Scale Insects on Ornamental Trees & Shrubs

Fact Sheet - IP4

1100 W. Moana Ln., Reno, NV 89509 11301 So. Virginia St., Reno, NV 89511

Key Points:

· Scale insects are often not recognized as insects.

· Scale insects typically reduce vigor and aesthetics but may also kill a plant.

· Two kinds of scale common in our area are armored and soft scale. Cottony scale is less prevalent.

· Soft scale will produce large quantities of honeydew similar to aphids.

· The most vulnerable stage of the scale insect’s life cycle is the crawler stage immediately after egg hatch. This is an active non-armored stage.

Identification

Scale insects are sucking insects which can be serious pests on all woody plants and shrubs. Adult female scales do not move and are hidden under a hard shell or waxy coating. They will occur on twigs, leaves, branches and fruit. Damage is a decline in vigor and a reduction in the aesthetics of the affected plant. Armored scales are less than 1/8 inch in diameter and have a plate like cover which is quite dense and provides a good protection for the insect inside. Soft scales may be smooth or cottony and are larger and more round. Armored scales hatch from eggs, settle down, lose their legs and form a hard cover over themselves that is separate from their bodies. Soft scales are able to move, but slowly. They also excrete large quantities of honeydew.

There are several types of scale in our area with Pine Needle Scale being the most serious and damaging species to conifers. This pest feeds on needles of most species of pine, spruce, and fir. During heavy infestations the needles may be nearly covered with scale giving an appearance that the plant has been spattered with white paint. These needles may drop prematurely and continued outbreaks could cause dieback of the trees.

Newly hatched eggs are called crawlers and resemble very tiny rosy-purple aphids. This is their only active period. Crawlers move about for just a few days before settling down and producing the golden brown shell or armor. One or two generations may be produced, depending on the season and elevation.

Striped-Pine Scale and Pinon Needle Scale especially will cause problems with our native Pinon Pines. The developing nymphs of Pinon Needle Scale resemble small black beans that are attached to the needles over the winter. As they mature in spring the adults lay cottony masses of eggs in March or April.

The most common insect pest of our woody plants is Oystershell Scale. This pest will attack more than 50 plant species, especially ash, cotoneaster, dogwood, lilac, poplar, and willow. Oystershell Scale outbreaks can even kill a plant when infestations are high. The adult mother scale that overwinters attached to the bark is about 1/8 inch long and is grey and brown banded. The shell resembles the general shape of an oystershell.

Eggs from all scale insects do not hatch at the same time and hatches may last over a period of two weeks. Oystershell Scale may produce two generations, the first being in late May thru June and the second hatch in July and August. Oystershell crawlers are pale and smaller than a pinhead. The crawler stage is the only mobile one and crawlers can find a suitable location in a shady area of the plant in only a few hours. Within a week they are covered with a waxy scale.

Controls

Pine Needle Scale has several natural enemies. The minute lady beetle is commonly found feeding on eggs and crawlers. Chalcid wasps are also commonly found. Pine Needle Scale is more readily controlled with horticultural oils than other armored scales such as Oystershell. Dormant oil should be used during the dormant period for pines. The next best control is during the crawler stage with insecticides such as Malathion. Some insecticidal soaps may also be used during the crawler stage and may be safer on spruce. Systemic insecticides can be effective for soft scale. Monitor the trees starting late April to late May for the appearance of the crawlers. Repeat applications of insecticides, especially soaps, may be necessary if several hatches are observed.

Dormant oil is the most effective control for armored scale during the dormant season. The use of the higher refined horticultural oils such as Ultra Fine oil for all scale control can be used all year round. Monitoring for crawlers begin with weekly exams in mid May. Shake an infested branch over a sheet of paper. Watch for crawling dust. A small branch can be put in a closed jar and monitored on the window sill, hatches outside should occur within a few days.

All Forms of Control

Cultural: Provide good cultural care and conditions for plant growth, especially proper irrigation, pruning heavy infestations out and pruning to open canopies to expose scale to heat.

Biological: The use of natural enemies. Allow these natural enemies to build up to control scale. If needed, spray to slow scale buildup using soap sprays.

Chemical: Dormant season application of supreme or Ultra Fine oils are effective against most scale pests of ornamentals, especially San Jose Scale but not against Oystershell Scale. Regular dormant oils can be used but only following the label very closely and during the dormant season. An application of oil or soap alone is usually adequate; the addition of insecticides such as Malathion does little to help effectiveness. Use of horticultural oil varies between dormant and spring/summer periods and scale pest. Be sure to study the label carefully, understand it, and then follow it.