Bt Research 2025, Vol.16, No.4, 168-181 http://microbescipublisher.com/index.php/bt 177 caused by sporadic resistant bollworms until around 2010, and it was basically limited to areas where asylum was not implemented properly. When pink cotton bollworms showed signs of resistance to the first generation of Cry1Ac cotton, the United States promptly introduced two-valent Bt cotton (Bollgard II) containing Cry2Ab and gradually phased out the unit price varieties. The popularity of bivalent Bt cotton has greatly delayed the progress of resistance, and the frequency of resistance individuals has decreased for a while (Tabashnik et al., 2012). In addition, American researchers have supplemented the comprehensive strategy of releasing sterile insects: breeding and releasing irradiated sterile male cotton bollworms on a large scale, and sterile offspring after mating with wild insects, and lowering the insect mouth from the population ecology level. This biological control measure was jointly applied with Bt cotton and successfully eradicated pink cotton bollworms in Arizona. 7 Ecological and Environmental Impact Assessment 7.1 Effect on non-target insects (such as natural enemies and pollinator insects) Whether Bt transgenic crops affect non-target organisms (especially beneficial insects) is one of the focus of public and regulators. A large number of studies and environmental release test results show that currently commercialized Bt crops have no direct adverse effects on most non-target arthropods. Bt toxic proteins have a high target specificity. For example, Cry1 toxins only act on the midgut receptors of specific Lepidopteran insects and are not toxic to predatory ladybugs, spiders, etc. due to the lack of corresponding receptors. Field comparison survey also supports this conclusion: there is no significant difference in the diversity and abundance of non-target insects such as predatory spiders and parasitic wasps in Bt and ordinary cotton fields. A study on Cry1Ah gene-resistant corn was used to monitor arthropod communities at soil surface and plant levels in the fields. It was found that the population richness and diversity index were basically the same as non-Bt corn fields, and there was no large reduction in a certain type of non-target insects due to Bt corn cultivation. In years of field experiments in Hubei, China, no obvious difference was found in comparison of community structures such as spiders and natural enemies of Bt rice fields and conventional rice fields. It should be pointed out that the indirect ecological effects of Bt crops mainly come from the beneficial effects brought about by the reduction of pesticide application. For example, because Bt cotton fields have almost no wide-spectrum insecticide spray, the number of natural enemies such as ladybugs and squids has increased significantly compared with conventional cotton fields, thus reducing the density of non-target pests such as cotton aphids (Ma et al., 2022). This shows that planting Bt crops is more friendly to non-target organisms than spraying, and is conducive to the development of the biological control system. 7.2 Stability and environmental risks of Bt toxins in soil During the growth process of Bt transgenic crops, part of the Bt toxic protein produced by them will enter the soil environment, such as through root secretion and decomposition of fallen leaves. Therefore, the retention and impact of Bt toxic protein in soil are also important contents of environmental safety assessment. Studies have shown that after entering the soil, Bt toxic protein will be rapidly adsorbed by soil particles and organic matter, and its activity will gradually decrease and will eventually be degraded by indigenous microorganisms. In most soils, the half-life of Bt protein varies from days to weeks, depending on factors such as soil type, temperature and humidity, and microbial activity (Liu et al., 2020). Under warm, humid and microbial active conditions, Bt protein degrades faster; it may last a little longer in cold or high organic matter soils, but generally does not accumulate for a long time. Several field tests have tested the content of Bt protein in soils grown by Bt corn and Bt cotton. As a result, trace amounts can often be detected during the crop growth season, but they will drop to undetectable levels within a few weeks to one or two months after harvest. 7.3 Long-term effects on agricultural ecosystems To evaluate the ecological and environmental impact of Bt genetically modified crops, long-term and large-scale effects need to be comprehensively considered. In an advantageous way, Bt crops promote pesticide reduction and biodiversity conservation, which are considered to contribute to the sustainability of agroecosystems. In areas where Bt cotton and Bt corn are grown for a long time, non-target organisms such as birds and bees are more abundant in the field. This is because pesticide pressure is reduced and food resources and habitat environment are
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