Extension News Releases

Warm temperatures and the arrival of foliage mean forest tent caterpillars have begun to hatch and begin their five to six-week cycle of feeding on a variety of trees and shrubs across Manitoba.

Both biological agents and conventional insecticides can be used to control caterpillars, but provincial apiarist Don Dixon has reminded residents of the importance of applying insecticides in ways that will not harm honey bees, honey production or the pollination of crops.

Even with complete defoliation, there is rarely any tree mortality directly attributable to caterpillar feeding and trees usually refoliate within two to four weeks, said Manitoba Agriculture and Food entomologist Brent Elliott.

"A good control method for caterpillars is the biological agent Btk because it poses no threat to bees. Btk must be sprayed on the foliage so that caterpillars will ingest it, resulting in death in a couple of days."

Conventional insecticides such as malathion and diazinon are also very effective but more caution is necessary since these products can also harm bees.

"Used improperly, conventional insecticides can have a substantial negative impact on honey bees, native bees, honey production and pollinators in general," said Dixon. If conventional insecticides are used, he suggested some basic guidelines to help protect honey bees:

• spray in the late evening when the bees are not active;
• delay spraying until after crops, cover crops or weeds have bloomed because of the attraction of bees to the flowers;
• avoid spraying in windy conditions to reduce the risk of drifting into non-target locations, particularly near commercial beekeeping operations;
• use insecticides which are the least damaging to bees; and
• consult with beekeepers to prevent losses.

"If larvae are not objectionable to property owners, it may be best to leave them alone," said Elliott. "If the decision is made to spray, Btk should be considered if bees are in the vicinity. However, if a conventional insecticide is used, planning and consultation with beekeepers should result in both control of caterpillars and protection of the honey bee population."