May 9, 2002
Clip 'N Keep:
Two common pests of evergreens

By Jennifer Hobson, OMAFRA Nursery Crops Specialist, Horticultural Science Division, University of Guelph

In 1999, we had an outbreak in populations of two major pests of evergreens: spruce spider mite and cedar leaf miner. These high pest populations, coupled with the hot, dry spring conditions, proved to be very injurious to many landscape evergreens. With the moderate winter temperatures we've been experiencing, we can expect quite a few of these pests to have overwintered successfully.

     Spruce spider mite (Oligonychus ununguis) is a very tiny mite (about 0.5 mm in length) that feeds on the foliage of many evergreens including Picea, Thuja, Tsuga, Juniperus, Pinus, Pseudotsuga, Abies, Larix and others. This mite overwinters as a tiny, reddish egg within the bud scales or underneath webbing on the foliage. The eggs hatch into small, red immature mites as the Acer platanoides is in early bloom. These larvae mature in about three to six days and the entire life cycle is completed every two to three weeks. They can produce about three generations per year. Both the immature and adult mites have piercing, sucking mouthparts that destroy plant cells on the surface, resulting in stippling and bronzing of the foliage (Figure 2-Thuja occidentalis). The oldest foliage at the bottom and inside of the plant is often the most severely damaged (Figure 1-Picea glauca 'Conica').

     Spruce spider mites can be found actively feeding during cool temperatures, in spring and again in the fall. Since they are difficult to see with the eye, a stereoscope or hand lens is often required for proper identification. Look closely for thin webbing, the tiny reddish eggs (or empty egg cases) (Figure 3) and of course, the fast-running reddish (immature) or greenish (adult) mites. To monitor for mites in the field, hold a white sheet of paper inside the plant's foliage and shake the above branches over the paper. Look for tiny specks motoring across the paper or try wiping your hand along the surface to see if any red streaks appear, a sure sign of the immature stage.

     Where populations are high, a pesticide application may be warranted. Newly hatched mites and adults are susceptible to chemical sprays whereas the eggs are resistant to many miticides and extreme environmental conditions. A dormant oil application just prior to bud break will help to reduce populations of this mite (avoid dormant oil applications on blue spruce). Subsequent applications of miticides, at 10-day intervals, may be required to knock down populations in mid to late spring. Be sure to coat all foliar surfaces since mites feed from protected crevices. Deep watering of the root zone and light fertilizing will help the plant to recover from feeding damage. Remember, there are many natural predator mites that feed on spruce spider mites, where pesticides are not being used.

     There are a few species of cedar leaf miner that can be found here in Ontario, the most common being Argyresthia thuiella. These leaf miners attack Thuja exclusively. The cedar leaf miner overwinters as a larva inside the leaf scales. This larva is yellow or green and feeds from the interior of the leaf scale in late winter/early spring. They pupate in mid to late spring, for several weeks. The adult moths are gray and white; about eight mm long and begin to appear around the foliage in mid to late June. When populations are high, these moths are very noticeable around cedar foliage, as they are easily disturbed. They lay their eggs on foliar tips of one- and two-year-old twigs. The newly hatched larvae bore into the leaf scales and feed until fall freeze-up.

     The larval stage of this moth can be found mining out the pith of foliar tips in the spring, and again in the fall. This causes browning of the foliar tips, similar to winter injury (Figure 4-Thuja occidentalis). The foliage damaged by the leaf miner is hollowed-out and easily crumbles between your fingers, whereas winter-injured foliage does not. Also, cedar leaf miner damage is denoted by a sharp margin between healthy green tissue and dead brown tissue (Figure 5). Most of the damage is caused by the older larvae found in the early spring. To monitor for cedar leaf miner larvae, gently tear the leaf scale along the margin between the green and brown tissue. Look for a tiny green or yellow larva (or pupa) in this area during the spring.

     Where cedar leaf miner populations are high and damage is significant, there are a few options for control. Populations of overwintering larvae can be reduced by shearing the infested tips (plus collecting and destroying those tips) before June. Some larval control can also be achieved through an application (e.g. soil drench) of dimethoate in late April/early May. Where adult populations are extremely high, an application of malathion or diazinon during peak flight may also aid in control. It should be noted that an established cedar might withstand up to 80 per cent foliar damage and still survive. Many parasitic wasps help keep populations in check where insecticides are not being used. Since insect injury is more pronounced in dry years, deep watering of the root systems and some light fertilizing can help most trees recover from pest damage.

     For specific pest control recommendations, consult OMAFRA publication 383, Nursery and Landscape Plant Production for 2000 ($12.00). (To order a copy, call 1-888-466-2372.)

Ms. Hobson can be reached at 519-824-4120 ext 2671, fax 519-767-0755, e-mail, nursery.html, or call the Nursery-Landscape Hotline at 1-888-290-4441.