Scale Insects

Scale on Areca

Scale on Citrus leaf

Scale on Citrus leaf

Severe scale infection

Scale on Citrus fruit

Common name: Scale/ Soft bodied scale/ Hard bodied scale

Super Family: Coccoidea

Physical description

Scale insects are small sap-sucking insects that secrete and are covered/ protected by a hard, waxy shell-like covering. Scale insects are divided into two groups depending on the hardness of the scale (hard and soft-bodied scale). The scales range in size from 1mm up to about 8mm and are normally light or dark brown in color. The shape of the scale depends on the species, it is generally rounded, but can also be ovoid in shape.

They are often found tightly packed/ clustered along the veins or mid-rib of leaves (upper and lower surface) and up stems as well. Insects will normally pick sheltered, slightly dark areas to breed in out of sight of predators. They are distantly related to Aphids and as with Aphids, once their mouthparts are extended into the plant they are anchored in place and cannot move, they will then secrete this waxy covering for protection (predators and desiccation). The small, immature insects are able to move short distances before feeding, so populations can develop quickly across the plant.

Description of damage

Scale insects feed off of the plant sap in the leaves of the plant, depending on the size of the insect population, this can cause yellowing of the leaves, stunting, and even disfigurement of buds or terminal foliage. Generally, yellow patches will develop on the leaves above where the insect is feeding as it removes plant sap and fluids from the leaf. In extreme cases, the plant will shed heavily affected leaves, which can cause the death of the plant - particularly small ones.

Life cycle description

Gregarious, large populations build up very quickly in sheltered, warm, humid conditions. A single female insect can produce tens or hundreds of eggs under the protection of her scale, these hatch and the "crawlers" are able to move to a new location before maturing into an adult. Most Sale insects will be female they only produce a limited amount of (winged and legged) males - the males don't feed, and their only purpose is to breed with the females.

Plants affected

Scale insects are found on both indoor plants as well as outdoors where breeding conditions are favorable. They will generally attack shrubs (not seasonal plants), and can also occasionally be found on trees.

General comments

As with Aphids, the excrement of the Scale insect is sometimes called “honeydew”, this is excreted from the rear of the body; ants are sometimes found in attendance and remove the honeydew due to its high sugar content. This is a symbiotic relationship, the ants tend and protect the Scale, and in return, they receive the honeydew. If the honeydew is uncollected it can form pools around the insect, or fall onto the leaves below where they are feeding, this then acts as a substrate for fungus or mold to grow on. This secondary condition of a fungus/ mold growing on the leaves below the Scale can also harm the plant as this blocks out light to the leaves and inhibits photosynthesis.

Control methods - Natural

The use of mineral plant oils (such as Neem) can be used to control the population, this will normally kill off any immature insects that are on the plant (through suffocation), but has limited effectiveness on an adult which is protected under their scale. Encourage natural predators such as Ladybugs if possible.

Control methods - Cultural

Outdoors - Elective pruning to open the plant canopy and allow air movement (and predators) into the center of the plants.

Control methods - Chemical

Systemic and contact insecticide products are very effective, but should never be used on edible crops.