A little combination of red and green for my Christmas week blog post.
Many things can cause browning of arborvitae, from cultural and environmental stresses to pests and diseases. When the tips become brown in a somewhat scattered pattern on the tree, one possibility is the false spider mite, Pentamerismus erythreus. This brightly colored, orange-red mite is hard to see without magnification but has been found on a regular basis associated with the symptoms shown below.
- The false spider mites are closely related to spider mites but differ from them in several ways. They do not produce strands of webbing and their bodies are more flattened. The life cycle is quite similar and includes the egg, larva, protonymph, deutonymph, and adult males and females (see diagram). False spider mites overwinter as either adult females or eggs, depending on the species. There can be 4-6 generations per year.
- Typical mite life cycle (http://biocontrol.ucr.edu/photos/mite/lifecycle.gif)
This pest is rather slow moving, even when disturbed, and tends to remain in a small, limited area for an extended period of time. Feeding injury results from a piercing and sucking mouthpart. Initial symptoms appear as tiny yellowish spots at the feeding sites and after a while (symptoms appear more slowly than with spider mites) browning can occur that resembles winter injury or fungal tip blights. The distribution of browning on the plant will be less uniform than if the cause was winter injury.
Check for false spider mites on arborvitae with these symptoms using a hand lens or by firmly tapping on the affected branches over a white paper plate or similar surface to dislodge them. Samples can also be sent to your university plant diagnostic lab for confirmation. If this is the problem, and it’s severe enough that you need to treat the plant(s), recommendations include horticultural oil and insecticidal soaps. These products only work on pests actually present at the time of treatment so it’s important to check for the mites first! Do not apply if temperatures are at or above 85°F to prevent plant injury.