Elm Spanworm (Ennomos subsignaria) defoliation makes the forest tent caterpillar defoliation pale in comparison. The spanworms are now fifth instar larvae and are defoliating trees, dropping from the branches and massing on objects in downtown Winnipeg and other areas of Manitoba. Outbreaks occur when weather conditions are favorable and parasitoid populations are low. Elm Spanworm populations are usually limited by mortality from egg parasitoids. The egg parasitoid, Ooencyrtus ennomophagus is credited with ending an outbreak in Connecticut in the 1970s. In Pennsylvania, the 1993 outbreak of elm spanworm completely defoliated over a million acres of forest.
The city will spray with the BT bacterial to kill caterpillars, but many spanworms have already formed their pupa stage or stopped eating and will not be affected by the spray. Moths will emerge later in the summer to mate and lay eggs. Residents hope the parasitiod populations will be high enough to stop the outbreak.