Scale insects: detection, prevention & control

Scale insects are unpleasant pests that need to be treated quickly and effectively. This article will show you how to combat these pests both naturally and chemically.

Scale bugs, just like other plant lice, absorb energy-rich plant sap which weakens the infested plant. Scale insects are often a major problem for indoor plants and to fully get rid of an infestation requires dedication and consistency.

Scale insects: an overview

The scale insect family (Coccoidea) is versatile and contains about 100 different species in Europe alone. Though the appearance of different scale insect sub-species can vary, there are general recognizable signs of an infestation. Scale insects like to infest woody plants and vines, but they are usually more dangerous for indoor plants. They prefer orchids, palms, fig trees, citrus trees, and oleanders as host plants.

Heavy infestations usually occur in winter and spring, when the plants are weak due to poor weather conditions. Scale insects only grow to be about 0.8 to 6 mm in size and multiply extremely quickly. They often hide at the bottom or beneath leaves and other hard-to-reach places on the plant. Honeydew is a reliable indicator of scale insect infestation: just like aphids, female scale insects also produce sugar-rich honeydew, which makes some leaves stick together. Signs of this plus spotting a few immobile female scale insects with their characteristic back shield you can be sure you are dealing with a scale insect infestation. Scale bugs often appear in great numbers of what may seem like entire colonies. Pustular formations are also indicative of scale insect presence.

Scale insects: a profile

Scale insects vary in appearance depending on their stage of development and sex. The most notable insects however are the older females which have a distinctive protective shield. Female scale insects are immobile from the second larval stage onwards and thus cannot escape predators, which is why this shield is so important for their survival. During this time the females are busy feeding on plant sap, laying eggs, and producing honeydew, a mixture of undigested sugar and water that has passed through the insect’s digestive system. Scale insects can reproduce through parthenogenesis, which means that female insects do not necessarily need male scale insects to reproduce, which is why these pests can reproduce so quickly.

Before young larvae hatch, scale bug eggs often lie protected near the mother or tucked underneath her. After they hatch, the larvae are mobile and can therefore look for a new place on the plant to feed on. From the second larval stage onwards, female larvae lose their mobility. Male scale insects, on the other hand, are always mobile and usually winged. Their sole purpose is to fertilise female scale insects, though the entire life span of a male scale insect is only a few days long.

Preventing scale insects

Because indoor plants such as orchids and palms are particularly susceptible to scale insect infestations, it is recommended to regularly search them for the suspicious shields of scale insects, especially in the winter. Most of the time, only the large female scale insects can be detected. The young larvae, which are less than one millimetre in size, are practically invisible to the human eye. Nevertheless, a closer look at newly acquired houseplants will not do any harm as infestations – once started – are hard to control.

Second to recognizing an infestation as early as possible, is the overall health of the plant. Scale bugs often reproduce rampantly on host plants that are weakened due to their location. The most common cause is lack of light in winter and spring. Light deficiency in combination with dry and overly warm air is stressful for most potted and indoor plants. Additionally, using a fertiliser with an excessive amount of nitrogen also will affect the plant’s vitality. All these factors can lead to a vulnerable plant that will struggle to defend itself against scale insects. Effective remedies can only be applied to a limited extent. In any case, nitrogen fertilisation should be reduced or completely avoided during the winter. If your indoor plants do not get enough light in winter, they should be kept as cool as possible. The cooler the temperature, the easier it is for the plant to adapt its metabolism to the poor light conditions. Colder temperatures also help slow down the development of scale insects.

Keep in mind: The plant should only be kept in the cold if it tolerates it!

Scale insect control

In most cases, scale infestation only becomes apparent once it is already severe. At this stage, trying to remove insects by hand is useless, some insects, the tiny larvae especially, will remain concealed and keep feeding and reproducing. The vicious cycle will only continue.

Natural treatment

Unfortunately, removing scale insects is only possible with specialised products. Natural solutions include oil preparations; natural treatments such as neem or pyrethrum do not work as poisons as scale insects’ shields protect the insect from the otherwise poisonous oil. Oil preparations (based on rapeseed oil for example), on the other hand, can combat scale insects effectively. The oil covers the insects with a thin film and suffocates them. Although these products are completely harmless to humans, the treated plants can react very sensitively to oil products. Before you apply the oil agent to the whole plant, test it on a small part of the leaves. If the plant does not react negatively and there is no damage, apply the solution to the rest of the plant. But remember that treated plants are more sensitive to light in the days following the treatment. Make sure to avoid putting them in direct sunlight.

Chemical treatment

If the oil preparation does not work on the infested plant, conventional products may be a good alternative. To find a proper pesticide, best consult a local garden centre to find a solution that best works in your area with as little environmental impact as possible.

Getting rid of scale insects with the help of other insects

Using beneficial insects is another method of removing scale insects. Different species of ichneumon fly, for example, can be used to control scale insects. Because beneficial insects have specific needs and target only individual scale insect species, they cannot be used to treat all scale insect species. Ultimately, identifying the exact species of scale insects is the basis for effective control. If you want to use beneficial insects to remove scale insects, consult an expert to determine the exact scale insect species beforehand. Often this service is offered by the distributor of the beneficial insects themselves.