Arborvitae (Thuja)-Leafminer

Argyresthia thuiella; other similar species of Argyresthia are reported in other states

Pest description and damage Moths are tiny, silvery to gray, with black and brown markings on forewings and tan underbelly and legs. They lay pinkish eggs on branch tips. Larvae are yellow-green and hairy. Tiny larvae tunnel into leaf scale and mine within the foliage, causing tips to turn brown. Occasional outbreaks may turn trees totally brown. Despite how dead the trees look, carefully timed pesticides can interrupt the cycle and the trees will green up. The tip browning is very similar to cypress tip moth.

Biology and life history Larvae leave the mines in May, drop on silken threads to pupate. They spin dirty white cocoons in leaf axils. Moths emerge from cocoons in late May or June, mate and lay eggs on the branch tips.

Pest monitoring Look for larvae on silk strands, dirty silken white cocoons in leaf axils, or moths flying around trees in June. This is not a pest known to kill trees. Tolerate moderate leafmining to supply food for parasitoids which keep the pest in check. Only spray during the occasional outbreak, but make sure there is a vulnerable life stage present. Look for small exit holes accompanied by white cocoons in leaf angles.

Management-cultural control

Lightly sheer branches in late April or early May and before leafminer adults emerge, then rake and remove trimmings.

Management-biological control

There are 26 species of parasitoids reported to prey on this insect in the US and Canada.

Management-chemical control

See:

See "Leafminer" in:

For more information

Childs, R. 2011. Arborvitae leaf miner (http://extension.umass.edu/landscape/fact-sheets/arborvitae-leafminer)

Cloyd, R.A. 2001. Arborvitae leaf miner (http://hyg.ipm.illinois.edu/pastpest/200108a.html)

WSU Hortsense. Arbovitae Leafminer (http://pep.wsu.edu/hortsense/scripts/query/displayProblem.asp?tableName=...)