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Scale insects: a formidable threat to your plants
The scale insect parasitizes a wide variety of indoor and outdoor plants, such as apple trees, lemon trees, and orchids. These insects measure between 0.5 and 10 mm and thrive in groups. They feed on plant sap and settle on all parts of their host: the trunk, branches, leaves, and young shoots.
Scale insects are found all over the world and comprise several thousand species. In France alone, there are 400 species, a quarter of which were introduced through trade. Very common—even invasive—they are considered pests and, in some cases, serious pests.
Among the best-known species are:
- the mealybugs, which attack green plants;
- armored scale insects, which prefer trees and shrubs;
- shield scale insects, which colonize trees, succulents, and cacti.
Some have a cottony appearance, while others produce toxic saliva. In any case, an infestation can quickly weaken a tree or plant.
Contributing factors
This pest develops most favorably in warm, confined and humid environments. The scale insect has numerous reproduction modes and cycles, sometimes even within the same species. It attacks indoor and greenhouse plants all year round. In the garden, its preferred period is from June to September.
How it works
Males can fly. They do not feed and survive only a few days, the time it takes to reproduce. Females live for several months, waiting to be fertilized. They remain attached to the plant and lay a large number of eggs, grouped together in cottony or waxy clusters. At their most advanced stage, the larvae move easily and can be transported by wind, insects or man. This is how the scale insect spreads.
Larvae and females feed by inserting their proboscis into the host plant. By pumping sap in this way, the mealybug injures the plant, depriving it of nutrients and weakening it.
Symptoms
Depending on the species, scale insects can be identified by:
· powdery clusters, brown shells, or sticky, transparent droplets on the affected plant;
· leaves that turn yellow prematurely and fall off;
· yellow spots appearing near the main vein of the leaf blades;
· foliage covered with a black, soot-like substance (this is a fungus, sooty mold, which inhibits photosynthesis);
· leaves that may be deformed, especially young shoots, due to the toxicity of the saliva;
· Slowed or even halted growth and flowering.
The insect itself can be white, brown, red, green, flat, round, or oval. These include:
· mealybugs, which are most commonly found on lemon and orange trees and orchids and have a soft body with a mealy or cottony appearance;
· Armored scale insects, which have a body protected by a fixed brown shell that is very difficult to remove. They are found on woody plants (trees and shrubs);
· shield scale insects, which also infest woody plants and have a waxy or downy shield that can be separated from their body.
Treatment
Natural treatments help target only the scale insect and spare beneficial insects.
Preventive treatment
It's essential to monitor your plants every time you water them: carefully inspect the stems, nodes, and leaves to detect the pest as early as possible. Airing out or taking your plants outside regularly during the warmer months helps prevent infestations.
Curative treatment
For plants with a mild infestation, remove the scale insects with a cotton swab or a cloth soaked in 90°C alcohol or soapy water. Then remove the waxy shells using a knife that isn't too sharp, sliding it between the shell and the leaf.
For a more intensive treatment:
· Mix one teaspoon of liquid black soap, one teaspoon of rubbing alcohol, and one teaspoon of vegetable oil. Spray this mixture onto the infested plant twice, 30 minutes apart, then once a week until the pests are gone;
· Place the plant out of direct sunlight so that the treatment does not damage the foliage.
Outdoors, you can:
· to rinse the less delicate plants with a water jet and brush the trunks to dislodge the scale insects;
· cut off infested branches and burn them;
· prevent infestations by setting up insect hotels and planting nectar-producing flowers to attract scale insect predators, such as ladybugs.
