An insect pest called Phenacoccus manihoti by experts and better known as the mealybug, although not native to many parts of Africa, is now doing very well there.
In just 10 years since the mealybug was first discovered there, the bug has spread over hundreds of thousands of square miles of Central and West Africa and today proves a serious threat to this area's most important food crop - cassava, also known as manioc, and the source of tapioca. It is the staple food for an estimated 200 million people in Africa.
Heavily infested fields appear to have been damaged by fire. The leaves of the cassava plants are bunched and stems are stunted - worse, the edible roots, normally two to three feet long, are the size of carrots.
Mealybugs - so-called because of their mealy or waxy secretions covering a soft, oval, wingless body 3 to 4 millimeters long - occur indigenously on all major landmasses except Antarctica, and on most oceanic islands as well. But until now the presence of mealybugs in the tropics has been rare; their normal habitat has been Europe, North America, and temperate parts of Asia.
In most parts of the world, they are kept under control by insect predators and other natural enemies. Apparently there are none of these in Africa. This helps explain the mealybug's rapid spread through Zaire, Angola, and Congo Brazzaville in Central Africa and Togo, Benin, and Nigeria in West Africa. Most recently it has been identified in Senegal and The Gambia.
No one really knows how much damage it inflicts. Cassava has not normally been included in official surveys. Scientific observers report that, in many devastated fields, damage reaches 80 to 90 percent. In fields under the control of national agricultural programs, 50 percent food losses are widely acknowledged. The potential for destruction - unless it can be controlled - is clearly great for an estimated 12 million acres of this food crop.
The cassava mealybug first caught scientists' attention when experts from the International Institute of Tropical Agriculture (IITA) in Nigeria were asked by the Zairian government to investigate cassava blight. They found that the disease was due to a bacteria, but they also found an incidence, over 90 percent in some fields, of unrelated symptoms. They found large colonies of cassava mealybugs. There also were reports of an insect problem in the Republic of Congo , and the bug had almost certainly reached northern Angola.
In both Nigeria and Zaire, cassava germ plasm is being evaluated for mealybug resistance. Experiments are also being carried out to see if the time of planting has a significant effect in reducing yield losses. Such work in Zaire has shown that losses are significantly lower when planting is done at the beginning of the rainy season. In Nigeria, the peak infestation of the mealybug occurs in February or March. At this time, early-planted cassava would be 10 months old and less susceptible to the insect pest. Unfortunately, cassava is generally planted late, because farmers are busy putting in their other crops during this earlier period.
Chemical control seems questionable. Although pesticides will control the mealybug, these chemicals are reserved for use on high-value crops such as cotton. Also, cassava is extremely sensitive to burn damage by chemicals. Furthermore, application would require great care to avoid leaving harmful residues on cassava leaves, which are eaten as a vegetable in some regions in Africa. These considerations are important in that continent. The technology of pesticides crosses borders and cultures much more quickly than the educational and legislative superstructure for their proper use and control. Some African nations have enacted virtually no legislation to govern the importation, registration, or handling of pesticides. Few countries maintain facilities for monitoring pesticide residues in the food or environment.The most hopeful proposal and the most promising work toward the control of the cassava mealybug is that of a Swiss entomologist, Hans R. Herren of the IITA. He plans to introduce the natural enemies of the cassava mealybug into Africa. ''Somewhere on cultivated or maybe wild cassava, Phenacoccus manihoti is living in a permanent struggle to survive, here attacked by a predator, there by a parasite, '' Dr. Herren says. ''Our job was to find them.'' If the introduction of natural enemies is successful, he explains, the parasite or predator population will increase rapidly, causing increasing percentages of hosts to be killed. They only feed on the mealybug. This is host specificity and will lead to a decline in the host population. Ideally, both host and parasite or predator would coexist in a stable interaction at very much reduced population densities.Dr. Herren points out that mealybugs have seldom been pests of major economic significance in areas where they are indigenous. He also notes that many outbreaks of imported mealybugs have been controlled by releasing their natural enemies.The first and most important task of biological control is to find the tools to work with - the natural enemies, Dr. Herren says. He chose the southern area of the United States and Central America and South America for exploration because of the density of their mealybugs species.According to Dr. Herren, there is also a great deal of literature showing Central America as the origin and distribution point of the cassava and some of its wild species. The closest known relatives of cassava among wild species originate in Central America in areas extending from Mexico to Panama. In addition, there are several other centers of origin of wild cassava species scattered from Mexico to the Brazilian-Paraguayan border. From field to field, scientists of the Commonwealth Institute of Biological Control, Trinidad, and the International Center of Tropical Agriculture, Colombia (CIAT), explored cassava in South America for mealybugs and their natural enemies.Dr. Herren, who drove nearly 30, 000 miles from southern California to Panama, says he reached a point where he could almost ''sense the presence of cassava'' growing along the main and back roads. He examined both wild and cultivated varieties because observations indicated that the association between mealybugs and cassava may be recent. During feeding, the mealybug injects a toxin into the cassava plant that induces severe growth disturbances. The tip of the cassava plant is totally deformed. If young, the plant may die. The strong proliferation of the cassava mealybug during the dry season shows a certain degree of inadaptability of the insect to the host plant habitat, but also to the host plant itself. The last two observations, low degree of tolerance from the host plant to the insect and insect adaptation to climatic factors, may have indicated a recent host plant switch.The program has been successful. Mealybugs have been found in the border area between Paraguay and Bolivia by CIAT scientists. Their parasites have been brought to Africa for multiplication and release.Man has again introduced a foreign insect. But this time it should serve a beneficial purpose.