little silk threads.- Mike Jenkins,the City of Edmonton's pestco-ordinator

Ash leaf cone rollers do much of the same to Edmonton's ash trees. The caterpillars roll leaves into a little cone "almost like an ice cream cone," said Jenkins, and they have a voracious appetite.

"That particular caterpillar starts out as a leaf miner, actually between the layers of the leaf itself,"Jenkins said. "And basically, on one day, all the caterpillars will come out and then they go bungee jumping on little silk threads."

Ash trees are able to survive the onslaught, said Jenkins. There is some research to suggest the caterpillars may help their hosts withstand drought conditions.

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Damage caused by the leaf-curling critters of all kinds may be unsightly, but an infestation usually isn't cause for big concern, said Jenkins.

Carefully identify the species first to determine if any damage is being done, and if a chemical remedy is really necessary, he advised. There area variety of web-spinning bugs.

"All of them are an important part of the ecosystem," Jenkins said. "You need to consider whether it's actually worth controlling it or not."