Bt Research 2024, Vol.15, No.2, 87-95 http://microbescipublisher.com/index.php/bt 88 2 Overview of Bt Toxins 2.1 Types of Bt toxins Bacillus thuringiensis (Bt) produces a variety of insecticidal proteins, primarily Cry and Cyt toxins, which are used in genetically modified (GM) crops to target specific insect pests. The most commonly used Bt toxins in agriculture include Cry1Ab, Cry1Ac, Cry2Ab, Cry3Bb1, and Cry3Aa. These toxins are highly specific to certain insect orders, such as Lepidoptera (moths and butterflies) and Coleoptera (beetles) (Federici, 2003; Kostov et al., 2014; Krogh et al., 2020). 2.2 Mode of action in target organisms Bt toxins function by binding to specific receptors in the gut cells of target insects, leading to cell lysis and death. When ingested by susceptible insects, the Cry proteins are solubilized and activated in the alkaline environment of the insect midgut. The activated toxins then bind to receptors on the midgut epithelial cells, forming pores that disrupt cell integrity, causing the insect to stop feeding and eventually die (Naranjo, 2009; Schrijver et al., 2015). This mode of action is highly specific, which is why Bt toxins are considered safe for non-target organisms, including humans and other vertebrates (Federici, 2003; Schrijver et al., 2015). 2.3 Use in agricultural practices Bt crops, such as Bt cotton and Bt maize, have been widely adopted in agriculture to control major insect pests, reducing the need for chemical insecticides. These crops express Bt toxins throughout their tissues, providing continuous protection against pests. The adoption of Bt crops has led to significant reductions in insecticide use, which has environmental and economic benefits (Federici, 2003; Naranjo, 2009; Kostov et al., 2014). For instance, Bt maize and Bt cotton have been shown to reduce insecticide applications by millions of kilograms globally, contributing to more sustainable agricultural practices. Additionally, Bt crops have been found to have minimal adverse effects on non-target invertebrates compared to conventional insecticide treatments (Marvier et al., 2007; Wolfenbarger et al., 2008; Yang et al., 2017). 3 Non-Target Invertebrates: Categories and Roles 3.1 Pollinators The ecological impact of Bacillus thuringiensis (Bt) crops on non-target invertebrates is a critical area of study, given the widespread adoption of these genetically modified organisms. Non-target invertebrates play essential roles in ecosystems, including pollination, soil health, aquatic systems, and biological control. This section studys the impact of Bt crops on various categories of non-target invertebrates. Pollinators are vital for the reproduction of many plants, including crops. The impact of Bt crops on pollinators has been a subject of concern, although studies specifically focusing on pollinators are limited. The general consensus is that Bt crops do not significantly affect pollinator populations. However, indirect effects through changes in plant-pollinator interactions or habitat alterations cannot be entirely ruled out and warrant further investigation. 3.2 Soil invertebrates Soil invertebrates, such as nematodes, earthworms, and mites, are crucial for nutrient cycling and soil structure. The impact of Bt crops on these organisms has been extensively studied. A systematic study and meta-analysis found no significant overall effect of Bt crops on soil invertebrate populations, although there was considerable variation among different orders of soil invertebrates (Shu et al., 2011; Krogh et al., 2020). Another study indicated that Bt rice did not affect nematode abundance or community composition but did enhance trophic connections within nematode communities, suggesting a complex interaction between Bt crops and soil ecosystems (Liu et al., 2018). 3.3 Aquatic invertebrates Aquatic invertebrates are essential for maintaining water quality and supporting aquatic food webs. The use of Bt var. israelensis (Bti) for mosquito control has raised concerns about its impact on non-target aquatic invertebrates. Long-term studies in French coastal wetlands have shown that Bti applications did not significantly affect the taxonomic structure or abundance of non-target aquatic invertebrate communities (Lagadic et al., 2014; Liu et al., 2018). These findings suggest that, when used according to recommended practices, Bti is environmentally safe for non-target aquatic invertebrates.
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