Don’t let lucerne crown borer overwinter in your soybean stubble

Once again, lucerne crown bor­er (LCB) (Zygri­ta diva) has been active in soy­beans this sea­son. Worst affect­ed regions were the North­ern Rivers and the Liv­er­pool Plains in NSW, but reports were also field­ed from the Bund­aberg, Fas­sifern and Dar­ling Downs regions in Queens­land. In some of the severe­ly affect­ed crops, over 80% of plants were infest­ed, and up to 70% gir­dled.

Girdling to soybean stem caused by lucerne crown borer (Joe Wessels, DAF). Note how the stem has browned off and died above the girdle.

Girdling to soy­bean stem caused by lucerne crown bor­er (Joe Wes­sels, DAF). Note how the stem has browned off and died above the gir­dle.

Girdling occurs when stem-bor­ing lar­vae near­ing pupa­tion ring­bark the plant’s stem inter­nal­ly, sev­er­ing the vas­cu­lar tis­sue. They then use this tis­sue to plug the stem’s pith canal, thus cre­at­ing a secure over­win­ter­ing cham­ber in the tap root.

While lar­val infes­ta­tion itself (tun­nelling in the pith) is unlike­ly to cause major yield loss­es, girdling is anoth­er mat­ter. If girdling occurs before pods are filled, there are obvi­ous yield loss­es. But loss­es can still occur when pods are ful­ly filled, as gir­dled stems are weak­ened and fre­quent­ly lodge, falling onto the ground where they are out of reach of har­vesters. Side branch­es can also lodge when their bases are weak­ened by lar­vae tun­nelling down to reach the main stem.

In gen­er­al, infes­ta­tion lev­els are high­er and girdling and lodg­ing more like­ly in ear­li­er (ear­ly Novem­ber) than lat­er (late Decem­ber) plant­ed crops. Girdling is also more like­ly when crops are drought stressed. This is because decreas­ing stem mois­ture effec­tive­ly mim­ics the pre-har­vest dry down stage, and lar­vae react accord­ing­ly by prepar­ing for over­win­ter­ing and pupa­tion.

Soybean stem with severe LCB damage (Natalie Moore NSW DPI)

Soy­bean stem with severe LCB dam­age (Natal­ie Moore NSW DPI)

Lucerne crown borer larva exposed in soybean tap root stubble (Natalie Moore NSW DPI)

Lucerne crown bor­er lar­va exposed in soy­bean tap root stub­ble (Natal­ie Moore NSW DPI)

Management options

Pes­ti­cide con­trol is dif­fi­cult because lar­vae feed inside the stem’s pith and can­not be reached by insec­ti­cide sprays, even ones with sys­temic activ­i­ty. Spray­ing the adults is also prob­lem­at­ic as crops are invad­ed over an extend­ed peri­od of time, and mul­ti­ple sprays would be required to give effec­tive con­trol. As well, even very low adult pop­u­la­tions can give rise sig­nif­i­cant lar­vae infes­ta­tion lev­els. A Kingaroy tri­al with less than 0.2 LCB adults per square metre had 45% of plants infest­ed and 30% of plants gir­dled at har­vest. Mul­ti­ple sprays would also great­ly increase the risk of flar­ing white­fly, mites and heli­cov­er­pa.

While seed treat­ments have poten­tial as a LCB man­age­ment option, they are not cur­rent­ly reg­is­tered and residue tri­als are required to deter­mine if the active ingre­di­ents are still present in the seeds at har­vest, or in plant tops where crop stub­ble is grazed. Addi­tion­al­ly, only prod­ucts that have no detri­men­tal impact on soy­bean nodu­la­tion would be suit­able.

This leaves cul­tur­al con­trol as the most viable cur­rent option. Strate­gies include:

1. Later plantings

Delay­ing plant­i­ng reduces the chances of LCB girdling plants before pods are filled, how­ev­er con­sult with your local agron­o­mist as poten­tial yields are some­times greater for ear­li­er plant­i­ngs and this must be bal­anced against the risk of greater LCB dam­age.

2. Targeted irrigation

Avoid­ing mois­ture stress is impor­tant as this can trig­ger pre­ma­ture girdling, which has been observed as ear­ly as Feb­ru­ary in recent hot sum­mers. Although not an option in dry­land crops, tar­get­ing an irri­ga­tion before the crop becomes stressed is an effec­tive pre­ven­tion strat­e­gy.

3. Control of alternate hosts

Con­trol­ling vol­un­teer soy­beans and weed hosts such as bud­da pea, ses­ba­nia and phasey bean is impor­tant to min­imise the pres­ence of alter­nate sources of LCB.

4. Strategic tillage

Using cul­ti­va­tion to bury and/or split the soy­bean stub­ble attacks the pest when it is most vul­ner­a­ble, i.e. when over­win­ter­ing in your pad­docks. Strate­gic tillage assist­ed by GPS steer­ing allows tillage to be con­fined to nar­row strips along the old plant rows. It is no coin­ci­dence that LCB activ­i­ty is great­est in zero till pad­docks. The bury­ing of the stub­ble to >10 cm also helps to sup­press soil borne dis­eases such as pho­mop­sis and char­coal rot, which are favoured by zero till.

Break­ing the life cycle by pre­vent­ing pupae from suc­cess­ful­ly over­win­ter­ing is an essen­tial step toward min­imis­ing the occur­rence of this pest.

Post-harvest soybean stubble (plant cut off near ground level) with the distinctive purple/orange frass made by LCB larvae. This stalk contained a LCB larva in the taproot below ground level. (Joe Wessels, DAF)

Post-har­vest soy­bean stub­ble (plant cut off near ground lev­el) with the dis­tinc­tive purple/orange frass made by LCB lar­vae. This stalk con­tained a LCB lar­va in the tap­root below ground lev­el. (Joe Wes­sels, DAF)

This work is forms part of the North­ern IPM Project (DAQ00196), fund­ed by GRDC.