The citrus mealybug (Planococcus citri) has a world-wide distribution and many and diverse host plants. In temperate regions of the world this mealybug poses a problem in greenhouse horticulture, and in the tropics and sub-tropics on outdoor crops. The insect causes damage especially in fruit trees and ornamental crops, particularly in pot plants such as ficus, palms, schefflera, croton and kalanchoë, but also in roses and gerbera. Additionally, the citrus mealybug (Planococcus citri) can also appear in cucumber, melon and aubergine.
Adult females of Planococcus citri are 2.5 - 4 mm long and 2 - 3 mm in breadth. Seen dorsally, they have an oval form; they are soft and covered with a fine waxy material. They move very little. They can be distinguished from other mealy bugs by their possession of 18 pairs of relatively short wax rods round the edge of the body, and two slightly longer ‘tail filaments’ . The tail filaments are always shorter than 20% of body length. They produce little wax, so that the light yellow to pink body is visible through its waxy covering. There is often a darker longitudinal stripe running over the body.
The adult males are short lived and can be hard to spot. They are smaller than the females, have two pairs of wings and two long tail filaments. Their sole task is to fertilize the females and as soon as a male emerges from its cocoon, it goes in search of a female. A fertilized female lays several hundreds of eggs in an elongated cottony egg sac composed of white waxy threads. Once the eggs are laid, the female shrivels up and dies. The first instar nymph develops from the egg. These first instar nymphs are known as ‘crawlers’. They are highly active in their search for a new feeding place and are capable of moving a reasonable distance over the plant. The male nymph attaches itself to the plant, whereas the females remain mobile throughout their entire development. After the second instar, a male nymph forms a dark brown ‘prepupa’ from which a pupa rapidly develops, inside a white cottony cocoon. Females undergo little changes in form, passing through a second and third instar after which they become sexually mature. Soon after becoming an adult the females start to release a sex pheromone to attract males. Males generally fly only in the early morning.
Although most species of mealybug feed on the aerial parts of the plant, some species extract their nourishment from roots, whilst others are gall-formers. A few species can also transmit harmful viruses.
Mealybugs inflict damage on the crop in various ways:
Nymphs and females extract the sap from the plant, stunting growth and causing deformation and / or yellowing of leaves, sometimes followed by defoliation. The overall effect is a reduction of photosynthesis and therefore the yield. Flowers and fruit often drop off.
Plant sap is rich in sugars, but low in proteins. In order to gain an adequate intake of protein, mealybugs must therefore ingest large quantities of sap, getting rid of the excess sugars in the form of honeydew. Characteristically, dark sooty moulds (Cladosporium spp.) are often found growing on this honeydew. Additionally, the white, waxy secretion of the mealybugs, reduces the ornamental value of the affected plants. Fruit and flowers are also fouled, rendering them unfit for sale, and the reduced level of photosynthesis in the leaves also reduces flower and fruit production.
In ornamental crops, the mere presence of mealybugs is sufficient to render the product unfit for sale. A very small population can thus cause considerable economic damage.