Bt Research 2025, Vol.16, No.5, 204-213 http://microbescipublisher.com/index.php/bt 208 proven to compensate for the weakness of a single dose—BtI has a fast onset but a short lasting period, and Bs has a slightly slower effect but a long residual effect. Studies have shown that after BtI and Bs are combined in an appropriate proportion, the quick killing rate and effectiveness period of Culex mosquito larvae are significantly improved, both with rapid knockdown and long-term inhibition, which is better than any single bacterial agent (Guidi et al., 2013). In addition, Bt and other bacteria can also be combined through engineering strain construction. Scientists have successfully introduced the insecticidal gene of Bt into Bs, creating engineered strains that have both Bt broad-spectrum toxicity and maintain Bs environmental adaptability (Figure 2) for mosquito control (Geng et al., 2023). In addition to Bs, some soil bacteria such as Pseudomonas and other species of Bacillus are also used in complex preparations. Figure 2 Electron microscopic observation of crystals produced by strains during sporulation (Adopted from Geng et al., 2023) Image caption: Crystals of recipient L. sphaericus G725D0498 (a), donor B. thuringiensis TAND672 (b), and transconjugant BS-pTAND672-2 (c to d). Red and green arrows indicate toxins specifically produced by B. thuringiensis and L. sphaericus, respectively, while yellow arrows refer to the spore-coat formed by Bacillus isolates. Scale bars are 500 nm (Adopted from Geng et al., 2023) 5 Integration with Nematodes and Other Antagonistic Organisms 5.1 Differences in the mechanism of action between insect pathogen nematodes and Bt Insect pathogenic nematodes (EPN) mainly include Steinernema and Heterorhabditis, which are microscopic nematodes that can infect and kill a variety of soil pests. Their insecticidal mechanism is completely different from Bt. The infection-stage larvae of EPN can actively find hosts in the soil, and after entering the pest through the mouthpart or valve, they release symbiotic bacteria. These bacteria reproduce rapidly in the blood cavity of the insect body, causing sepsis to cause the host to die within several days. By contrast, Bt lacks the ability to actively find a host (Yan et al., 2014), and can only enter the digestive tract and act on the midgut through insect feeding. Therefore, EPN is more suitable for controlling pests living in soil or hidden environments, such as underground root borers, soil-dwelling larvae, etc., while Bt is more effective for feeding leaf-eating pests exposed to leaves. Both have their own strengths in the environment of action and the target stage: EPN can penetrate soil and litter, attack larvae and pupae on the rhizosphere or soil surface; Bt can cover the surface of the plant by spraying to target exposed feeding larvae (Abdolmaleki et al., 2017).
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