Etiella alert for mungbean on the Darling Downs, Goondiwindi and Northern NSW growing regions

A sig­nif­i­cant etiel­la out­break was recent­ly report­ed in pod-fill­ing mung­bean in the Moree region of North West­ern NSW. In this par­tic­u­lar crop, it is esti­mat­ed that etiel­la lar­vae have infest­ed pods on 50% of plants. Etiel­la eggs, at low den­si­ties, were also recent­ly observed in an ear­ly flow­er­ing crop near Bun­gun­ya (west of Goondi­win­di, Qld).

Iden­ti­fi­ca­tion
Etiel­la (Etiel­la behrii) is also known as the Lucerne seed web moth. Ellip­ti­cal shaped eggs are laid on pods and flow­ers or under bracts and are very hard to detect. New­ly-hatched lar­vae bore straight into pods leav­ing a near-invis­i­ble entry hole. The lar­vae are pale cream/orange when small while larg­er lar­vae are cream or green with pink stripes. Lar­vae near­ing pupa­tion often have a pink tinge all over. Etiel­la can be dif­fer­en­ti­at­ed from bean pod­bor­er which has dis­tinc­tive black spots.

Etiella moth - 12 mm long

Etiel­la moth — 12 mm long

Etiella eggs (0.6 mm) on rattle pod

Etiel­la eggs (0.6 mm) on rat­tle pod

Medium etiella larva (5 mm), with typical pale green colour and pink stripes.

Medi­um etiel­la lar­va (5 mm), with typ­i­cal pale green colour and pink stripes.

Dam­age at bud­ding and flow­er­ing
Etiel­la lar­vae feed on buds and flow­ers, dam­ag­ing these struc­tures. Larg­er lar­va may also tun­nel into plant stems. Major infes­ta­tions in crops at the budding/flowering stage will like­ly reduce yield how­ev­er there are no val­i­dat­ed thresh­olds for etiel­la at this crop stage. For this rea­son, researchers in GRDC’s North­ern Grains IPM project would wel­come any reports of, and sam­ples from cur­rent­ly infest­ed crops and again at har­vest.

Etiella damage to mungbean buds.  Look for dead buds, fine webbing and frass

Etiel­la dam­age to mung­bean buds. Look for dead buds, fine web­bing and frass

Etiella damage to mungbean stem

Etiel­la dam­age to mung­bean stem

Crops with ade­quate mois­ture will most like­ly com­pen­sate for low to mod­er­ate lev­els of ear­ly dam­age but major dam­age would most like­ly reduce crop yield.
Pre­lim­i­nary data in soy­bean sug­gests this crop can com­pen­sate for ear­ly stem dam­age by set­ting addi­tion­al side branch­es. How­ev­er, it is unknown how well, or if mung­bean can com­pen­sate for this type of dam­age.

Dam­age at late pod-fill
Late dam­age at the pod-fill stage will reduce yield because of the direct con­sump­tion of seeds. Data from soy­bean shows each etiel­la lar­vae con­sumes only 0.2 gram which is equiv­a­lent to one soy­bean seed. Because mung­bean seeds are small­er, it is like­ly that each etiel­la will con­sume more seeds, pos­si­bly as many as 4 to 5. How­ev­er, the yield loss in mung­bean is like­ly to be sim­i­lar to that in soy­bean, which is 2kg/ha for every lar­va per square metre. This equates to a the­o­ret­i­cal thresh­old of 30 to 40 larvae/m2. This is a very high thresh­old but keep in mind that etiel­la lar­vae are much small­er than heli­cov­er­pa lar­vae and there­fore con­sume pro­por­tion­al­ly less. Also remem­ber that this is a the­o­ret­i­cal thresh­old as tri­als last year sug­gest that once lar­vae are inside pods, they are very dif­fi­cult if not impos­si­ble to con­trol.

Large etiella larva (13 mm) in soybean pod.  Note pinkish colour typical of large larvae approaching pupation

Large etiel­la lar­va (13 mm) in soy­bean pod. Note pink­ish colour typ­i­cal of large lar­vae approach­ing pupa­tion

Qual­i­ty issues
Qual­i­ty may be an issue if a sig­nif­i­cant num­ber of seeds only have par­tial dam­age, and they are not lost out the back of the head­er at har­vest. The risk of par­tial seed dam­age could increase in black pods where, as the seeds dry down, lar­va feed­ing could con­ceiv­ably slow down and lar­vae fail to reach full size by har­vest.

Assess­ments of dam­aged sam­ples from infest­ed mung­bean crops will enable researchers to mea­sure the impact of etiel­la dam­age on mung­bean seed qual­i­ty, and estab­lish thresh­old guide­lines. Researchers are also keen to eval­u­ate insec­ti­cides reg­is­tered in the crop against oth­er pod-bor­ing cater­pil­lars (heli­cov­er­pa and bean pod­bor­er).

Etiella damaged mungbean pod with frass inside

Etiel­la dam­aged mung­bean pod with frass inside

Etiella damaged mungbean seeds

Etiel­la dam­aged mung­bean seeds

Any­one with sus­pect­ed etiel­la dam­age,  please con­tact Hugh Brier, DAFF Kingaroy on 0428 188 069.