The characteristic leaf-rolling behaviour of this insect does not seriously hurt the tree.
What is it?
The adult of the ash leaf cone roller (Caloptilia fraxinella) is a small gray moth with a 12 mm wingspan. This insect is native to North America.
Since the summer of 1999, many of the leaflets of Edmonton ash trees have been rolled up into the shape of a cone by the activity of this caterpillar.
What's the problem?
Early stages of the caterpillar are leafminers, feeding on the tissue between the upper and lower surfaces of the leaflet. In early to mid June, the caterpillar migrates on a silk line to a different leaflet. Once on a new leaflet, the caterpillar folds the leaflet into the characteristic cone, and spins its cocoon inside.
What can I do?
Spraying is not required as this rolling up of leaflets appears to have little effect on the health of the ash trees. The rolled leaflets remain attached to the tree. While a small part of a rolled leaflet will rot, most of it appears to remain undamaged.
The City of Edmonton Pest Management group is attempting to reduce pesticide use and has decided against spraying for this insect on the basis that:
- A tiny parasitic wasp attacks and kills a significant proportion of the caterpillars. These parasitic wasps are beneficial insects that do not sting people. Together with perhaps other naturally occurring enemies of the caterpillar, like birds, fungi or viruses, the population of these parasitic wasps will likely build up, thereby reducing that of the ash leaf-cone caterpillar.
- Any damage caused by this insect is cosmetic and does not warrant a pesticide response. These caterpillars are protected inside leaves almost all their lives, so a contact insecticide application is likely to kill off more natural enemies than caterpillars.
The University of Alberta is currently doing research to develop a "bait and kill" station, which employs the mating pheromone to draw male moths to their death.
The City of Edmonton pest management staff will continue to monitor this insect and impacts on its population by natural enemies like the parasitic wasp.