Box 16.4. Taxonomy and biological control of the cassava mealybug

Cassava (manioc, or tapioca — Manihot esculenta) is a staple food crop for 200 million Africans. In 1973 a new mealybug (Hemiptera: Pseudococcidae) was found attacking cassava in central Africa. Named in 1977 as Phenacoccus manihoti, this pest spread rapidly until by the early 1980s it was causing production losses of over 80% throughout tropical Africa. The origin of the mealybug was considered to be the same as the original source of cassava — the Americas. In 1977, the apparent same insect was located in Central America and northern South America and parasitic wasps attacking it were found. However, as biological control agents they failed to reproduce on the African mealybugs.

Working from existing collections and fresh samples, taxonomists quickly recognized that two closely related mealybug species were involved. The one infesting African cassava proved to be from central South America, and not from further north. When the search for natural enemies was switched to central South America, the true P. manihoti was eventually found in the Paraguay basin, together with an encyrtid wasp, Apoanagyrus (formerly known as Epidinocarsis) lopezi (J.S. Noyes, pers. comm.). This wasp gave spectacular biological control when released in Nigeria, and by 1990 had been established successfully in 26 African countries and had spread to more than 2.7 million km2. The mealybug is now considered to be under almost complete control throughout its range in Africa.

When the mealybug outbreak first occurred in 1973, although it was clear that this was an intro- duction of neotropical origin, the detailed species- level taxonomy was insufficiently refined, and the search for the mealybug and its natural enemies was misdirected for three years. The search was redirected thanks to taxonomic research. The savings were enormous: by 1988, the total expenditure on attempts to control the pest was estimated at US$14.6 million. In contrast, accurate species identification has led to an annual benefit of an estimated US$200 million, and this financial saving may continue indefinitely.

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