Molecular Entomology 2024, Vol.15, No.5, 200-208 http://emtoscipublisher.com/index.php/me 201 resistance development, such as intercropping Bt and non-Bt cotton or using crop refuges, aiming to deepen the understanding of effectively integrating genetically modified crops into sustainable agricultural practices for long-term productivity and environmental safety. 2 Biology and Behavior of Cotton Bollworms 2.1 Life cycle of cotton bollworms Cotton bollworms, particularly Helicoverpa armigera and Pectinophora gossypiella (pink bollworm), undergo a complete metamorphosis, which includes four stages: egg, larva, pupa, and adult. The life cycle duration varies depending on environmental conditions, particularly temperature and food availability. Eggs are laid on the host plants, and larvae hatch within a few days to feed on the plant tissues. The larval stage, lasting two to three weeks, is the most destructive, as the larvae burrow into buds and bolls. The pupal stage is completed in the soil, lasting approximately 7 to 10 days before the adult moth emerges to continue the cycle (Tabashnik and Carrière, 2019). 2.2 Feeding patterns and damage to crops Bollworms cause significant damage during the larval stage, with larvae feeding on the cotton bolls, squares (flower buds), and leaves. Helicoverpa armigera prefers to feed on the reproductive parts of the plant, leading to direct damage to cotton lint and seeds. Infestations can lead to over 50% yield loss if not properly managed. Larvae also create entry points for secondary infections by fungi and bacteria, compounding the damage. Bt cotton varieties expressing Cry proteins have been effective in reducing bollworm feeding, but the development of resistance has become a concern in recent years (Shera and Arora, 2016). 2.3 Reproductive strategies and population dynamics The reproductive capacity of cotton bollworms is high, with adult females laying several hundred eggs over their short lifespan of approximately one week. This high reproductive rate contributes to rapid population growth, particularly in favorable environmental conditions. Bollworm populations tend to spike during periods of high temperature and humidity, which provide optimal conditions for larval development. In Bt cotton fields, reduced population growth has been observed, but the evolution of resistance to Cry proteins in certain regions has allowed bollworm populations to resurge, particularly in India and China (Naik et al., 2021). 2.4 Adaptations to various environmental conditions Cotton bollworms exhibit a remarkable ability to adapt to diverse environmental conditions. Their capacity to survive in both arid and humid climates, as well as to develop resistance to chemical insecticides and Bt toxins, has made them a persistent pest across the globe. Larvae can tolerate a range of temperatures and humidity levels, and adult moths are capable of long-distance migration, which allows for rapid spread in favorable conditions. In Bt cotton systems, bollworms have developed genetic resistance mechanisms, such as mutations in the cadherin gene (PgCad1), which confers resistance to Cry1Ac in pink bollworms (Fabrick et al., 2023). 3 Genetically Modified Crops for Cotton Bollworm Management 3.1 Bt cotton: Mechanism of action and development Bt cotton, a genetically modified variety, expresses Bacillus thuringiensis (Bt) proteins, specifically Cry proteins, which are toxic to lepidopteran pests like cotton bollworms. These Cry proteins bind to specific receptors in the gut cells of the larvae, causing cell lysis and eventually death (Li, 2024). Bt cotton was first introduced in the mid-1990s, and different varieties have since been developed, such as Bollgard I and Bollgard II, which express different combinations of Cry toxins (Cry1Ac and Cry2Ab). This innovation has revolutionized pest control, significantly reducing the reliance on chemical insecticides and enhancing crop protection (Tabashnik and Carrière, 2019). 3.2 Resistance management in Bt cotton Despite its effectiveness, Bt cotton faces challenges due to the evolution of resistance in pests like the pink bollworm (Pectinophora gossypiella). Resistance management strategies include planting non-Bt refuges, rotating crops, and employing Integrated Pest Management (IPM) approaches to delay resistance. In India, where pink bollworm resistance to Cry1Ac and Cry2Ab in Bollgard II has been documented, IPM practices such as reducing
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