Soybean moth active again in 2017 in coastal Queensland

Major soy­bean moth out­breaks have recent­ly been report­ed in some coastal Bur­nett soy­bean crops, and less­er out­breaks in crops in the Bur­dekin. Soy­bean moth is gen­er­al­ly a minor soy­bean pest but major out­breaks have been report­ed peri­od­i­cal­ly in all soy­bean grow­ing regions. In extreme cas­es, crops can be total­ly defo­li­at­ed.

Feeding damage

Lar­vae feed most­ly with­in the leaves, mak­ing dis­tinc­tive pale leaf mines, and often dis­tort­ing leaves with fine web­bing.

Soy­bean moth is usu­al­ly present in low num­bers in most soy­bean crops. How­ev­er, in some sea­sons, a pop­u­la­tion explo­sion can lead to huge num­bers, some­times with thou­sands of lar­vae per square metre. Such large pop­u­la­tions can total­ly defo­li­ate a crop if not detect­ed and act­ed upon in time.

Severe dam­age through­out the crop near Bund­aberg (pho­to by Matt Leighton)

Severe soybean moth damage (photo by Hugh Brier)

Severe soy­bean moth dam­age (pho­to by Hugh Brier)

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.  Infes­ta­tions may also be high­er along the edge of the crop adja­cent to trees.

Identification

Soy­bean moth lar­vae are small, reach­ing only 7 mm in length and are grey-green, but turn pale cream before pupat­ing in fold­ed leaves. Lar­va should not be con­fused with the much larg­er legume web-spin­ner (14 mm) which is a brighter green, webs leaves togeth­er, and feeds exter­nal­ly with­in the rolled up leaf, rather than min­ing with­in the leaf. The oth­er species that they may be con­fused with is the soy­bean leaf min­er, which also mines in leaves, but their lar­vae are much small­er, only reach­ing 2–3 mm in size, and have a notice­able thick­en­ing of the body behind the head.

Soy­bean moths are small (6 mm long) with dark fold­ed wings with a white band. The legume web-spin­ner moth is much larg­er (13 mm wingspan) and is brown with yel­low mark­ings. The soy­bean leafmin­er also has fold­ed wings but is very small (2 mm long) with shin­ing orange wings with white spots ringed with black.

Thresholds

Thresh­olds are based on per­cent­age defo­li­a­tion. In veg­e­ta­tive crops, 33% leaf tis­sue death (min­ing) is allow­able before yields are com­pro­mised. How­ev­er, the tol­er­a­ble per­cent­age defo­li­a­tion falls to less than 20% dur­ing pod-fill.

Control

Soy­bean moth lar­vae 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 (see image below) 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.

Pre-pupal larvae and soybean moth pupa. The pupae above them has been parasitised

Pre-pupal lar­vae and soy­bean moth pupa on edge of leaf. The pupae above them has been par­a­sitised

Arti­cle by Hugh Brier and Liz Williams. Images by Joe Wes­sels unless oth­er­wise spec­i­fied.

Contact:

Hugh Brier, Senior Ento­mol­o­gist DAF Kingaroy Q.
Email  [email protected]    Phone 0741 821 840 or Mob 0428 188 069

Liz Will­liams Research Scientist/Entomology DAF Kingaroy Q.
Email  [email protected]   Phone 0741 821 818 or Mob 0476 850 415