Mealybugs: a fearsome pest for your plants

Scale insects parasitize a wide range of indoor and outdoor plants such as apple trees, lemon trees and orchids. These insects measure between 0.5 and 10 mm and live in groups. They feed on plant sap and settle on all parts of their host: trunks, branches, leaves and young shoots.

The cochineal is found all over the world, and comes in several thousand species. In France alone, there are 400 species, a quarter of which are commercially imported. Very common, even invasive, it is considered a pest.

The best-known species include :

mealybugs, which attack green plants;
carapace scale, which prefers trees and shrubs;
shield mealybugs, which colonize trees, succulents and cacti.

Some have a cottony appearance, while others have a toxic saliva. In all cases, an invasion can rapidly weaken the 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 seen in :

- mealy clusters, brown hulls or sticky, transparent drops on the affected plant ;

- early yellowing and leaf drop;

- yellow spots appearing near the midribs of leaf blades;

- foliage covered with a black, sooty substance (caused by a fungus, fumaginia, which prevents photosynthesis);

- leaves that may be deformed, especially young shoots, due to the toxicity of the saliva;

- growth and flowering may be slowed or even stopped.

The insect itself can be white, brown, red, green, flat, round or oval. We can distinguish :

- mealybugs, most often found on lemon, orange and orchid trees, with a soft body and a mealy or cottony appearance;

- carapace scale, whose body is protected by an attached brown carapace that is very difficult to separate. They are found on woody varieties (trees and shrubs);

- shield mealybugs, which also invade woody species and have a waxy or downy shield that can be dissociated from their body.

Treatment

Natural treatments help to target only the mealybug and spare auxiliary insects.

Preventive treatment

It's essential to monitor your crops every time you water them: observe stems, nodes and leaves carefully to detect the pest as early as possible. Aeration or taking plants outside regularly during the summer prevents infestations.

Curative treatment

For lightly invaded plants, remove scale insects with a cotton bud or cloth soaked in 90°C alcohol or soapy water. Then remove the waxy hulls with a not-too-sharp knife, passing it between the hull and the leaf.

For a more intensive treatment :

- mix a teaspoon of liquid black soap, a teaspoon of methylated spirits and a teaspoon of vegetable oil. Spray this mixture on the infested plant twice at 30-minute intervals, then once a week until the parasites have disappeared;

- place the plant out of direct sunlight to avoid damaging the foliage.

Outdoors, you can :

- spray less fragile plants and brush trunks to dislodge scale insects;

- cut off infested branches and burn them;

- prevent invasions by installing insect hotels and melliferous flowers to attract mealybug predators such as ladybugs.