A BAsic Guide To Scale Insects

What are scale insects?

These are sap-sucking pests that appear as shell-like bumps on plants. They are often somewhat rounded or oval but not always. Different species can be white, black, orange or a colour that blends with the plant. Scale insects are closely related to aphids, whiteflies and mealybugs. Most scale insects are very small and almost always appear in clusters. They adhere to stems, branches and leaves of plants to feed and their shell-like bump appearance causes them to be mistaken for a plant part, fungal or bacterial disease. Unlike other pests, scale insects are immobile once they lock themselves on a feeding site.  There are more than 7,000 species of scale insects. Different species of scale favor different plants. There are two types of scale insects; armored (hard) scale insects and soft scale insects.

Armored scale insects secrete a hard protective covering over themselves which is not attached to the body. The hard scale lives and feeds under this spherical covering and does not move about the plant. These type of scale insects do not excrete honeydew. The hard shell serves as protection from predators and it also makes it difficult to kill these pests with pesticides. These types of scale are generally smaller than soft scale insects and are not raised and bumpy like soft scales. They are found mostly on lower leaf surfaces and stems forming thick crusts.

Soft scale insects secrete a waxy film that is part of the body. They vary in shape from flat to almost spherical.  Even though they are able to move short distances they rarely move. They are usually found on the underside of leaves and stems but some species may occur on upper leaf surfaces. They produce large amounts of honeydew.  Soft scale is easier to kill than armored scale.

The covering which protects the insect underneath it may be flattened oval, dome-shaped, oyster shell-shaped or have a fluffy coating. Female scales are usually immobile, wingless and usually with no visible legs or antennae while male scales look somewhat like tiny gnat but doesn’t have mouthparts and cannot feed.  Male scales are rarely seen. Some species of scale insects are hermaphroditic while others reproduce asexually. Scale insects feed on a wide range of host plants and are common pests of many indoor and outdoor plants including trees and shrubs. Most species, however, are restricted to particular host plants or plant groups.  Scale insects thrive in warm, dry environments.

Lifecycle

Female scale insects lay eggs under their bodies which hatch into the first instars called crawlers. They lay between 50-2000 eggs or live young depending on the species.  The eggs hatch in 1-3 weeks. Crawlers have legs and are mobile. They move around searching for a favourable feeding site then they settle and begin producing their scale coverings. The nymphs go through three instars. Once female scales become adults they are covered by a waxy scale covering which adheres tightly to their body and cannot be separated from it. Female soft scales don’t lose their antenna and legs completely but are greatly reduced which restricts their movement.  For hard scales, females lose their legs and antenna during the first molt. The legless adult female remains under the scale

How does scale insect damage look like?

Infestations can build up unnoticed before plants begin to show damage. The large amounts of honeydew produced by the soft scale brings about black sooty mould which blackens leaves thus affecting photosynthesis. Hard scales inject toxins in the plant while feeding killing leaf tissue cells around the feeding site to die, causing a yellow or brown ring. Heavy feeding can disrupt nutrient flow enough to cause premature leaf drop or branches to die.  Large populations may result in poor growth, reduced vigour and yellowed leaves. An infested plant may become so weak that it dies.

Control and prevention

Adult scale insects are firmly attached to their host plants and covered by their protective coating making them difficult to eradicate. Control measures are most effective during the crawler stage because the nymphs do not have a protective covering. This is the time when they can be effectively killed with pesticides. Scale insects can be combated in a number of ways.  Since scale are difficult to eradicate some methods may need to be used together to get rid of them effectively.

1.   When scale numbers are low they may be rubbed or picked off plants by hand.
2.   Prune infected plant parts. Regular scouting of plants will help to catch the infestation while it is still light. After pruning, inspect the plant and adjacent plants to ensure that all affected parts have been removed.  The pruned parts should be thrown I n the trash or burned, do not compost them.
3.   If possible, affected plants should be isolated to prevent the pest from spreading to healthy plants.
4.   Companion planting, especially with flowering plants helps to attract the natural predators of scale insects such as ladybirds and lacewings, which attack the crawlers.
5.   Controlling the ants which usually accompany scale infestations.  These ants feed on the honeydew that soft scale produce and protect the bugs from their enemies to ensure this food supply. Ants can be controlled by either destroying their nests or by excluding ants from the plant by applying a grease barrier around the stems or trunks of the plants.
6.   Spraying using insecticidal soap or soapy solution made from organic liquid soap.

Mix 1 tablespoon of organic liquid soap with 1 litre of water. Apply on the infested plants thoroughly. Aim to spray directly on the pests. Spraying will need to be repeated every 6-7 days to eradicate scale insects. About 3 treatments should be enough to control an infestation. Do not use washing detergents.

7.       Neem oil works really well against scale insects because it kills both adults and nymphs. It disrupts the growth and development of scale insects. Being an oil, it also smothers the pests. Neem oil can be applied in two ways, either as a foliar spray or as soil drench which then makes it a systemic insecticide.