The glistening sap falling from trees onto Vancouver sidewalks and roads this summer is the work of aphids and their appetite for sucking juices from leaves.
Aphids are tiny insects that feed by sucking sap from plants. In Vancouver, they’ve grown attached to linden trees, also known as tilias. The pests secret a sticky substance that coats pavement and car windshields.
“Basically, it’s honeydew liquid, it’s a sugar rich substance that the aphids secrete out,” said Amit Gandha, a city arborist. “The leaves themselves will have this glistening glow to them.”
Around 10,000 lindens were planted in the city from the 80s to the early 2000s until urban forestry workers like Gandha discovered that aphids were attracted to them.
Gandha said the aphid problem isn’t worse than other years, but the city is monitoring the insects’ prevalence by counting them on leaves and using insecticidal soap to wash them off trees.
“Because these concerns were starting to come forward and we didn’t want to go down that road of obviously continuing to plant something that we would have these challenges with,” explained Gandha “So we haven’t been planting any tilias or lindens since then.”
In the past, the city bought large numbers of ladybugs and released them in areas where there are many aphids. Ladybugs eat the aphids, but Gandha says now there are too few ladybug suppliers
“We’ve had an extremely difficult time sourcing ladybugs,” he said.
Wasps are also predators, but the city does not use them due to safety concerns.
“Nobody’s going to want to handle them.”
Gandha is encouraging residents to blast aphids off trees with water.
“Some of the trees are quite large for your average home owner, a 40- to 50-foot tree,” he said. “Sometimes it doesn’t feel like you’re doing anything by getting some of them off but it does help, it does lower the numbers.”
Please water trees
He also wants residents and businesses to water trees where they can, as continued hot dry weather is stressful for the trees and can make them more susceptible to disease and decline.
“Tilias, I mean they seem to resist and hang on, they don’t, so far from our experience here in Vancouver, these infestations haven’t impacted them,” said Gandha.
Meanwhile if the city can’t manage its aphid problem, Gandha says one option may be to remove the linden trees.
However he says that won’t happen without plenty of consultation with residents.
“It does change the landscape of a major block where you could have 20 to 30 of these trees planted, he said. “So that would be quite a change. We need to come up with a plan if it does go down that road, how do we do it?”