by Lee Townsend, Extension Specialist
University of Kentucky College of Agriculture
Several species of soft scales, referred to as Lecanium scales feed on sap from shade trees and woody ornamentals. They are more or less hemispherical or turtle-shaped, brown, and range in size from 1/8 inch to 1/6 inch. While there are several very similar species, all have one generation a year and a general set of recommendations can be used to manage them.
Lecanium scales spend the winter on twigs and branches in an immature or nymphal stage. Development resumes in the spring and mature females produce large numbers of eggs which are protected by their soft waxy covering. Crawlers that hatch from these eggs move to leaves, settle, and feed on sap during the rest of the summer. They move back to twigs and branches prior to leaf drop and settle for the winter.
As with many species of soft scale, these insects produce large volumes of a liquid waste called "honeydew". Deposits of this sugar-rich material gives leaves a shiny appearance. Limbs of heavily infested trees may be blackened by the growth of sooty mold fungus.
Scale control can be challenging and may need to be repeated over several seasons. Proper timing of insecticide applications is a major key to success. Applications must target newly hatched scale crawlers which are active in June and July. They are very susceptible to control measures while moving over plant surfaces to find a feeding spot. Once settled, they begin to secrete a waxy covering that shields them from sprays.
Alternatives for crawler control
• Cultural control
Scales tend to thrive on stressed plants. Following a recommended fertility program and watering regime will promote plant health. However, over-fertilization favors scale buildup. If practical, improve plant sites to reduce stress and promote growth. Severely prune back heavily infested branches and protect new growth with insecticide applications.
• Insecticidal Sprays
Horticultural oils kill by suffocation or after penetrating over-wintering stages of the insect. Consequently, they may not be effective where several layers of scale coverings have accumulated.
Dormant oils are typically applied during February or March but may not be very effective against armored scales. Highly refined supreme, superior, or summer oils can be used on many trees and shrubs during the growing season. Read the product label for guidelines on plant sensitivity and temperature restriction before buying and using these products.
Insecticidal soaps are long chain fatty acids that kill susceptible insects through direct contact. Like horticultural oils, they require thorough coverage. Soaps leave no residue so repeated applications may be needed for some pests. These products may burn the foliage of sensitive plants, such as Japanese maple, so check the label for information about the plant species that you intend to treat.
A variety of natural and synthetic insecticides are labeled for use as sprays to control scale crawlers on landscape trees and shrubs. While the residual life of these products is generally longer than oils and soaps, timing, coverage, and precautions on damage to some plant species are very similar to those for oils and soaps.
• Systemic insecticides
Imidacloprid (Bayer Advanced Garden Tree & Shrub Insect Control Concentrate) is applied as a drench around the root zone of infested plants. This water soluble insecticide is taken up by the roots and transported throughout the plant where it is ingested by sap feeding insects. This provides a means of scale control without reliance on sprays. However, it may need to be applied several weeks before crawlers are active for best results.
|Representative products for scale crawler control.|
|Insecticide common name*||Representative brand names|
|Acephate||Orthene Turf, Tree & Ornamental Spray
Ortho Systemic Insect Killer
Gordon's Garden Guard Liquid Insecticide
|Cyfluthrin||Bayer Advanced Garden Multi-Insect Killer Concentrate|
|Lambda-cyhalothrin||Spectracide® Triazicide® Soil & Turf Insect Killer|
|Dimethoate||Dragon Cygon 2E Systemic Insecticide|
|Esfenvalerate||Ortho Bug-B-Gon Garden & Landscape Insect Killer Concentrate|
|Malathion||Ortho Mosquito-B-Gon Tree & Shrub Spray
Bonide Malathion Insect Control
|Permethrin||Ortho Mosquito-B-Gon Tree, Shrub & Lawn Spray
Spectracide® Bug Stop® Multi-Purpose Insect Control Concentrate
Bonide Borer-Miner Killer
|*All insecticides have unique common names that can be found just below the brand name on the product label. You may be able to find other brand name products for scale control that contain these active ingredients. Be sure that the product you select is labeled for the plants that you intend to spray.|
The success or failure of control efforts may not be readily apparent but here are some things to check.
- Dead soft scales often fall off of the plant. Live scales should produce a liquid when mashed, dead scales will be dry and not "bleed" when crushed.
- New foliage of infested plants should have a healthier appearance once the scale burden has been removed. Buds should break a little earlier than when the plant was infested and expanded leaves should have normal color and turgor.
- Sooty mold and shiny leaves should gradually disappear from plants that were infested with soft scales.
Scale insects can be attacked by a variety of lady beetles, predatory mites, and small parasitic wasps. Lady beetle adults and larvae can be seen but mites and parasitic wasps are very difficult to see. You can conserve natural enemies by using insecticidal soaps and oils which have limited impact on beneficial species in comparison to other control alternatives.
CAUTION! Pesticide recommendations in this publication are registered for use in Kentucky, USA ONLY! The use of some products may not be legal in your state or country. Please check with your local county agent or regulatory official before using any pesticide mentioned in this publication.
Of course, ALWAYS READ AND FOLLOW LABEL DIRECTIONS FOR SAFE USE OF ANY PESTICIDE!