Soybean moth active in North Queensland

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Sig­nif­i­cant soy­bean moth activ­i­ty has recent­ly been report­ed in some Bur­dekin soy­bean crops. Soy­bean moth is a gen­er­al­ly a minor soy­bean pest but sig­nif­i­cant out­breaks have been report­ed peri­od­i­cal­ly in all soy­bean grow­ing regions.

Feed­ing dam­age
Lar­vae feed most­ly with­in the leaves, mak­ing dis­tinc­tive pale leaf mines, and often dis­tort­ing leaves.

Soybean moth larvae and typical leaf mining damage

Soy­bean moth lar­vae and typ­i­cal leaf min­ing dam­age

Soy­bean moth is present in low num­bers in most crops. How­ev­er, in some sea­sons a pop­u­la­tion explo­sion can lead to huge num­bers with over 1000’s of lar­vae per square meter. Such large pop­u­la­tions can total­ly defo­li­ate a crop.

 

 

Reg­u­lar crop sam­pling will pick up the ear­ly stages of dam­ag­ing pop­u­la­tions, specif­i­cal­ly the numer­ous pale small feed­ing mines per leaflet. Stressed sec­tions of a crop are often more heav­i­ly infest­ed.

Soybean moth damage

Soy­bean moth dam­age

Severe damage throughout the crop

Severe dam­age through­out the crop

Iden­ti­fi­ca­tion
Soy­bean moth lar­vae are small, reach­ing only 7 mm in length and are grey-green. Lar­va should not be con­fused with the much larg­er legume web-spin­ner (14 mm) which is a brighter green and webs leaves togeth­er, rather than min­ing with­in the leaf. Note that soy­bean moth lar­vae in the pre-pupal stage turn pale before pupat­ing in fold­ed leaves.

Pre-pupal larvae and pupa. Top pupa is parasitised

Pre-pupal lar­vae and pupa. Top pupa is par­a­sitised

The emerg­ing moths are small (6 mm long) with dark fold­ed wings with a white band. The legume web-spin­ner moth is much larg­er (18 mm wingspan) and is brown with yel­low mark­ings.

Soybean moth adult

Soy­bean moth adult

Thresh­olds
Thresh­olds are based on per­cent­age defo­li­a­tion. In veg­e­ta­tive crops, 33% leaf tis­sue death (min­ing) is allow­able before yields are com­pro­mised. How­ev­er, the tol­er­a­ble % falls to less than20% dur­ing pod-fill.

Con­trol
Soy­bean moth are eas­i­ly con­trolled with Abamectin at 300 mL/ha under per­mit 14288 (valid until 30 June 2017). Check that lar­vae are not in the pale pre-pupal stage as they will have stopped feed­ing and will not be con­trolled by the insec­ti­cide. Feed­ing lar­vae die inside the leaves and turn an opaque grey colour.

Arti­cle by Hugh Brier and Kate Charleston