Green vegetable bug (GVB) in late cotton

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There have been sev­er­al reports of GVB occur­ring in late cot­ton crops. This has led to some con­fu­sion in the indus­try as to whether GVB needs to be con­trolled at this late stage in cot­ton.  Research done by DEEDI ento­mol­o­gists has shown that younger bolls are more sus­cep­ti­ble to lint dam­age caused by GVB feed­ing. These fac­tors need to be con­sid­ered when mak­ing man­age­ment deci­sions regard­ing the con­trol of GVB.
 
When are bolls sus­cep­ti­ble to GVB dam­age?

Bolls aged 20 days and over (these bolls are hard and about ¾ inch­es in size) do not incur sig­nif­i­cant lint dam­age (brown coloured lint or tight lock) even if fed on by GVB. This means that even if GVB are present in the field, if the  youngest bolls are old­er than 20 days GVB con­trol is not war­rant­ed.

 Bolls younger than 7 days may shed as a result of feed­ing by GVB.              

Thresh­olds for GVB

Action thresh­olds for GVB in cot­ton are

  • 1 bug per metre when sam­pling with a beat­sheet
  • 0.5 bug per metre for visu­al sam­pling     

Par­a­sitism by Tri­chopo­da — do par­a­sitised GVB cause any dam­age?

Research shows that a GVB par­a­sitised by Tri­chopo­da can con­tin­ue to cause dam­age for up to 2 weeks after being par­a­sitised.

Tri­chopo­da gia­comel­li is an impor­tant nat­ur­al ene­my of GVB. This fly par­a­sitis­es main­ly adult GVB and in some cas­es late instar nymphs. Female Tri­chopo­da lay eggs on GVB and when the lar­vae hatch, they bur­row into the bug and feed on its inter­nal organs and body flu­ids. With­in 2 weeks, final-instar Tri­chopo­da lar­va emerges from the GVB and pupates in the soil.

Although Tri­chopo­da does not kill the GVB imme­di­ate­ly and dam­age can still occur, they do con­tribute to GVB mor­tal­i­ty and help reduce the size of fol­low­ing GVB gen­er­a­tions, poten­tial­ly reduc­ing the risk offu­ture dam­age.

              Par­a­sitised GVB adult           

 Which life stage of GVB caus­es max­i­mum dam­age?

Late instars nymphs (4th and 5th instars) cause sim­i­lar dam­age as the adults.   How­ev­er third instars will cause about half the dam­age as an adult while  1st and 2nd instars cause up to one third the dam­age.

 

 Oth­er fac­tors to con­sid­er when con­trol­ling GVB in late cot­ton

Good spray cov­er­age is a pre­req­ui­site to suc­cess­ful man­age­ment of GVB. Unfor­tu­nate­ly there are no selec­tive insec­ti­cides avail­able for the con­trol of GVB. Hence any chem­i­cal used to con­trol GVB will dis­rupt pop­u­la­tions of ben­e­fi­cial insects. This is an impor­tant con­sid­er­a­tion espe­cial­ly when sil­ver­leaf white­ly or aphids are also present in the field. Dis­rupt­ing the nat­ur­al ene­mies of these pests can lead to big­ger prob­lems such as lint con­t­a­m­i­na­tion.
 
Arti­cle by Moazzem Khan