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    ‘Su­per­fly’ threatens fu­ture of mil­lions re­ly­ing on cas­sava

    Sci­ent­ists are sound­ing the alarm on a tiny, fast breed­ing in­sect that wipes cas­sava in minutes which they say could threaten mil­lions of Africa, with cas­sava being the third most im­port­ant source of cal­or­ies after maize and rice.

    The Be­misia tabaci also known as ‘super fly’ and one of sev­eral white­fly spe­cies,

    Ac­cord­ing to the UN's Food and Ag­ri­cul­ture Or­gan­iz­a­tion (FAO), cas­sava 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 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.


    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.

    "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," said James Legg, a lead­ing cas­sava ex­pert at the In­ter­na­tional In­sti­tute of Trop­ical Ag­ri­cul­ture (IITA).

    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 said 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.
    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.

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