Centers Declare Victory Over the Cassava Green Mite
The Cassava Green Mite (CGM) is a pest responsible
for between 30 and 50 percent yield loss of cassava, a starchy
root crop in tropical Africa. Cassava is a staple food for more
than 200 million people in sub-Saharan Africa. Rich in calories,
highly drought tolerant, thriving in poor soils and easy to store
in the ground, cassava is popularly called "the staff of
life" for the poorest of the poor in Africa.
A breakthrough in CGM control was made possible through
identification and mass release of an effective natural enemy
of the CGM in Africas cassava regions. Peter Neuenschwander,
Director of Plant Health Management at the International Institute
of Tropical Agriculture (IITA) based in Ibadan (Nigeria), says:
"The Institute had chosen the option of classical biological
control strategy to fight crop pests and diseases because it constitutes
the most environment-friendly technology."
In the early 1990s, IITA had already achieved a substantial
breakthrough by introducing a biological control method which
virtually freed sub-Saharan Africa of another important pest,
the cassava mealybug. "The new breakthrough in GCM control
through the use of predatory mites constitutes another milestone
in the classical biological control of crop pests and diseases,"
says Neuenschwander. "However, the green mite has been
much more difficult to control than the mealybug -- using the
same biological control methods -- because so little is known
about mites in general, and especially mites on cassava."
When both the cassava mealybug and the CGM appeared
in the African continent in the 1970s, causing widespread damage
and loss, the lives and livelihoods of millions of people were
threatened. The first outbreak of the CGM, Mononychellus tanajoa,
(in French: acarien vert) was noticed in Uganda in 1970. The pest
now covers virtually all cassava producing areas of sub-Saharan
Africa. Its attack on cassava shoots is most damaging during the
dry season when its population explodes; it travels and multiplies
unimpeded by rainfall mortality. "In the absence of natural
controls, the mite swept through the 27 countries of Africas cassava
belt," says IITA Acarologist Steve Yaninek who is leading
a team of scientists at the IITA Biological Control Center in
Both pests originated in South America, the genetic
ancestral home of cassava, where they do not cause much damage
because natural enemies help suppress their populations make their
presence largely insignificant. After the mealybug had been brought
under control in Africa by introducing the parasitoid Apoanagyrus
lopezi from South America, the battleground shifted to the CGM.
Control of the pest through application of toxic
chemicals was ruled out because of possible adverse effects of
chemicals on illiterate farmers and the environment. Also, disease
pathogens and pests tend to gradually develop over time resistance
to chemicals pesticides. Moreover, chemicals most pesticides are
not selective and might destroy the natural enemies pests together
with the peststheir natural enemies.
To effectively control cassava pests and ensure better
living standards for cassava farmers in sub-Saharan Africa, an
inter-continental integrated pest management research project
was launched several years ago. The project involves international
agricultural research centers in Africa and Latin America, national
agricultural research systems, and farmers in a joint effort to
develop an ecologically sustainable cassava plant protection strategy.
Under the arrangement, IITA is collaborating with the International
Center for Tropical Agriculture (CIAT), in Cali, Colombia, the
Brazilian Agricultural Research Corporation (EMBRAPA) in Brazil,
and several African national research systems.
The United Nations Development Programme provided
four-year funding for this project to help scientists at the centers
and in national institutions in four West African countries (Benin,
Cameroon, Ghana and Nigeria) to work directly with farmers and
extension agents. The project also enjoys support from the International
Fund for Agricultural Development (IFAD), Denmark and Germany.
The goal was to develop, test and implement ecologically sound
cassava plant protection technologies.
Great emphasis was placed on farmer participatory
research, a method in which farmers and scientists work as partners
in identifying local problems and finding possible solutions.
The Ecologically Sustainable Cassava Plant Protection (ESCaPP)
project, known as PROFISMA in Brazil, brought together a large
variety of scientists from the two continents, working directly
with farmers and extensionists in addressing the daily problems
faced in cassava production.
After evaluating many species of predatory mites
-- natural enemies of CGM -- over a period of ten years, EMBRAPA
identified several species from Brazil that can survive in Africa.
One of them, Typhlodromalus aripo, was found to reduce pest populations
by as much as 90 percent in the dry season when pest populations
are usually high; in the wet season, pest attacks are not as severe
and therefore the reduction in GCM populations is less dramatic.
After its introduction to the Africa's cassava belt
in 1993, T. aripo found a conducive environment to prey on CGM.
Impact assessment studies carried out by IITA at the sites where
the CGM natural enemies had been released revealed that cassava
yields increased by 35 percent within one season. Farmers are
gaining about 70 US-dollars per hectare of cassava planted. In
West Africa alone this adds up to a total profit per planting
season of about $60 million for the cassava farmers.
According to Steve Yaninek, T.aripo was first released
on cassava farms in 1993 in Benin after it had been transported
from Brazil. It has subsequently been released in 11 countries
and is now confirmed as established in all of them, except Zambia.
T. aripo has also spread into Togo and Côte d'Ivoire from
neighboring countries. It spreads about 12 km in the first year,
and as much as 200 km in the second year. Today, the CGM predator
has been established on more than 400,000 square kilometers of
Africas cassava growing areas.
"T. aripo is able to spread quite easily,"
says Steve Yaninek, "because it has many food sources --
for instance red mites, whiteflies, maize pollens, honey dew,
and plant exudates -- on which it can survive. However, in order
to reproduce it requires mite prey." Tests have shown that
in the absence of the CGM, T. aripo either disperses to find
CGM or goes extinct locally, thus not becoming an ecological nuisance.
(International Institute of Tropical Agriculture (IITA)/Centro Internacional de Agricultura Tropical (CIAT)