Dan­ger­ous daisy meets its wee­vil night­mare

Bi­o­log­i­cal con­trols are be­ing re­leased to halt the in­va­sion of famine weed on farms and re­serves

Mail & Guardian - - News - Sipho Kings

One of South Africa’s most de­struc­tive weeds is wip­ing out farm­lands, poi­son­ing cat­tle and mak­ing peo­ple sick. It’s been do­ing this for more than a cen­tury. Now an army of wee­vils, fun­gus and beetles is be­ing bred to fight back.

You’ve prob­a­bly seen the plant, whose Latin name, Parthe­nium hys­teropho­rus (Aster­aceae), has been swapped out for famine weed be­cause of its ef­fect on food crops. It grows to the height of an adult and spreads in vast clumps, with the weeds wrap­ping around each other. In spring and sum­mer it sprouts de­cep­tively beau­ti­ful white flow­ers. Those stick to ev­ery­thing, so tourists carry them across the coun­try and work­ers take them back home. They love the mud un­der­neath cars and use them to spread along mo­tor­ways, and then into the veld.

A na­tive of Cen­tral and South Amer­ica, famine weed was first recorded here in the late 1800s.

Its evo­lu­tion­ary ad­van­tage is a chem­i­cal it leaks out that stops other plants from grow­ing. This mech­a­nism al­lows it to take over fields, farms and game re­serves in a sin­gle sum­mer. The an­i­mals and ecosys­tems that rely on indige­nous plants and crops ei­ther starve or move away. It thrives in sub­trop­i­cal parts of the world and has in­vaded 48 coun­tries.

Much of what we know is thanks to a 40-year bat­tle Aus­tralia has fought against the weed. There, it de­stroyed large tracts of land that cat­tle would graze on. Where famine weed took hold, food for cat­tle dropped by be­tween 25% and 80%. The meat from cat­tle that eat the weed has to be de­stroyed.

In In­dia and Ethiopia, famine weed has crip­pled food pro­duc­tion in ar­eas. Dur­ing the past decade, Ethiopia’s yield from sorghum has dropped by be­tween 45% and 80% in places where famine weed has in­vaded. Some 15 coun­tries on the African con­ti­nent are thought to have famine weed in­va­sions. Many of them have no plan to deal with it.

The weed also tar­gets hu­mans. The small flow­ers cause hay fever and asthma, and touch­ing the plant cre­ates sores, rashes and skin le­sions. These only get worse the longer peo­ple are ex­posed to them. In South Africa, that’s a prob­lem be­cause famine weed thrives in ar­eas where the land is overused. Where farm­ers plough al­ready dam­aged soil and the veld is over­grazed, the soil is vul­ner­a­ble to a famine weed in­va­sion. Large patches of farm­land in Mpumalanga and KwaZulu-Natal have been worst hit.

Famine weed’s Afrikaans name, De­moina bossie, tips a hat to the first big in­va­sion in 1984. Dam­age caused by Cy­clone De­moina, and the ex­ces­sive rain­fall that came with it, cre­ated per­fect grow­ing con­di­tions.

That’s when the Agri­cul­tural Re­search Coun­cil got fund­ing to build a lab­o­ra­tory in KwaZulu-Natal to re­search famine weed, and other in­va­sive plants. A quar­an­tine fa­cil­ity was added in 2008. A clus­ter of ac­cess-con­trolled green­houses act as the nerve cen­tre for the fight to de­feat this weed.

Lor­raine Strathie works at the lab­o­ra­tory and fo­cuses her full at­ten­tion on famine weed. “All coun­tries have dif­fer­ent con­di­tions but we have been re­ally helped by all the lessons learned in Aus­tralia,” she says.

By 2008 the fo­cus was on try­ing to stop famine weed from spread­ing.

This in­volved mostly man­ual meth­ods such as spray­ing sec­tions of game re­serves and farms to act as bar­ri­ers. Cars and peo­ple mov­ing in and out of in­vaded ar­eas were cleaned.

The way to con­trol an in­va­sive species is to get rid of its ad­van­tage. Famine weed — and other in­va­sives — pro­lif­er­ate be­cause they have no preda­tors. Aus­tralia turned to bi­o­log­i­cal con­trol agents. Eleven nonindige­nous nat­u­ral preda­tors were in­tro­duced to eat the plant. Each one goes for a dif­fer­ent part of the weed, ei­ther the stem or its leaves.

In South Africa, the prob­lem was that only one in­tro­duced preda­tor had come across with the weed. Strathie and other sci­en­tists needed to bring in more preda­tors. The worry is that the preda­tors might then go on and be­come in­va­sive species in their own right, do­ing dam­age that no­body had thought about.

Strathie says: “We only in­tro­duce bi­o­log­i­cal con­trol agents when we are 100% con­vinced that they won’t af­fect any­thing else.”

This has in­volved the re­searchers car­ry­ing out tests on fun­gus, wee­vils and other bugs that are in quar­an­tine at the KwaZulu-Natal green­houses. Each con­trol agent has its own area where noth­ing is al­lowed to con­tam­i­nate the re­search.

South Africa has a cen­tury-long record of not mak­ing er­rors with this type of ex­per­i­men­ta­tion, and the re­searchers have to shower and change ev­ery time they en­ter or leave their lab­o­ra­tory.

For be­tween three and five years, the re­searchers watch whether the con­trol agent eats any­thing be­sides famine weed. The agents tend to stick to the weed be­cause they have evolved to do so.

When the con­trol agents pass the test, they are bred in larger num­bers and then re­leased into the wild, af­ter the na­tional en­vi­ron­ment depart­ment signs off on the tests.

The first batch of famine weed con­trol agents were re­leased just be­fore the drought that started in 2014. Many wee­vils and beetles died. They weren’t ready for such in­tense weather.

But some hardy vari­ants thrived. About 40 000 have now been re­leased at 330 sites around the coun­try.

The mix of in­tro­duced species is im­por­tant. Strathie says: “It’s un­usual to find a sil­ver bul­let be­cause plants have many nat­u­ral preda­tors.” And it takes an army of bi­o­log­i­cal con­trol agents to take down one of South Africa’s most dan­ger­ous in­va­sive species.

That army is en­joy­ing suc­cess. Aus­tralia has man­aged to shrink the famine weed in­va­sion down to man­age­able lev­els — once a species in­vades it is al­most im­pos­si­ble to kill off — and this is where South Africa is headed. By at­tack­ing each weed with a com­bi­na­tion of preda­tors, pop­u­la­tions are be­ing brought un­der con­trol in KwaZulu-Natal and Mpumalanga.

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