To verify current thresholds for Bollgard II® cotton in Australia, the impact of Helicoverpa spp. (Lepidoptera: Noctuidae) larvae on yield, development, and quality under various infestation intensities and durations, and stages of growth, was tested using small plot field experiments over two seasons. Infestation with up to 80 eggs m&amp;#8722;1 of Helicoverpa armigera (Hübner) and Helicoverpa punctigera Wallengren showed that species, infestation level, and stage of growth had no significant effect on yields of seed-cotton or lint and on maturity and fibre quality. The duration of infestation of white flowers with H. punctigera neonates (maximum of every day for up to 4 weeks) had no impact on the yield of seed-cotton or lint, maturity, and fibre quality, but when 100% of flowers were infested (compared with 0 or 50%), seed-cotton and lint yields were significantly reduced and maturity was delayed. Infestation with up to 18 medium H. armigera larvae m&amp;#8722;1 at several plant stages did not significantly affect yields of seed-cotton and lint, maturity, and fibre quality. A heliocide spray applied on a commercial farm at the current threshold resulted in a significantly higher lint yield, compared with a farm where no spray was applied. In conclusion, Bollgard II® cotton is highly resistant to Helicoverpa spp. infestation.