Multiline seed mixtures in Transgenic cotton ( Gossypium hirsutum L.): an alternative refuge strategy for managing insect resistance in Bt varieties

In straight varieties, where conventional refuge deployment is difficult, insect resistance management (IRM) becomes essential for sustaining Bt cotton (Gossypium hirsutum L.). The present study evaluated a multiline seed-mixture approach as a built-in refuge strategy for managing insect resistance in cotton. Four phenotypically similar cotton lines differing in Bt background: non-Bt H07-1 (10%), Cry1Ac (25%), Cry2Ab (25%), and Cry1Ac + Cry2Ab (40%) – were mixed to constitute multiline generations MLT-G1 and MLT-G2. The lines were evaluated through three seasons for agronomic traits, field pest reaction, and boll damage. Component proportions were verified by ELISA using 900 plants per generation and compared with the intended 10:25:25:40 ratio using Chi-square analysis. Boll damage caused by cotton bollworm (Helicoverpa armigera) and pink bollworm (Pectinophora gossypiella) was also evaluated under replicated laboratory infestation. The non-Bt refuge component was maintained at 8.22% in MLT-G1 and 8.78% in MLT-G2, with no significant deviation from the intended ratio. The multiline generations showed lower pink bollworm and boll damage than the non-Bt parent. MLT-G1 and MLT-G2 recorded mean single-plant yields of 86.26 and 89.09 g, respectively, while major fibre traits remained relatively consistent. Overall, the findings demonstrate the potential of multiline seed mixtures to maintain a non-Bt refuge component while providing acceptable agronomic performance and reduced boll damage. The approach may offer a practical built-in refuge option for straight Bt cotton varieties and high-density planting systems.