Cedar (Thuja) Leafminer Damage

Thuja occidentalis (Arborvitae) is incredibly disease resistant, and there are only a handful of pests that are of any concern to the bonsai grower. One of those pests is the larvae of a tiny moth called the cedar leaf tip miner.

The following excerpt is taken from a University of Guelph Pest Diagnostic Clinic Factsheet on Thuja occidentalis:

Cedar leafminers are tiny moths native to Canada. Damage is caused by the small larvae feeding within the scale-like leaves of cedar. An infestation is usually first noticed in the spring when the tips of some branches begin to bleach and brown. Heavy infestations can cause severe thinning of the foliage.

The article goes on to say that the disease is not a major concern but, for bonsai growers, it can be aesthetically displeasing and can also weaken the tree if left untreated.

The tree below was collected in late April 2011. I suspect that its recovery left its defenses down, and by August the leafminer had established itself.

Cedar Leafminer Damage on a small (25 cm) Thuja occidentalis. The damage is minor, but enough to have affected the recovery of this recently collected tree.

Treatment of leafminer is relatively simple. Prune away the damaged foliage, and keep the tree in full sun to promote the recovery of the plant’s defenses. A light application of a systemic insecticide such as imidacloprid will seal the deal.

As a preventative measure, Reiner Goebel recommends treating Thuja with a systemic in the spring to avoid unexpected infestations of leafminer.

One month later, the tree is sparse, but happy. The tree should be much stronger through 2012, and I doubt the leaf miners will return. It may even be ready for its first wiring by the summer. Had it not been for the leaf miner, the tree probably could have been wired this spring.

2 Responses

  1. crust

    I will be giving my cedars a insecticide treatment this spring. I think I may have been struck by miners last year although it is hard to really tell because they were barely fertilized for a few years while I worked overseas and their soil became alkalized due to my water (PH 7.9). Anyway, I like this ratty thing you have pictured.

    March 9, 2012 at 9:48 pm

    • That is a pretty high pH for conifers. I am lucky to have good water in this area (Lake Ontario) and have never had the need to monitor it.

      I like this ratty little tree too. It is my smallest cedar, but it has everything the big wild ones have. The top jin really reminds me of the tortured thuja that grow in hellish environments. I look forward to working on it.

      Thanks for the comment!

      March 9, 2012 at 10:24 pm

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