SCALE INSECTS ON OUTDOOR ORNAMENTALS

Oyster shell scale (armoured scale) http://wlapwww.gov.bc.ca/vir/pp/ipm/insects/oyster.html

There are two groups of scales that are a concern on ornamentals: armoured scales (Family: Diaspididae) and soft scales (Family: Coccidae). Armoured scales are the most common scale found on trees and shrubs. The females are small, soft bodied and are concealed under a scale covering that is usually free from the body of the insect beneath. The scale is formed of wax, secreted by the insect, along with cast skins of its early instars. Armoured scales vary in length and diameter from 1-3mm. The body shapes vary with species - circular, elongate or pear shaped.

Overwintering females of armoured scales lay their eggs under their scale. The first instar larvae or "crawlers" which hatch from these eggs, are active and can travel quite a distance without any food. This is how the scale insect disperses itself throughout a tree. "Crawlers" can also be windblown to adjacent trees. Once a suitable location is found, the crawlers settle down, insert their mouthparts into the host plant to feed, and begin to produce their scale covering. The females remain sessile for the remainder of their life, whereas the males are winged.

Unarmoured or soft scales have a life cycle similar to that of the armoured scale described above. The difference between the two scales is that the unarmoured female scale is flattened, elongate and oval, that can be exposed or enclosed in a waxy or cotton-like secretion.

CONTROL

Low populations of scales are usually kept in check by birds, other insects and natural predators. Pesticide application may be required, however, to control high populations. High populations of scales may occur if excess nitrogen is available to the plant, or when the ornamentals are under environmental stress such as too much or too little water. Prune out severely infested limbs. Low toxicity pesticides registered in Ontario for the control of scales on outdoor ornamentals are available in most garden centres. However, insecticides used to control "crawlers" also kill the scales predators and parasites but not the adult scales whose number then increase rapidly.

There are two strategies for the control of scale insects. The first involves the use of dormant oil or dormant lime sulphur in the spring, usually late April, against overwintering, adult scales. Applications should be made before leaf buds break, when cold nights are not expected. Applications should also be carried out in the morning so the spray has a chance to dry quickly. Some ornamentals such as Japanese maple, sugar maple, white pine, and yews are sensitive to oils and should not be sprayed. Always check the label of the product for listings of sensitive plant species.

The second strategy is the use of insecticides, during the growing season, against the newly hatched larvae or "crawlers" as they spread throughout the tree. Correct timing of the pesticide spray is essential for each particular species. Usually 1-3 applications are needed to control the crawlers, since hatch of the crawlers is spread over a period of time. Spraying around the time of hatch and every 7-10 days thereafter until 3 applications are made, usually give effective control. Since adult scales are protected by their waxy covering, pesticides are relatively ineffective against them.

The following chart will help to identify and control scales on ornamentals in your yard:

ARMOURED SCALES

SOFT OR UNARMOURED SCALES

For further control measures refer to Ontario Ministry of Agriculture and Food (OMAF) Publication #64 "The Gardener’s Handbook". This publication can be ordered through your local OMAF office or through the OMAF website.

When using pesticides, please read and follow all label instructions regarding rates and timing and methods of application. Also, you should take notice of any mention of plant species that would be sensitive to the sprays.

L. Dennie

PDCF-064 (Revised 2003/11/07)