Yield, development, and quality response of dual-toxin Bt cotton to Helicoverpa spp. infestations in Australia Baoqian Lu 1,2,3§ , Sharon Downes 2,3 *, Lewis Wilson 3,4 , Peter Gregg 1,3 , Kristen Knight 5 , Greg Kauter 6 & Bruce McCorkell 7 1 School of Rural Science and Agriculture University of New England, Armidale, NSW 2351, Australia, 2 CSIRO Ecosystem Sciences, Australian Cotton Research Institute, Locked Bag 59, Narrabri, NSW 2390, Australia, 3 Cotton Catchment Communities Cooperative Research Centre, Australian Cotton Research Institute, Locked Bag 1001, Narrabri, NSW 2390, Australia, 4 CSIRO Division of Plant Industry, Australian Cotton Research Institute, Locked Bag 59, Narrabri, NSW 2390, Australia, 5 Monsanto Australia Research Laboratory, PO Box 92, Harlaxton, Toowoomba, QLD 4350, Australia, 6 Cotton Australia Suite 4.01, 247 Coward Street, Mascot, Sydney, NSW 2020, Australia, and 7 NSW Department of Primary Industries, Tamworth Agricultural Institute, 4 Marsden Park Road, Calala, NSW 2340, Australia Accepted: 18 June 2012 Key words: Helicoverpa armigera, Helicoverpa punctigera, Bollgard II ® , cotton yield, fibre quality, cotton maturity, Lepidoptera, Noctuidae, Malvaceae Abstract To verify current thresholds for Bollgard II ® cotton in Australia, the impact of Helicoverpa spp. (Lepi- doptera: 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 À1 of Helicoverpa armigera (Hu ¨bner) 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 À1 at several plant stages did not sig- nificantly 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. Introduction Transgenic insecticidal plants have revolutionized agricul- ture by providing opportunities for pest control with reduced reliance on insecticide sprays (Weil, 2005; Fitt, 2008). However, this advantage brings new challenges relating to threshold levels used to determine whether treatment for control of any surviving target pests on transgenic plants is necessary. This decision-making can be complicated by apparent temporary breakdowns of toxin efficacy at certain times such that transgenic insecti- cidal plants can act like non-transgenic plants in terms of insect control (Fitt et al., 1998; Greenplate et al., 1998; Adamczyk et al., 2001; Olsen et al., 2005). Some of the most successful examples of transgenic insecticidal crops involve insertions of proteins from the soil bacterium, Bacillus thuringiensis Berliner (Bt) (Bene- dict & Ring, 2004). A first-generation one-toxin (Cry1Ac) Bt-cotton known as Ingard ® (Bollgard in USA) was intro- duced in Australia during the 1995–1996 season (Fitt & Wilson, 1999). Young plants provided excellent control of the primary pests in cotton, Helicoverpa punctigera Wallengren and Helicoverpa armigera (Hu ¨bner) (Lepido- ptera: Noctuidae), but a decline in toxin expression in older plants meant that later in the growing season larvae survived (Fitt, 2008). Thus, in terms of insect management *Correspondence: Sharon Downes, CSIRO Ecosystem Sciences, Australian Cotton Research Institute, Locked Bag 59, Narrabri, NSW 2390, Australia. E-mail: Sharon.Downes@csiro.au § Current address: Ministry of Agriculture, Chinese Academy of Trop- ical Agriculture Sciences, Environment and Plant Protection Institute, Key Laboratory of Monitoring and Control of Tropical Agricultural and Forest Invasive Alien Pests, Danzhou, Hainan 571737, China © 2012 The Netherlands Entomological Society Entomologia Experimentalis et Applicata 1–10, 2012 Entomologia Experimentalis et Applicata © 2012 The Netherlands Entomological Society 1 DOI: 10.1111/j.1570-7458.2012.01313.x