The boxelder leafroller can be seen in downtown Kamloops in May and June.

(CARLI BERRY / iNFOnews.ca)

Kamloops residents may notice wriggling green caterpillars hanging from threads in Riverside Park as a city arborist says there's a high population of them in the area this year.

The boxelder leafroller is a caterpillar that feeds on the leaves, webbing them together with silk. If disturbed, the larva quickly drops from the tree, suspended by a silken thread. The caterpillars have caused problems in other cities like Edmonton as heavy infestations can result in the complete defoliation of trees.

“The annual population of this pest cycles between low, moderate and high numbers depending on weather and other variables throughout the year. The extensive defoliation of Manitoba maple trees in Riverside Park and throughout Kamloops suggests there is a high population of the leafrollers in Kamloops this year,” said Brian Purves, an arborist with the City of Kamloops, via email.

They are common throughout parts of the United States and Canada, according to Utah State University.

Eggs hatch with warmer temperatures in early May and will form cocoons and emerge as adult moths in mid-July.

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The Manitoba maple tree is very resilient and does not seem to be affected in the long-term by the caterpillars, Purves said.

“The main concern and reason why we receive many calls regarding the pest is the unsightly look of a tree with no leaves and green caterpillars dangling on silk threads. The caterpillar stage of the pest's life cycle causes the visible damage in May and June. Once the caterpillar stage is finished, defoliated healthy trees usually produce a second set of leaves and appear nearly normal later in the summer,” Purves said.

The city does not spray the trees with pesticides to control these bugs, but continues to monitor the long-term health of the trees.

“Usually these larvae will pupate in rolled leaves (not many left to actually use) and then develop into a smallish brown moth with a wingspan of about two centimetres. Some of these appear to already be flying about,” said TRU bug expert Rob Higgins, via email.

It’s not unusual to see these caterpillars dangling on their silk strings and they’re harmless to people, he said.

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