Photo 3. Cassava mealybug, Phenococcus marginatus, distorting terminal shoot of cassava. The leaves are short, distorted and bunched.
Widespread. Asia, Africa, South America. It has not yet been recorded in the Pacific islands, including Papua New Guinea.
Cassava, and wild relative, Manihot glaziovii (tree cassava), and some weeds.
The mealybug sucks plant sap, causing leaves to distort, fall, and stems to dieback (Photos 1-5). As the mealybugs feed, honeydew is expelled, falling onto leaves which become colonised by sooty mould fungi; these reduce the exchange of gases and photosynthesis, so weakening the plants further.
Females develop eggs without mating, and lay up to 500 on shoot tips, on the underside of leaves and on leaf stalks. The eggs hatch and the nymphs or 'crawlers' disperse to the top of the plant, settle, moult twice, before becoming adult (Photo 4). Adults are pink, covered in white wax secretions, 1.10-2.6 mm long and 0.5-1.40 mm wide.
Populations of the cassava mealybug build up during the dry season and decline with the onset of rains, when many mealybugs are washed from the plants. The life cycle last about 50 days, with temperatures of 28°C being optimal.
Spread occurs when crawlers move by themselves over short distances, or longer when carried by wind currents, vehicles, animals, birds, on clothing, and during exchanges or distributions of cuttings.
The impact of the mealybug in Africa has been considerable. One review said: "Cassava mealybug spread across the width of Africa in about 15 years. Its accidental introduction damaged a staple crop that is particularly important in times of drought, during a time of drought, leading to famine"1. Shoots became heavily infested resulting in low root yields. The loss of leaves was also important as they are a major leafy vegetable in Africa. In addition to the immediate damage, stunting resulted in weak cuttings for the next crop.
Estimation of losses caused by the mealybug (and the green spider mite, Mononychellus species) has been put at $2 billion a year, until both were brought under control using natural enemies.
Look at the undersides of leaves and the shoot tips for colonies of the mealybug. However, there are other species which are similar, and because of this identification by experts is essential.
Although widely distributed throughout Asia and Southeast Asia, Phenacoccus manihoti has not yet reached Papua New Guinea, or other Pacific island countries. Therefore, it is necessary to prevent this mealybug from extending its range. Biosecurity authorities need to be alert to the potential pathways that may take this pest across national borders, in particular the movement of cassava shoots for leafy vegetables, planting material, and people.
Natural enemies of the cassava mealybug include ladybird beetle generalists, e.g., Hyperaspis, Exochomus and Diomus species. There are also several specific parasitoid wasps, among which Apoanagyrus lopezi (Epidinocarsis lopezi) has been the most effective parasitoid controlling the cassava mealybug since its introduction to Africa and Asia.
Note, ants tend mealybugs for their honeydew. By doing so, they protect the meanlybugs from the activities of parasites and predators. To manage mealybugs, it is important to remove the ants, so that biological control can operate.
The release of the parasitoid Apoanagyrus lopezi has proved very successful wherever introduced against the cassava mealybug; once released, insecticides should be avoided. Their use can make the problem worse by destroying biocontrol agents, parasites and predators. If a pesticide is required, the following are recommended:
AUTHOR Grahame Jackson
Information from Herren HR, Neuenschwander P (1991) Biological control of cassava pests in Africa. Annual Review of Entomology 36:257-283; and CABI (2017) Phenacoccus manihoti (cassava mealybug) Crop Protection Compendium. (www.cabi.org/cpc); and from Cassava mealybug (Phenococcus manihoti) Plantwise Knowledge Bank. (http://www.plantwise.org/KnowledgeBank/Datasheet.aspx?dsid=40173). Photos 1-5 Kris Wyckhuys (CIAT) and Phanuwat Moonjuntha (Thai DoA). Photos 6&7 Georg Goergen, IITA-Benin.
Produced with support from the Australian Centre for International Agricultural Research under project PC/2010/090: Strengthening integrated crop management research in the Pacific Islands in support of sustainable intensification of high-value crop production, implemented by the University of Queensland and the Secretariat of the Pacific Community.
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