Seedling thrips in spring mungbean crops

There have been sev­er­al reports of seedling thrips in spring mung­bean crops in the Goondi­win­di region. Seedling thrips (Thrips tabaci) are also known as win­ter cere­al thrips or cot­ton seedling thrips. The thrips move out of win­ter cere­als when these start to dry off into new green spring growth such as mung­bean, navy beans or cot­ton. Spring plant­ed crops, espe­cial­ly those in close prox­im­i­ty to matur­ing cere­al crops are at great­est risk. In most cas­es, ear­ly thrips dam­age does not trans­late to yield loss. How­ev­er if you fear an extend­ed thrips inva­sion and you do spray, please leave some unsprayed strips to deter­mine if the spray was worth­while.

Descrip­tion
Adult thrips are 2 mm long and are dark, cig­ar shaped and have nar­row feath­ery wings fold­ed along their back. Lar­vae are small­er, lack wings and are pale.

Adult seedling thrips. Image by Lewis Wilson

Adult seedling thrips.
Image by Lewis Wil­son

Dam­age symp­toms
Adult thrips can infest a seedling’s grow­ing point as soon as it emerges from the ground. In crack­ing soils, seedlings may even be infest­ed before they emerge. Lar­vae feed inside veg­e­ta­tive ter­mi­nals. Pop­u­la­tions typ­i­cal­ly peak with­in 4 weeks of plant emer­gence.
Thrips attack the seedlings grow­ing point and dam­age the embry­on­ic leaves. How­ev­er, in mung­bean, the dam­age is usu­al­ly not man­i­fest­ed until the first tri­fo­li­ate leaves open. How­ev­er in some of the Goondi­win­di crops, there also appears to be dam­age to the uni-foli­ate leaves. Dam­aged leaves can be severe­ly dis­tort­ed and dis­coloured and may resem­ble her­bi­cide (2,4-D) dam­age.

In most cas­es thrips dam­age is large­ly cos­met­ic and will not com­pro­mise yield or matu­ri­ty. In a DAFF tri­al on the Downs, seedling thrips pop­u­la­tions peaked at over 6 per plant ter­mi­nal at 9 days after emer­gence (DAE), but crashed to less than 0.5 per plant by 37 DAE. Dam­age symp­toms on new­ly expand­ed tri­fo­li­ate leaves in untreat­ed con­trol plots peaked at severe lev­els at 16 DAE but crashed to zero in new tri­fo­li­ate leaves by 37DAE as the crop ‘grew away’ from the ear­ly dam­age. In this tri­al and a sim­i­lar­ly-infest­ed crop at Kingaroy, thrips had no effect on yield or plant matu­ri­ty (i.e. on time to flow­er­ing or har­vest), despite seedlings dis­play­ing severe leaf dis­tor­tion.

Seedling thrips damage on the first true leaves in spring mungbeans at Goondiwindi. Image by Andrew Walker

Seedling thrips dam­age on the first true leaves in spring mung­beans at Goondi­win­di.
Image by Andrew Walk­er

Seedling thrips damage to tri-foliate leaves in spring mungbean

Seedling thrips dam­age to tri-foli­ate leaves in spring mung­bean. Image by Hugh Brier

How­ev­er if thrips from sur­round­ing cere­al crops keep invad­ing mung­bean over an extend­ed peri­od of time, they could pos­si­bly impact on plant growth and ulti­mate­ly yield.

Man­ag­ing thrips
In cold springs, where the mean tem­per­a­ture is below 18°C, slow plant growth and stunt­ing is often wrong­ly attrib­uted to thrips. Cool weath­er can how­ev­er exac­er­bate thrips dam­age while warm weath­er will help plants grow away from the dam­age hence the risks are high­er in the cool­er regions.

Mon­i­tor for thrips by check­ing the plant’s grow­ing points. A good hand lens should be used as the pest is very small. Grow­ing points can be sub­merged in alco­hol to dis­lodge the thrips.

There are no thresh­olds for seedling thrips in mung­bean. Thresh­olds in seedling cot­ton of 10 thrips (adult and lar­vae) and 80% dam­age to leaves may pro­vide a guide. Remem­ber that thrips dam­age to new leaves is inflict­ed in the ter­mi­nal before they emerge and expand. So spray­ing spray now won’t undo cur­rent dam­age but would pre­vent future dam­age. How­ev­er the pesticide’s impact may not be obvi­ous imme­di­ate­ly, as leaves yet to emerge may have already been dam­aged.

Before imple­ment­ing any chem­i­cal con­trol, con­sid­er that thrips are also impor­tant preda­tors of spi­der mites, oth­er thrips and small eggs.

Thrips can be con­trolled with dimethoate at a rate of 800mL/ha, under per­mit PER13155. Apply a nar­row band spray over the seedlings to reduce the impact on preda­tors such as spi­ders in the inter-row. In crops where there is an edge effect (more dam­age clos­est to the cere­als), con­sid­er only spray­ing the severe­ly dam­aged pro­por­tion of the crop.

Seedling thrips leaf dam­age can be great­ly reduced if dimethoate is applied with­in 3 days after plant emer­gence. Spray­ing after dam­age symp­toms are man­i­fest­ed is usu­al­ly too late to reduce dam­age symp­toms, unless thrips keep invad­ing from sur­round­ing cere­als over an extend­ed peri­od of time.

No seed dress­ings are cur­rent­ly reg­is­tered for thrips con­trol in mung­bean. If pos­si­ble, do not plant mung­bean crops adja­cent to win­ter cere­als. Avoid spring mung­bean plant­i­ngs in regions where cool spring weath­er is like­ly, as low tem­per­a­tures have a far greater impact on mung­bean growth than seedling thrips.

Mon­i­tor the out­come and com­pare with unsprayed strips
Where pos­si­ble, leave some unsprayed strips to see whether pes­ti­cide appli­ca­tion was real­ly need­ed and how crops recov­er from thrips dam­age. It would also be use­ful to pho­to­graph tagged plants over time in sprayed and unsprayed parts of the crop to ascer­tain whether there is a vari­a­tion in time of flow­er­ing and matu­ri­ty, and yield.

Please report any out­breaks to Hugh Brier on 0741 600 740 or 0428 188 069 or email: [email protected]

For infor­ma­tion about thrips in seedling cot­ton vis­it the beat sheet arti­cle on seedling thrips in cot­ton

Arti­cle by Kate Charleston and Hugh Brier