|Mealybugs on citrus
|© A.M. Varela, icipe
Mealybugs damage plants by sucking sap from roots, tender leaves, petioles and fruit. They excrete honeydew on which sooty mould develops. Severely infested leaves turn yellow and gradually dry. Severe attack can result in shedding of leaves and inflorescences, reduced fruit setting and shedding of young fruit. The foliage and fruit may become covered with sticky honeydew, which serves as a medium for the growth of sooty moulds.
Honeydew, sooty mould and waxy deposits may cover leaves reducing photosynthetic efficiency and may lead to leaf drop. Contamination of fruit with honeydew and with sooty mould reduces its market value. The honeydew attracts ants, which collect the honey and protect indirectly mealybugs from natural enemies. Some mealybugs inject toxic substances while feeding causing deformation of the plant (e.g. the cassava mealybug). Some species transmit viruses (e.g. the pineapple mealybug).
The most important species of mealybugs and their major host crops in Africa are:
- The cassava mealybug (Phenacoccus manihoti) attacks cassava
- The citrus mealybug (Planococcus citri) attacks a wide range of crops such as cocoa, bananas, tobacco and coffee and wild trees such as Ceiba pentandra and Leucaena.
- The long-tailed mealybug (Pseudococcus longispinus) is widespread and common on many crops but it is usually not a serious pest. Major hosts plants of the long-tailed mealybug are citrus, taro, avocado, guava, eggplant and grapevine.
- The mango mealybugs (Rastrococcus iceryoides and R. invadens) have been reported on a number of economically important plants, but there are reports of economic damage only on mango and citrus
- The pineapple mealybug (Dysmicoccus brevipes) attacks pineapple, and other crops including avocado, banana, celery, citrus, clover, cocoa, coconut, coffee, custard apple, figs, ginger, guava, maize, mango, oil palm, orchids, groundnut, peppers, plantain, potato and sugarcane.
- The Kenya mealybug (Planococcus kenyae) attacks coffee and a large number of wild and cultivated plants including yam, pigeon pea, passion fruit, sugarcane and sweet potato
- The pink sugarcane mealybug (Saccharicoccus sacchari) is found primarily on sugarcane and its wild relatives (Saccharum spp.). It has been recorded occasionally on sorghum, rice and other grasses
- The striped mealybug (Ferrisia virgata). It is widespread and common on many crops but it is usually not a serious pest.
|Severe infestation of pineapple mealybugs on the fruit
|© Courtesy EcoPort (http://www.ecoport.org): Bedford ECG, de Villiers EA
Mealybug infestations of above-the ground plant parts start with the appearance of crawlers (the first-instar nymphs) on the underside of the leaves on terminal shoots, stems and other plant parts. Heavy mealybug attack appears as white, waxy masses of mealybugs on stems, fruits and along the veins on the underside of leaves. Heavy infestations usually result in coating of adjacent stems, leaves and fruits with honeydew and sooty mould. Severely infested plants may wilt due to sap depletion; leaves turn yellow, gradually dry and ultimately fall off. Feeding on fruit results in discoloured, bumpy, and scarred fruit, with low market value, or unacceptable for the fresh fruit market.
Mealybugs injecting toxic substance while feeding cause plant deformation. This is the case of the cassava mealybug; feeding of this mealybug on cassava plants causes stunting, leaf distortion, shortening of the internodes and loss, dieback and weakening of stems used for crop propagation.
Mealybugs attacking roots, as is the case of the citrus mealybug on coffee and the pineapple mealybug, cause stunted roots, rotting of roots and subsequent wilting of the plants. Roots of coffee plants attacked by the citrus mealybug are often encased in a thick case of greenish-white fungal tissue; if the fungal coat is pealed off, the white mealybugs can be seen.
Affected plant stages
Seedling stage, vegetative growing stage, flowering stage and fruiting stage.
Affected plant parts
Growing points, leaves, roots, stems and whole plant.
Growing points: deadheart, abnormal forms.
Leaves: abnormal colours, abnormal forms, abnormal leaf fall, wilting, yellowed or dead, honeydew or sooty mould.
Roots: reduced root system, fungal growth (coffee).
Stems: abnormal forms, abnormal growth, dieback.
Fruits: scarring, discolouration, honeydew, sooty mould.
Wholeplant: wilting, plant dead, dieback, dwarfing.