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"Super-fly" threatens "Rambo" cassava, food security
lead photo
JOHANNESBURG, 7 May 2013 (IRIN) - A tiny, rapidly breeding
cyanide-munching insect, dubbed a "super-fly" by scientists, is
threatening the food security of millions of Africans.
The Bemisia tabaci - one of several whitefly species - carries lethal
viruses that cause cassava brown streak disease (CBSD) and cassava
mosaic disease (CMD), which have decimated the hardy cassava plant.
Cassava, a tropical root crop, is the third most important source of
calories in the tropics, after rice and maize. According to the UN's
Food and Agriculture Organization (FAO), it is the staple food for
nearly a billion people in 105 countries, where it comprises as much as
a third of daily calories consumed. The cheapest known source of
starch, cassava is grown by poor farmers - many of them women - often
on marginal land; for these people, the crop is vital for both food
security and income generation.
The threat to cassava is particularly alarming as the plant is often
called the "Rambo" root
-security-say-scientists> for its ability to withstand high
temperatures and drought. With climate change expected to take a major
toll on maize in the coming decades, many hope cassava will offer an
alternative route to food security in Africa. Cassava may also prove to
be an important source of biofuel.
Experts plan to take aim at the whitefly this week, at a conference of
the Global Cassava Partnership for the 21st Century (GCP21), at the
Rockefeller Foundation Bellagio Center in Italy. The conference is
dedicated to "declaring war on cassava viruses in Africa."
Pandemics
From the 1980s to the mid-2000s, CMD ravaged more than 4 million square
km in Africa's cassava-growing heartland, stretching from Kenya and
Tanzania in the East to Cameroon and the Central African Republic in
the West. But in recent years, the scientific community developed
cassava varieties resistant to CMD.
James Legg, a leading cassava expert at the International Institute of
Tropical Agriculture (IITA), who works out of Tanzania, told IRIN, "The
premature celebrations for this apparent victory were very soon
squashed, however, as sinister new reports were received of the
occurrence and apparent spread of CBSD in southern Uganda."
geID=201305061401550951&width=490>
Photo: Neil Palmer/CIAT
Bemisia tabaci on a cassava leaf
Until then, scientists had assumed that the viruses causing CBSD could
not spread at medium-to-high altitudes; the disease had previously only
been reported in coastal areas of East Africa and the low-altitude
areas around Lake Malawi. "The spread recorded from Uganda instantly
cast doubt of the validity of that earlier theory," said Legg. "Worse
still, the disease spread out from Uganda over following years, and
into the neighbouring countries of Kenya, Tanzania, Burundi and
Rwanda."
CBSD is now a pandemic, threatening Nigeria, the world's largest
producer and consumer of cassava. The cassava starch industry in
Nigeria generates US$5 billion per year and employs millions of
smallholder farmers and numerous small-scale processors.
Only in 2005 were scientists able to confirm that the whitefly
responsible for spreading CMD was also responsible for spreading CBSD.
"With this realization, it became clear that the spread of these two
disease pandemics was really only a consequence of the fact that East
and Central Africa was experiencing a devastating outbreak of the
whitefly that transmits both of them," explained Legg.
He told IRIN that in the 1980s, researchers recorded an average of less
than one fly per plant, but by the mid-1990s, the number of whiteflies
had increased a hundredfold.
"These insects also seem to have a close relationship with the viruses
that they transmit, and some evidence has shown that the insects do
better on virus-diseased plants"
Arms race
It seems Bemisia tabaci has been assisted by climate change: The warmer
temperatures occurring in higher altitudes have created optimal
conditions for the insect to breed rapidly, speeding its adaptation and
evolution. More importantly, said Legg, is the fact that these flies
seem to have worked out how to do better on cassava plants, whose
cyanide production deters all but a very small group of insects. As the
whitefly population has exploded, rapid spread of the viral diseases -
CMD and CBSD - was an inevitable consequence.
What makes a bad situation even worse, however, is that these diseases,
in turn, may promote the whitefly. "These insects also seem to have a
close relationship with the viruses that they transmit, and some
evidence has shown that the insects do better on virus-diseased plants,
leading to an 'I scratch your back, you scratch my back' type of
mutually beneficial relationship," Legg said.
Scientists are working towards solutions. A member of Legg's team is
examining the impact of climate change on the whitefly in search of
ways to deal with the pest. Other planned projects are working to
control whiteflies directly, either through introducing other
beneficial insects that kill whiteflies, or through producing varieties
that combine whitefly and disease resistance.
Efforts to breed high-yielding, disease-resistant plants suitable for
Africa's various growing regions will involve going to South America,
where cassava originated, and working with scientists at the cassava
gene bank of the International Center for Tropical Agriculture (CIAT),
IITA's sister organization, in Colombia. CIAT is the biggest repository
of cassava cultivars in the world.
Experts at the conference in Italy will also discuss a more ambitious
plan to eradicate cassava viruses altogether. The aim will be to
develop a regional strategy that gradually replaces farmers' infested
cassava plants with virus-free planting material of the best and most
disease-resistant cultivars. Approaches to developing these cultivars
will include new molecular breeding and genetic engineering
technologies to speed up selection. The hope of the team is that by
joining forces, and employing the whole range of technologies
available, a lasting impact will be made in tackling a crop crisis that
poses the single greatest challenge to the future of Africa's cassava
crop.
jk /rz
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