Question
I live in NE PA and had several Emerald Arborvitae planted last Aug.  I watered them thru Sept. and then stopped for the winter. I did not fertilize at that time.  They now have brown patches.  These are not just at the bottom so I've discounted dogs urinating. What can I do to rectify? I have another group about thirty feet from these that do not have any signs of this,however these are about three years old.  There are trees in the neighborhood that also have brown patches.  Can this be attributed to insects?  

Answer
Diseases usually do not infect these often, There area couple of insects that can cause this type damage. These are bagworms and spider mites.
Bagworms live in a cone shaped bag and will eat the needles making the tree look like the needles have turned. You should see the bags on the twigs and/or foliage.
Spider mites are tiny and are not as easy to see. You can take a white piece of paper and hold it under a "brown" limb and shake the limb. Spider mites should fall on the paper and look like tiny reddish dots that move.

These are bagworms. Bagworms are caterpillars that make distinctive spindle-shaped bags on a variety of trees and shrubs throughout the  USA. They attack both deciduous trees and evergreens, but are especially damaging to juniper, arborvitae, spruce, pine and cedar. Large populations of bagworms can strip plants of their foliage and eventually cause them to die. Infestations often go unnoticed because people mistake the protective bags for pine cones or other plant structures.

Bagworms pass the winter as eggs inside the bag that contained the previous year抯 female. In mid to late May the eggs hatch, and the tiny larvae crawl out from the end of the bag in search of food. By using silk and bits of plant material, they soon construct a small bag around their hind part that looks like a tiny, upright ice cream cone. As the larvae continue to feed and grow, they enlarge the bag enabling them to withdraw into it when disturbed. Older larvae strip evergreens of their needles and consume whole leaves of susceptible deciduous species, leaving only the larger veins. The bag is ornamented with bits of whatever type of vegetation they are feeding upon.
By early fall, the bags reach their maximum size of 1-1/2 to 2 inches. At this time the larvae permanently suspend their bags (pointing downward) from twigs, and transform into the pupa or resting stage before becoming an adult.

If only a few small trees or shrubs are infested, picking the bags off by hand and disposing of them may afford satisfactory control. This approach is most effective during fall, winter or early spring before the eggs have hatched.
When many small bagworms are infesting evergreens, an insecticide may be needed to prevent serious damage. The best time to apply an insecticide is while the larvae are still small (less than 1/2-inch long). Small larvae are more vulnerable to insecticides, and inflict less damage. Carefully inspect susceptible landscape plants, especially evergreens, for last year抯 bags. Young bagworms are harder to see; look closely for the small, upright bags which have the appearance of tiny ice cream cones constructed of bits of plant material. Preventive treatment is often justified on plants that were heavily infested with bagworms the previous year.



Several products are available for homeowner and professional use. For homeowners, conventional insecticides such as Sevin, Dursban, diazinon, and malathion, or the microbial insecticide Bacillus thuringiensis (BT) provide satisfactory results. The BT products have very low mammalian toxicities, but are only effective against younger larvae. If large bagworms are present (more than about 3/4-inch long), a conventional insecticide probably will provide better results.
Check with your local nursery for these insecticides. Check the adjacent plants and if they also have bagworms spray them also.  


Spruce Spider Mite: Mites are not insects but are more closely related to spiders. Spruce spider mites (Oligonychus ununguis) are occasional pests of arborvitae. They are very small and not seen easily with the naked eye. They have piercing mouthparts that they use to suck plant sap. Their feeding results in speckling (formation of tiny yellow spots) on needles. Some needles may turn brown and drop off usually in the lower limbs.. With heavy infestations, fine webbing may be seen on the plant. Several seasons of heavy mite feeding may kill an arborvitae. Although most spider mites increase in numbers during hot, dry weather, spruce spider mites are cool-weather mites. Their populations peak during spring and fall, but drop dramatically during the heat of summer when predators feed on them.

To determine whether insecticide use is needed, it helps to know how many mites are present. Hold a white sheet of paper under a branch and strike the branch. The mites that are knocked off will be seen crawling around on the paper. If dozens of mites are seen per whack, serious damage can result. Continue to check population numbers at 7- to 10- day intervals. Pesticides labeled for homeowner use against spruce spider mite include insecticidal soaps and acephate + fenbutin oxide (Ortho Systemic Insect Killer or Ortho Orthenex Garden Insect & Disease Control Concentrate). Completely cover the foliage with the spray. As with any pesticide, read and follow all label directions and precautions before using.