The cottony cushion scale (Icerya purchasi) is a troublesome pest of citrus plants. It is easily recognizable by its distinctive appearance, resembling a cotton ball. This mealybug infests citrus trees grown in specialized orchards and domestic settings, including potted ones. It must be carefully controlled by farmers since extensive infestations can jeopardize the plant’s survival and certainly affect the quality of production.
Let’s examine the characteristics and damages caused by the cottony cushion scale and how to eliminate it from trees.
Identification of the cottony cushion scale
Icerya purchasi is an insect belonging to the order Hemiptera, family Monophlebidae. It is one of the many species of mealybugs found in our country, even though it is of foreign origin. The cottony cushion scale variety is originally from Australia and was first reported in Italy in 1900, in Portici (NA).
Today, it is present throughout the country, although it prefers areas with a milder climate, particularly in the central-southern regions.
Description of the cottony cushion scale
The cottony cushion scale is a hermaphroditic species, meaning it is self-sufficient in terms of reproduction, and no male individuals are conventionally present.
As an adult, this mealybug measures between 10 to 15 mm in length, with a conical body shape resembling a seashell. The terminal part is reddish, while the rest of the body is whitish. In reality, the insect’s final part is the egg sac, very voluminous, waxy in texture, cottony in appearance, marked longitudinally by 16 distinct ridges. Due to these ridges, Icerya purchasi is also known as the grooved cottony cushion scale.
The young forms, called nymphs, are orange in color and are immediately covered by a abundant white waxy coating.
Biological Cycle of cottony cushion scales
The cottony cushion scale generally completes three generations in a year. At the end of winter, in February, the first generation begins with the production of several hundred eggs inside the egg sac. In early spring, the nymphs make their appearance, gradually leaving the egg sac to attach themselves to the vegetation, typically along the central vein on the underside of leaves. Following this first generation, two more generations appear, with nymphs emerging at the end of June and September, and in exceptionally warm years, there can even be a fourth generation.
cottony cushion scales overwinter on the host plant itself, at any stage of development.
Direct Damage Caused by cottony cushion scales
As the name suggests, cottony cushion scales primarily damage citrus trees, especially oranges, lemons, kumquats, mandarins, chinotto, grapefruits, and finger limes. Infestations primarily occur on the underside of leaves but can also affect the trunks and developing branches. In general, mealybugs prefer shaded parts of the tree. Direct damage is caused by sap removal, leading to leaf drop, branch desiccation, stunted growth, and compromised fruit production.
Indirect Damage
cottony cushion scales produce abundant honeydew, which provides an ideal substrate for sooty mold formation. This sooty mold can blacken the vegetation and fruit, further impairing the tree’s health due to inhibited photosynthesis. Therefore, young trees attacked by cottony cushion scales can easily perish.
Biological Control with Natural Predators
Cottony cushion scale parasitized by Rodolia
Fortunately, in Italy, there is a natural predator of the cottony cushion scale, known as the Rodolia Cardinalis ladybird. This beneficial insect was introduced to our country by the Institute of Agricultural Entomology at the University of Naples. It’s somewhat similar to what’s happening recently with the introduction of the samurai wasp, a natural enemy of the brown marmorated stink bug. Rodolia Cardinalis is highly prolific, adapts well to different environments, and exhibits excellent predatory activity. In orchards where both species are present, no further interventions are necessary as a natural balance is maintained. Biological control involves releasing Rodolia Cardinalis into the citrus grove during the summer. This is because both mealybug eggs and nymphs are present on the plants.
How to Eliminate cottony cushion scales
The number of plants infested by cottony cushion scales determines the type of intervention needed. The approach will differ depending on whether a few or many plants are affected.
On a Few Plants
Suppose you have 2-3 potted lemon trees on your balcony. Cottony cushion scales are not very mobile. Therefore, you can manually remove them. Use a soft-bristle brush or cotton soaked in alcohol for this purpose. Afterward, rinse the vegetation with potassium soap, which will wash away mealybug remnants, honeydew, and sooty mold.
If you prefer to avoid manual cleaning, you can directly wash the vegetation with potassium soap. In this case, take extra care to reach all mealybugs. Repeat the treatment every few days if you notice any remaining insects. You can find potassium soap in specialized stores.
On Many Plants
Let’s now understand how to eliminate cottony cushion scales in case of severe infestations on a large number of trees, such as in a substantial family orchard or a dedicated citrus grove. The recommendation is to use white mineral oil, a biological product that is non-toxic and highly effective against mealybugs. You can also purchase online, and it does not require a pesticide license. Usually, this treatment is carried out in the middle of winter when mealybugs are in a dormant state, and beneficial insects are not present. The low temperatures also prevent damage to the vegetation from the mineral oil. Following this, you can use potassium soap to thoroughly clean the vegetation of the pest.
