How do susceptible 'Helicoverpa' larvae behave on Bollgard II?

Shortly after the widespread adoption of Bollgard II cotton in Australia there were reports that Helicoverpa larvae were surv1vmg in these crops at levels that exceeded the recommended threshold levels for control. In response to these reports, Crop Consultants Australia members were surveyed for data from the 2005-06, 2006-07, and 2007-08 seasons (see the June 2009 issue of The Australian Cottongrower for a full report). This work showed that in any year an average of 15 per cent of the Bollgard U area planted carried larvae at above threshold levels, and that survivors were present in some fields in all cotton growing regions. Survival always occurred during flowering, and could follow low or high egg pressure but was always within the range of one to three larvae per metre. Within a region. survivors were localised to particular fields, and within a field the distribution of larvae could be uniform or patchy. But in general only between 20-60 per cent of plants in a field sustained damage from surviving larvae. Mediumsized larvae were most commonly found on bolls, but were also found in disproportionately large numbers on flowers of Bollgard U cotton. These have relatively low levels of Bt in their components.