A number of residents have noticed the cankerworm infestation impacting trees in the Rosedale ravine, or have seen related media reports. NRRA Board Member Norm Mierins contacted the Forest Health Care Inspector for the City of Toronto, Joel Harrison-Off to review the City’s assessment and planned response, and to provide some practical guidance to residents.
Cankerworms are also known as inchworms. Their numbers have spiked significantly this year. In addition, some neighbourhood trees are being impacted by blight – or more specifically, anthracnose.
Mr. Harrison-Off believes that the stress on the trees will start to abate in July as the rain subsides. The City is monitoring the situation closely.
None of these threats to trees is new. In the case of cankerworm, serious infestations are not unusual and take place every 7 to 10 years. However, the last outbreak occurred nearly 17 years ago.
The cankerworm suddenly increases in abundance to a very high level, and then suddenly crashes. The preferred hosts are Manitoba maple, red and black oak and crab apple. Caterpillars feed on the tree leaves from April to mid-June. By that time defoliation is nearly complete and the caterpillars are moving back to the soil to pupate. Healthy trees will put on new leaves during the summer.
The City is monitoring the situation closely and may consider preventative action such as “banding” to protect a stricken tree.
Specific Management Practices for Control of the Cankerworm
- Trap wingless female moths by installing a band around the main stem of host trees in October. This physical barrier prevents egg laying in the upper crown.
- Attract birds and other beneficial organisms to your own yard – such as ground beetles – by planting appropriate plants (herbs, flowers, ground covers and shrubs).
- Increase the number of beneficial organisms by releasing insect attacking nematodes and/or parasitic wasps. Check your local Garden Centre for availability.
- Discourage the use of synthetic pesticides, as spray drifts are harmful not only to humans and pets, but also to many beneficial organisms that naturally help control infestations.
- Apply Bacillus thurigiensis var. kurstaki (Btk) in severe infestation at the early larval or instar stages of development, when the host leaves first start unfurling. This bacterial insecticide affects only actively feeding moth and butterfly caterpillars.
The City has extensive information on the cankerworm. Please follow this link:
The wet spring has encouraged the growth of another threat to our trees – anthracnose.
Anthracnose is a group of fungal diseases that affect a variety of plants in warm, humid areas. Commonly infecting the developing shoots and leaves, anthracnose fungi (usually Colletotrichum or Gloeosporium) produce spores in tiny, sunken, saucer-shaped fruiting bodies known as acervuli. Symptoms include sunken spots or lesions (blight) of various colours in leaves, stems, fruits, or flowers. Some infections form cankers on twigs and branches.
Anthracnose causes the wilting, withering, and dying of tissues, though the severity of the infection depends on both the causative agent and the infected species. Severity can range from mere unsightliness to death. Shade trees such as sycamore, ash, oak, and maple are especially susceptible, though the disease is found in a number of plants, including grasses and annuals.
The fungus overwinters on infected twigs and fallen leaves and is activated in the spring by cool and wet weather.
Specific Management Practices for Control of Anthracnose
- Raking leaves in the fall, and pruning dead or dying branches helps reduce infection the following year. Leaves may be composted by City composting programs.
- When watering, avoid wetting the crown of the tree.
For more information, please follow this link to the City’s Urban Forest website: