Bt Research 2024, Vol.15, No.2, 87-95 http://microbescipublisher.com/index.php/bt 89 3.4 Predators and parasitoids Predators and parasitoids are important for natural pest control. The impact of Bt crops on these functional guilds has been mixed. Meta-analyses have shown that Bt crops can reduce the abundance of certain parasitoids, particularly those that are specialists on target pests, while having less consistent effects on generalist predators (Dang et al., 2007; Wolfenbarger et al., 2008; Duan et al., 2009). Laboratory studies have generally been conservative, often predicting greater impacts than those observed in the field (Duan et al., 2009). This discrepancy highlights the need for comprehensive field studies to accurately assess the ecological risks of Bt crops. 4 Field Studies and Real-world Impacts 4.1 Long-term field monitoring Long-term field monitoring is essential to understand the real-world impacts of Bt crops on non-target invertebrates. Studies have shown that Bt crops can influence the abundance and diversity of non-target invertebrates over extended periods. For instance, a meta-analysis of 42 field experiments indicated that non-target invertebrates are generally more abundant in Bt cotton and Bt maize fields compared to non-transgenic fields managed with insecticides, although certain taxa were less abundant in Bt fields compared to insecticide-free control fields (Marvier et al., 2007). Another study conducted over four consecutive years in French coastal and continental wetlands found no immediate or long-term detectable effects of Bt formulations on the taxonomic structure and abundance of non-target aquatic invertebrate communities (Yaqoob et al., 2016; Zhou et al., 2020). These findings suggest that while Bt crops can have some impact on non-target invertebrates, the effects may vary depending on the specific taxa and environmental conditions. 4.2 Comparative studies with non-Bt areas Comparative studies between Bt and non-Bt areas provide valuable insights into the ecological impacts of Bt crops. The study and meta-analysis comparing soil invertebrates in Bt and conventional crop fields found no significant effect of Cry proteins on soil invertebrates across different orders (Wolfenbarger et al., 2008). Similarly, a meta-analysis on the effects of Bt cotton, maize, and potato on non-target arthropod functional guilds revealed no uniform effects, with insecticide use having a more significant impact than Bt crops themselves (Figure 1) (Dang et al., 2007; Li et al., 2022). In China, a meta-analysis of Bt rice showed that while parasitoid populations decreased slightly, detritivores increased, indicating that Bt rice poses negligible risks to non-target functional guilds in large-scale agroecosystems (Kostov et al., 2014). These comparative studies highlight the importance of considering both Bt and non-Bt areas to isolate the specific effects of Bt crops. 4.3 Observed ecological changes Field studies have documented various ecological changes associated with Bt crops. For example, a study on the decomposition of Bt maize residue found that Bt maize decomposed similarly to non-Bt maize in large mesh bags, but decomposed faster in small and medium mesh bags during winter, suggesting potential seasonal effects on decomposition processes (Zwahlen et al., 2007). Another study on the effects of Bt maize on non-target Lepidoptera reported adverse effects on caterpillars in both laboratory and field settings, although the overall risk to butterflies and moths remains inconclusive due to limited data and the need for more ecologically realistic experiments (Lang et al., 2016). Additionally, a study of Bt rice's impact on spiders, major predators in rice fields, indicated that Bt protein can transfer and accumulate in spiders, affecting their physiology and spreading to higher trophic levels (Yang et al., 2017). These observed ecological changes underscore the complexity of Bt crop impacts and the need for comprehensive, long-term studies to fully understand their ecological implications. 5 Mechanisms of Non-target Effects 5.1 Direct toxicity Direct toxicity refers to the immediate harmful effects of Bt crops on non-target invertebrates due to the ingestion or contact with Bt toxins. Laboratory studies have shown that exposure to high doses of Bt Cry proteins can reduce the survival rates of non-target Lepidoptera and other invertebrates (Duan et al., 2009; Marroquin et al., 2020). However, field studies often reveal a more nuanced picture. For instance, while laboratory tri-trophic
RkJQdWJsaXNoZXIy MjQ4ODYzNA==