Threats to Trees

Bees, Wasps and Hornets

Bees, wasps and hornets have a dangerous reputation due to their ability to sting, and while that danger is of particular concern to those who are hypersensitive, the beneficial aspects of these insects are often overlooked.  Bees are important pollinators and honey producers.  Wasps and hornets are considered beneficial insects because they feed on a large number of insect pests.  They are also scavengers, feeding on carcasses.

Bees, wasps and hornets are social insects and live in colonies. They nest in sheltered places such as hollows of trees, stumps, in the ground or in constructed nests often suspended from the branches of trees, eaves or other parts of buildings.  These insects tolerate human approach, but will attack if their nest is disturbed.  Most nests built by stinging insects are placed high enough in trees to not threaten public safety.  Nests situated in the tree higher than 3m (approx. 10 ft) are typically not removed.  However, if the nests are within the public right-of-way and close enough to the ground to threaten public safety, City of Toronto, Urban Forestry will inspect the site and decide on further action. 

Bald-faced hornet nest attached to a branch

If a nest in a City owned tree is causing a problem in the summer, residents may request an inspection.  Many residents notice these nests in the fall, when the leaves fall from the tree.  Removal of nest at this time is not necessary since the wasps, which remain in the nest, do not survive the winter season.  Only the queens overwinter in sheltered locations outside of the nest.

Given the value of many wasp species for natural control of insect pests, it is the policy of the City of Toronto, Urban Forestry to leave nests in the trees where possible.  Do not disturb the nests under any circumstances.  If the nest needs to be removed, seek help from a professional.

Specific Management Practices for Control of Bees, Wasps and Hornets

To keep bees, wasps and hornets away from your surroundings:

  • Do not leave fallen fruit and food debris lying around.
  • Make sure that garbage bins are covered with tight lids.
  • Do not keep uneaten pet food outside.
  • Remove water from ponds, puddles, birdbaths and any other source of standing water on your property.  Bees, wasps and hornets are often attracted to standing water.

 

Wasp and hornet nests can be treated from a relatively safe distance (2-4m) with a foam jet, insecticide spray sold in a pressurized can. The foam released from the can covers the opening of the nest while the insecticide kills the insects trapped inside. This ready-to-use product provides contact and residual control and is sold at most garden centres.  Read the label carefully before using any pesticide.  Application to the nest should be made during early morning or late evening when the insects are inactive.

Where the nest is situated in a City owned tree below 3m (10 ft) and is a public safety concern call 311.


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