A bat­tle of good ver­sus wee­vil


Re­searchers at the Univer­sity of South­ern Queens­land are ex­plor­ing ways of us­ing fun­gus to safe­guard the world’s sweet potato har­vest. BELINDA SMITH re­ports.

You might catch more flies with honey, but you can dis­patch more wee­vils with fungi.

Re­searchers at the Univer­sity of South­ern Queens­land, funded by the Aus­tralian Cen­tre for In­ter­na­tional Agri­cul­tural Re­search, are find­ing ways to har­ness na­ture’s chem­i­cal weapons to fight the sweet­potato wee­vil (Cy­las formi­car­ius), an in­nocu­ous-look­ing bug that can dev­as­tate en­tire crops.

Some 95% of sweet pota­toes (Ipo­moea batatas) are grown in de­vel­op­ing coun­tries, where they are the fifth most im­por­tant food crop. Not only rich in vi­ta­mins, they’re also chock full of car­bo­hy­drates. A farmer grow­ing sweet pota­toes can pro­duce more ed­i­ble en­ergy per hectare per day than they could with rice or cas­sava.

Sweet pota­toes are hardy, too, hap­pily grow­ing in dry soil with lit­tle fer­tiliser or ir­ri­ga­tion. But they’re not tough enough to with­stand a sweet potato wee­vil on­slaught. Adult wee­vils lay eggs in the stems and roots. Af­ter hatch­ing, the grubs grow into adults which gnaw their way out, rid­dling the sweet pota­toes with holes and ren­der­ing them ined­i­ble.

By the time a crop is in­fested, it’s usu­ally too late to do any­thing about it. Man­u­fac­tured in­sec­ti­cides only kill the adult wee­vils, not the lar­vae al­ready en­sconced. In coun­tries such as Pa­pua New Guinea, where sweet potato is the pri­mary food source, farm­ers de­pend solely on “cul­tural con­trol”, such as crop ro­ta­tion and san­i­ta­tion, to stop the wee­vil’s spread. But the nat­u­ral world has its own ar­se­nal: the fun­gus Me­tarhiz­ium aniso­pliae.

Found in soils around the world, M. aniso­pliae is en­to­mopathogenic, mean­ing it only in­fects in­sects. Sim­ply com­ing into con­tact with fun­gal spores is enough for in­fec­tion to take place. The species bur­rows into the un­for­tu­nate in­sect, re­pro­duces in­side its body, and bursts out again, killing the host – if it’s not dead al­ready. And M. aniso­pliae counts sweet­potato wee­vils in its range of hosts.

So re­searchers such as Bree Wil­son at the Univer­sity of South­ern Queens­land in Toowoomba are find­ing ways to use it to the sweet­potato’s ad­van­tage.

One pos­si­bil­ity is a “lure and kill” ap­proach: male wee­vils, at­tracted to baits laced with com­mer­cially man­u­fac­tured sweet­potato wee­vil fe­male sex pheromone and M. aniso­pliae spores, could pick up the lethal fun­gus and trans­fer it to fe­males be­fore dy­ing.

But when Wil­son and her col­leagues added par­tic­u­larly vir­u­lent M. anisopi­lae strains in their baits, which would kill more in­sects faster, the wee­vils steered clear. The re­searchers sus­pect those strains se­crete cer­tain volatile com­pounds that sweet­potato wee­vils can de­tect and know to avoid.

“While we haven’t iden­ti­fied the volatiles re­spon­si­ble for avoid­ance in our iso­lates, this is next on our cards,” Wil­son says.

She adds this will be helped by a new and “very fancy ol­fac­tome­ter” which will sniff out re­pelling volatiles. And when they fig­ure out which genes are re­spon­si­ble for the wee­vil-de­ter­ring ef­fects, they’ll use CRISPR-CAS9 gene edit­ing tech­nol­ogy to snip them out of the fun­gus genome.

The new, re­pel­lent-free ver­sion of the fun­gus will then be tested in glasshouses and in the field to see if it re­tains its lethal­ity. An­other op­tion, she says, is to man­u­fac­ture and spray the re­pelling volatiles around a crop to pro­duce a smelly wee­vil-proof bar­rier.

Sweet­potato wee­vils aren’t the only pest in Wil­son’s sights. Her big­gest chal­lenge will be the root knot ne­ma­tode ( Meloidog­yne species) – round­worms that in­fect roots, weak­en­ing or killing the plant.

The main de­fences against root knot ne­ma­todes are ex­pen­sive and nasty, so she hopes to ex­plore how Pas­teuria species of bac­te­ria – which stop the worm re­pro­duc­ing – could help.

“I’m look­ing for­ward to work­ing with the Aus­tralian and Pa­pua New Guinean grow­ers to test from of this re­search to of­fer gen­uine al­ter­na­tives to con­trol these pests,” she says.

Cy­las adults on sweet pota­toes

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