Etiella in mungbeans and soybeans – do we manage the threat or the pest?

There is an under­stand­able lev­el of con­cern about how best to man­age etiel­la (Etiel­la behrii) in sum­mer mung­bean crops fol­low­ing the unex­pect­ed­ly high etiel­la infes­ta­tions (and cor­re­spond­ing seed dam­age and con­t­a­m­i­na­tion) in spring mung­bean crops on the Downs and in Cen­tral Queens­land and North West­ern New South Wales.

Respond­ing to the etiel­la threat with­out react­ing pre­ma­ture­ly and cre­at­ing more pest prob­lems.
It is unclear whether the etiel­la pop­u­la­tion will remain high and pose a major threat to the sum­mer crop. Depart­ment of Agri­cul­ture, Fish­eries and Forestry (DAFF) ento­mol­o­gist Hugh Brier and Tech­ni­cal Offi­cer Adam Quade have recent­ly (10 Feb­ru­ary) mon­i­tored soy­bean crops in the Branchview region, and mung­bean crops (rang­ing from veg­e­ta­tive to ear­ly pod fill) in the Bowenville and Jim­bour regions. This mon­i­tor­ing work includ­ed crops close to ear­li­er-infest­ed spring crops (now har­vest­ed). No sig­nif­i­cant or obvi­ous etiel­la dam­age or moth activ­i­ty was observed in these crops and unlike in 2014, etiel­la activ­i­ty in Jan­u­ary sam­ples from the Branchview soy­beans was extreme­ly low. One point that needs to be made is that crops in the veg­e­ta­tive and flowering/early pod­ding stages are at much low­er risk of ‘sur­prise attack’ than crops at the late pod­fill stage. This is because the ear­ly signs of dam­age are man­i­fest­ed by dead or dying axil­liary buds, where­as in pod­ding crops, the pest’s pres­ence is often not detect­ed until lar­vae exit the pods just pri­or to pupa­tion.

Etiella moth - 12 mm long

Etiel­la moth — 12 mm long

An ear­ly indi­ca­tion of etiel­la activ­i­ty in mung­beans pri­or to budding/flowering is the num­ber of moths vis­i­ble in the canopy. If num­bers are high, the moths will be eas­i­ly vis­i­ble fly­ing in the crop as you walk through. Etiel­la moths are small, slen­der and grey with very pro­nounced, for­ward pro­ject­ing mouth­parts. The moths come to rest with their wings fold­ed back along the body. The forewing has a dis­tinc­tive white stripe along the lead­ing edge (see image). A sweep net can be used to col­lect moths from the canopy for iden­ti­fi­ca­tion.

The oth­er indi­ca­tor of etiel­la activ­i­ty is dam­age to the plant’s axil­liary buds (the pre­cur­sor to the flo­ral buds). Dam­aged buds turn pale grey or straw in colour, and may be cov­ered with fine web­bing and frass.

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

As the flo­ral bud­ding stage approach­es, be on the look­out for sim­i­lar dam­age caused by bean pod­bor­er, the web­bing and frass of which will be coarser/larger. Bean pod­bor­er have already been observed in spring crops in Cen­tral Queens­land, and may con­tin­ue to be active if favourable (rainy) grow­ing con­di­tions are expe­ri­enced for the remain­der of the mung­bean grow­ing sea­son. Lar­vae of the two species, etiel­la and pod­bor­er, are quite dif­fer­ent, the for­mer being green/cream with pink stripes, and the lat­ter very pale cream with numer­ous black spots.

Etiella damaged mungbean pod with frass inside

Etiel­la dam­aged mung­bean pod with frass inside

 

 

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.

Options and con­sid­er­a­tions for min­imis­ing etiel­la dam­age.

In south­ern Aus­tralia where etiel­la is a major pest of lentils, it is man­aged by spray­ing syn­thet­ic pyrethroids to kill moths when they are active in the crop. The tim­ing of these treat­ments is guid­ed by a mod­el that pre­dicts the start of etiel­la activ­i­ty in spring. The syn­thet­ic pyrethroids pro­vide short-lived knock­down of moths in the crop.
In a mung­bean crop, the pro­phy­lac­tic use of syn­thet­ic pyrethroids increas­es the risk of flar­ing heli­cov­er­pa and mites because ben­e­fi­cial insects are killed off. Be aware that syn­thet­ic pyrethroids pro­vide lim­it­ed con­trol of mirids because they have knock­down effect, but no resid­ual con­trol. The effi­ca­cy of syn­thet­ic pyrethroids on heli­cov­er­pa will also be lim­it­ed by the lev­el of resis­tance and size of lar­vae in the crop.
Where Alta­cor® is used for heli­cov­er­pa and/or pod­bor­er con­trol in mung­beans, it might also be expect­ed to give con­trol of etiel­la lar­vae as they hatch and enter buds, flow­ers or green pods. The effi­ca­cy of Alta­cor® against etiel­la is cur­rent­ly being inves­ti­gat­ed by Hugh Brier. Alta­cor® has proven to be very effec­tive against pod­bor­er feed­ing inside mung­bean buds and flow­ers, so it is rea­son­able to pre­dict it will also be effec­tive against etiel­la lar­vae feed­ing inside buds and flow­ers.
How­ev­er, once etiel­la are inside pods, DAFF tri­als and mon­i­tor­ing of etiel­la-infest­ed spring crops sug­gest Alta­cor® (and most like­ly any oth­er prod­uct) will have no impact on this pest. In con­trast, tri­als sug­gest heli­cov­er­pa in pod­ding crops will be con­trolled. This is because small lar­vae can­not pen­e­trate pods and feed ini­tial­ly on buds/flowers, while larg­er lar­vae tend to move in and out of pods and are thus exposed to more insec­ti­cide.
If Alta­cor® proves to be effec­tive in pre­vent­ing the estab­lish­ment of etiel­la lar­vae in flow­ers and pods, it will pro­vide a more tar­get­ed and less dis­rup­tive option than pro­phy­lac­ti­cal­ly spray­ing crops to kill moths with syn­thet­ic pyrethroids.

Etiella damaged mungbean.

Etiel­la dam­aged mung­bean.

Etiel­la seed dam­age, qual­i­ty issues and thresh­olds

In late-pod­ding mung­beans, etiel­la can par­tial­ly dam­age as many as 4–5 seeds per pod. Many of these seeds are car­ried through into loads of har­vest­ed seeds deliv­ered to the pack­ing sheds. This results in grade outs and qual­i­ty down­grades. In late pod­ding soy­beans the sit­u­a­tion is sim­pler, as the major­i­ty of lar­vae eat only one seed, equat­ing to about 0.2g/larva grain loss. As seeds are near­ly total­ly con­sumed, the seed rem­nants are lost at har­vest and the pest’s impact is most­ly con­fined to yield loss. Thresh­old cal­cu­la­tions for soy­beans sug­gest the­o­ret­i­cal thresh­olds are very high at 35–40 lar­vae per square metre, assum­ing lar­vae can be killed. The catch is that they won’t be killed once they are inside pods, so any thresh­olds in pod­ding crops will have to be based on moth activ­i­ty – watch this space.