Diapause tool to identify helicoverpa risk

While there was a hefty pres­ence of corn ear­worn or cot­ton boll­worm, Heli­cov­er­pa armigera, in the ear­ly and mid­dle part of the 2007-08 sea­son, pest activ­i­ty has declined in recent weeks and for the most part they appear to pose no major risk.

What is dia­pause?
This is the time of the year when a pro­por­tion of mature lar­vae going to ground to pupate enter a hiber­na­tion phase termed dia­pause or over­win­ter­ing. This dor­man­cy strat­e­gy allows the pest to sur­vive the win­ter months in tem­per­ate regions when host plants are scarce and tem­per­a­tures are gen­er­al­ly too low to allow suc­cess­ful devel­op­ment. The trig­gers to enter dia­pause are decreas­ing daylength and tem­per­a­ture as expe­ri­enced dur­ing late sum­mer and autumn.

Pic­ture of heli­cov­er­pa pupa in earth­en cell.

The pro­por­tion of pupae enter­ing dia­pause increas­es from low lev­els in March, to high lev­els, almost 100%, by late April. The rate of dia­pause induc­tion varies from sea­son to sea­son, and region to region. Know­ing when dia­pause is induced is use­ful for iden­ti­fy­ing ‘high risk’ fields i.e. those fields most like­ly to have dia­paus­ing pupae.

A web tool is avail­able on the Cot­tas­sist web­site to help cal­cu­late the like­ly rate of dia­pause induc­tion for your area, based on local cli­mate data. The tool is also able to com­pare the results for the cur­rent sea­son with the long term aver­age and hot­ter than aver­age and cool­er than aver­age sea­sons.

How are dia­paus­ing pupae con­trolled?
Over­win­ter­ing pupae are very impor­tant because they con­tribute to the spring pop­u­la­tion and may take with them the resis­tance genes enabling them to tol­er­ate con­ven­tion­al insec­ti­cides and the Bt trans­genic tox­ins found in Boll­gard II®.

It is for this rea­son that full soil sur­face cul­ti­va­tion to 10 cm depth (also known as pupae bust­ing) is so impor­tant. When car­ried out prop­er­ly, pupae bust­ing can reduce sur­vival of over­win­ter­ing pupae to less than 5%.

Pupae bust­ing is manda­to­ry for all Boll­gard II® fields; it is a require­ment of the Boll­gard II® licence.

Some relax­ation of pupae bust­ing require­ments has been intro­duced for con­ven­tion­al cot­ton fields. Sprayed con­ven­tion­al cot­ton crops defo­li­at­ed after 9 March are more like­ly to har­bour insec­ti­cide resis­tant H. armigera pupae and should be pupae bust­ed as soon as pos­si­ble after pick­ing and no lat­er than the end of August.

The same holds true for the major­i­ty of grain crops on the Downs. Crops that were mature on or before 9 March are unlike­ly to har­bour over­win­ter­ing pupae. Con­ser­va­tion tillage (zero till or min-till) can be con­fi­dent­ly used on these fields. Late sea­son grain crops that could sup­port devel­op­ment of lar­vae after 9 March are ‘high risk’ of har­bour­ing over­win­ter­ing pupae, and each field should be judged on its mer­its. The deci­sion on whether to pupae bust will be influ­enced by the den­si­ty of lar­vae present in the crop, the age of the lar­vae, and the tim­ing of their pres­ence in the crop.

Par­a­sites will also influ­ence sur­vival of over­win­ter­ing pupae, with esti­mates from a 5 year study on the Dar­ling Downs show­ing 44% of over­win­ter­ing pupae were par­a­sitised. The two-toned cater­pil­lar par­a­site, Het­ero­pel­ma scapo­sum, was the most abun­dant par­a­site species record­ed.

What is the cur­rent sea­son­al out­look for dia­pause in cot­ton and grain crops?
Out­put for the cur­rent sea­son at Dal­by, which is cool­er than aver­age, indi­cates a high­er than aver­age pro­por­tion of pupae have entered dia­pause.

All cot­ton crops, and a large pro­por­tion of the grain crops on the Downs, and in most oth­er areas, will be har­vest­ed lat­er than mid March. This means it is worth check­ing and record­ing whether these crops are host­ing lar­vae that will dia­pause. Crops deter­mined to be ‘high risk’ will war­rant pupae bust­ing.