ARBORVITAE LEAFMINERS
(Lepidoptera: Yponomeutidae)

Typical arborvitae leafminer damage on cedar. Image source: Common Pests of Trees in Ontario, Ministry of Natural Resources, 1989.

The arborvitae leafminer, Argyresthia thuiella (Packard), is a tiny native moth distributed over the entire range of its host plant, the eastern white cedar. Infestations of this insect on ornamental white cedar hedges and specimen trees reduce the aesthetic value of these types of plantings.

Small larvae, feeding within the scale-like leaves of the cedars, are responsible for most of the damage. An infestation is usually first noticed in the spring when the tips of leafy branches begin to brown and later shed. Heavy infestations can cause severe browning and thinning of the foliage. It appears that open plantings such as hedges and groupings of ornamental trees are more susceptible.

Other forms of injury to white cedar are sometimes confused with a leafminer infestation. These include winter drying and salt damage in areas near highways and roads with heavy high-speed traffic. Both can cause a dieback of branches. The presence of small pink or green larvae, exit holes 1-2 mm in diameter, hollowed out leaflets, or excrement particles, are all signs of the presence of the arborvitae leafminer.

Close up of arborvitae leafminer larva Image source: Howard Russell, MSU Diagnostic Services.

A. thuiella overwinters as small larvae inside the flat scale-like leaves of cedars. The larvae feed on the leaves throughout the spring, pupate in the hollowed out leaves in late May and June and emerge as adult moths from early June until July. The tiny grey moths are weak fliers and usually remain near the trees from which they emerged. They may be carried by wind for considerable distances to other cedars. The moths mate and lay their eggs under the leaf scales of the new foliage. These hatch and the small larvae begin feeding to complete the life cycle. There is only one generation per year. Other species of Argyresthia also attack white cedar. Their life cycle and controls are similar to those of A. thuiella.

CONTROL

White cedars can withstand considerable injury from leafminers before significant damage occurs. In most years, control will not be necessary. If need be, efficient control of the leafminer on small ornamentals may be accomplished simply by pruning out and destroying infested branches before the moths emerge.

If damage is severe, the systemic insecticide dimethoate can be used as a foliar spray to control feeding larvae and adult moths. An application in early May, and again in late June is recommended. If necessary another application after mid-August can be made in order to control the newly hatched larvae. The contact insecticides malathion or carbaryl can be applied in June and July to kill the adult moths, thus preventing them from laying eggs.

Follow label directions to ensure the safety and efficacy of each product

M.K. Sears/M.D. Dykstra/M. Sabourin

PDCF-077 (Revised 2003/11/07)