Bean podborer harassing mungbeans

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Dam­ag­ing bean pod­bor­er (Maru­ca vit­ra­ta) pop­u­la­tions of up to 100+ larvae/m2 have been observed in flow­er­ing and pod­ding mung­beans in the South Bur­nett, Daw­son Cal­lide and Cen­tral High­lands. Although pod­bor­er is not an uncom­mon pest in these regions dur­ing wet sum­mers, very high pop­u­la­tions can inflict dev­as­tat­ing dam­age with zero pod set observed in some crops where the pest is uncon­trolled. In wet­ter sea­sons, the pest has also been report­ed on the Dar­ling Downs, albeit usu­al­ly in low­er num­bers. Oth­er crops at risk from pod­bor­er attack are adzu­ki beans, navy beans and pigeon pea.

Typ­i­cal bean pod­bor­er dam­age to flow­er­ing mung­beans. Note the web­bing sur­round­ing the dam­aged buds and flow­ers.

Dam­age

Ear­ly warn­ing signs are large num­bers of the dis­tinc­tive moths fly­ing in the crop. As the moths are flighty and often dif­fi­cult to see when at rest, con­sid­er using a sweep net to catch them and to con­firm the species. Flow­ers and buds that are webbed togeth­er are often the first vis­i­ble sign of lar­val dam­age. How­ev­er flow­ers infest­ed with small lar­vae (<5 mm) may show no vis­i­ble signs of dam­age until they are cut open. After ini­tial­ly feed­ing inside the flow­ers, lar­vae move to adja­cent pods. Lar­vae are a pale translu­cent cream with rows of dis­tinc­tive black spots.

Bean pod­bor­er moth Maru­ca vit­ra­ta in typ­i­cal pose with body raised at front and wings out­stretched. (25 mm wingspan).  

Thresh­olds

The cur­rent thresh­old is a nom­i­nal 3 lar­vae per square metre (based on expe­ri­ence, not research tri­als How­ev­er, sam­pling for the pest is prob­lem­at­ic as beat sheet sam­pling can under­es­ti­mate bean pod bor­er pop­u­la­tions by a fac­tor of 5. Because sam­pling is so dif­fi­cult, the pro­posed thresh­old is prob­a­bly very con­ser­v­a­tive (erring on the side of cau­tion).
 

Mon­i­tor­ing for pod­bor­er

The most reli­able way to esti­mate pod­bor­er num­bers is to:
1)      deter­mine the num­ber of infest­ed flow­er­ing racemes on 10–20 indi­vid­ual plants from dif­fer­ent areas of the field. Often the dam­age is obvi­ous, webbed buds/flowers, but open un-webbed flow­ers to check for small lar­vae.

2)      Then mul­ti­ply the mean num­ber of infest­ed racemes per plant by the num­ber of plants per square metre, assum­ing one lar­va per infest­ed raceme.

 Many severe­ly infest­ed crops found by Ento­mol­o­gist Hugh Brier have pop­u­la­tions up to 50 times the cur­rent nom­i­nal thresh­old.

Man­age­ment

Bean pod­bor­er can be con­trolled with the reg­is­tered pes­ti­cide methomyl®, but where there is sus­tained pod­bor­er pres­sure, repeat­ed sprays are often nec­es­sary. Pod­bor­ers are also (co-inci­den­tal­ly) con­trolled by indox­acarb (Stew­ard®) sprays tar­get­ing heli­cov­er­pa. How­ev­er, recent DEEDI tri­als and in-crop inspec­tions show the syn­thet­ic pyrethroids (deltamethrin* and alpha-cyper­me­thrin* do not con­trol bean pod­bor­er (*reg­is­tered in mung­beans against green veg­etable bug (GVB), and GVB and small heli­cov­er­pa respec­tive­ly). This is despite pre­vi­ous tri­als (in the 1990’s) show­ing good pod­bor­er con­trol with syn­thet­ic pyrethroids.

 
Note the key to man­ag­ing this pest is con­trol­ling ear­ly before lar­vae enter the pods.
 
Pod­bor­er can attack as soon as the first ear­ly buds appear but infec­tions are typ­i­cal­ly most obvi­ous at full flow­er­ing.  Hugh Brier is keen for any feed­back regard­ing the suc­cess or oth­er­wise of pod­bor­er con­trol in crops, and also the loca­tion of sig­nif­i­cant out­breaks.
 
DEEDI ento­mol­o­gists are cur­rent­ly eval­u­at­ing new gen­er­a­tion pes­ti­cides for pod­bor­er and heli­cov­er­pa con­trol in pulse crops. These tri­als aim to iden­ti­fy new pes­ti­cides with (a) greater effi­ca­cy, (b) a longer peri­od of crop pro­tec­tion, and © less impact on ben­e­fi­cial insects than pes­ti­cides cur­rent­ly reg­is­tered in mung­beans. The lat­ter is impor­tant as tri­al data also sug­gest that more-selec­tive pes­ti­cides sig­nif­i­cant­ly reduce the risk of flar­ing heli­cov­er­pa. Results to date are promis­ing.
 
Fund­ing for this project comes from the GRDC North­ern Grains IPM project DAQ00153
Arti­cle by Hugh Brier