Reports of medium to large Helicoverpa caterpillar survivors in Bollgard II cotton crops in the Emerald Irrigation Area are being investigated by Queensland's Department of Primary Industries and Fisheries and the Cotton CRC.
DPI&F/Cotton CRC cotton extension officer, Susan Maas, said surviving larvae had been collected and tested but there was no evidence of any Bt resistance.
"We are continuing our quest for answers after up to after up to 1.5 medium to large larvae per square metre were reported in December and all affected fields were at the mid- to late-flowering stage of crop growth," she said in a statement from the DPI&F.
The DPI&F says that of the 85 Helicoverpa
larvae collected from the Bollgard II plants at Emerald, 31 were Helicoverpa puntigera and 54 were Helicoverpa armigera
but there is no evidence to suggest that either species may be differentially surviving.
"Moths from a sample of Helicoverpa
eggs collected last month have been submitted for second generation Bt resistance," Ms Maas said.
"While scientists have identified moths carrying the resistance gene, they have found no evidence of resistance in the field."
Ms Maas said it was possible to test plants for the presence or absence of the Bt genes.
"One theory is that at the peak flowering stage, there could be a dip in expression of the plant gene conferring the toxin," she said.
The threshold for Bollgard II cotton is the same as for Ingard cotton, and equates to two larvae greater than 2mm/metre in two consecutive checks or 1 larvae greater than 8mm/m.
Further research on the thresholds for Bt susceptible Helicoverpa
survivors in Bollgard II was being undertaken this season by Cotton CRC scientist Boaqain Lu.
Throughout the flowering period at St George this season, CSIRO researcher Sharon Downs is monitoring levels of expression for the Bt genes.
Ms Maas said the 2008-09 Central Highlands cotton planting of 12,000 hectares was generally looking good.
"Unfortunately, there is likely to be significant boll damage on the Bollgard II crops impacted by the Helicoverpa
survivors as the plants were at the flowering cut out stage with no compensatory boll set," Ms Maas said.
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