Recurring Issues With Thujas (Arborvitaes)
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Oldest needles undergo natural browning and shedding each year. Photo: Gene Fox, NCCES.
If you asked me what I got the most calls about as the Consumer Horticulture Agent in Brunswick County, I would undoubtedly answer Thujas commonly known as Arborvitaes (referred to as for the remainder of the article). Arborvitaes are a commonly planted landscape tree in Brunswick County. The tree is a quick growing evergreen that provides a natural screening effect. It is often planted closely together in rows on the perimeter of homeowners properties as a pseudo privacy fence, however these close plantings cause future problems.
Regardless of the season, our office constantly gets calls from concerned clients about browning Arborvitaes. Despite being an evergreen tree, a certain amount of browning is to be expected. This natural browning occurs annually and is referred to as internal browning or shading out. This seasonal needle drop is characterized by the older (innermost) needles and/or twigs yellowing, browning, and dropping at multiple heights on the tree. Environmental stressors can also contribute to needle drop and discoloration. Internal browning/shading out and environmental stress typically is not a major cause for concern overall health of the tree as long as the needles on the outer growth remain green indicating a healthy evergreen.
There are other reasons why an arborvitae might be dropping needles and browning to a concerning level. Arborvitaes are highly susceptible to insect and disease pests. Some of these pests are easy to identify while others require sampling for advanced analysis by a pathologist or entomologist. An easy to identify and common arborvitae pest that causes browning is bagworms. Bagworms are spindle-shaped silk bags covered with pieces of plant foliage hanging from twigs; when bagworms are numerous, much of the plant may be defoliated and may die. The bagworms can be mistaken for the fruiting structure of the arborvitae as well. 
An indicator of bagworms is browning and defoliation all over the tree without a noticeable pattern, along with coils of silken threads embedded below or in swelling branches.
Late May or early June is a perfect time to spray for bagworms as then the caterpillars are tiny and very susceptible to pesticides. At that time, use a pyrethroid such as permethrin or bifenthrin because pyrethroids have a longer residual life than most other insecticides. If the bagworms are first noticed in late summer, it is too late to do anything but physically remove the bags. By then they have already mated and females have already laid eggs inside their bags for next year’s generation. It’s best to use a very sharp knife or utility razor to cut the silk band that bagworms wrap around and around the twig just before they pupate inside. If you merely pull the bag to remove it, the silk band may slide along the twig and shear off the needles. If not removed, the silk bands sometimes girdle the twig, which causes it to eventually die and break off at that point. For specific recommendations on insecticides please contact the Brunswick County Extension office at (910)-253-2610.
As mentioned earlier, other arborvitae issues are a lot harder to identify. They often require a microscope or soil/tissue sample analysis by trained pathologists or entomologists. These issues include but are not limited to Armillaria root rot or Phytophthora root rot, spruce spider mites, juniper scales, or cankers. If you suspect your arborvitaes are suffering from one of these issues please contact the Brunswick County Extension office at (910)-253-2610.
Sources and additional information can be found at:
https://content.ces.ncsu.edu/bagworms
https://pdic.ces.ncsu.edu/decision-guide-for-cypress-problems/