Is the threshold for 'Helicoverpa' spp. larvae in Bollgard II adequate?

Abstract

We investigated if Helicoverpa larvae surviving in Bollgard II cotton crops could cause economic damage, with the aim of refining thresholds. We stocked cotton with a range of densities of eggs, very small or medium-sized larvae to create naturally occurring damage but even at extremely high levels of pressure there were few survivors. We therefore inflicted artificial damage at intensities and distributions which mimicked those in Bollgard II fields with survivors, and also at levels higher than this. We also evaluated the effect on yield of 'surviving' larvae in two commercial fields. Artificial damage experiments showed that, when damage was inflicted early in flowering, there was no effect on yield but the loss of 50 or 100 per cent of squares delayed maturity by between five and 10 days. When damage was inflicted at peak flowering, yields were significantly reduced only when 100 per cent of squares were removed from all plants, either alone or in combination with 30 per cent boll damage. When damage was inflicted late in flowering, only the highest levels of damage intensity that we inflicted (30 per cent bolls damaged across 50 per cent or all plants) reduced yield. The commercial field experiments showed that when larvae were not controlled at a density of three per metre (3 X the current threshold) yield was reduced but if they were controlled at this level there was no yield loss. In conclusion, simulated pest damage showed that only the heaviest damage treatments, at each stage, would be expected to reduce yield. These levels of damage were in excess of any damage we observed in surveys of commercial fields. Similarly, if larvae at three times the threshold were controlled promptly no yield loss occurred-hence it is likely that the much lower current threshold of one medium larvae per metre for Bollgard II is conservative.