Bt Research 2025, Vol.16, No.4, 157-167 http://microbescipublisher.com/index.php/bt 163 Vip3Aa toxin. As a result, in the highly resistant cotton bollworm fields, the survival rate of cotton bollworms dropped significantly to below 5% after administration of this engineered strain, while the use of traditional preparations containing only Cry1Ac was minimal. This shows that the Bt-engineered strain designed for "symptomatic treatment" of resistant pests can re-establish control effects in the field (Hemthanon et al., 2023). 6 Assisted Strategies to Improve Bt Effectiveness 6.1 Combination with RNA interference (RNAi) technology By targeting key genes of silencing pests, RNA interference technology has become a new biological control strategy that has emerged in recent years and is regarded as a beneficial supplement to Bt insecticidal protein. Combining Bt strains with RNAi technology can improve the prevention and treatment effect at multiple levels. Delaying resistance production: The common mechanism by which pests develop resistance to Bt toxins is receptor mutation or downregulation of expression. Targeting resistance-related genes in silenced pests through RNAi means can partially restore their sensitivity to Bt toxins. There were studies on bollworms that were already resistant to Cry1Ac and fed feed carrying the Cad receptor gene dsRNA. As a result, the expression of Cad in the midgut of bollworms was reduced and they showed a higher sensitivity to Cry1Ac again. It is inferred from this that the simultaneous application of Bt bacteria agent and specific RNAi preparation in the field can break the "shield" of pests' existing resistance, thereby extending the effective life of Bt bacteria agent. Enhanced insecticide spectrum and virility: For pest species with unsatisfactory Bt poisoning, RNAi can be designed to attack other important physiological targets, thus forming a "double blow" with Bt. Genetic engineering fusion: The cutting-edge strategy is to use synthetic biology to introduce RNAi system into Bt strains. It has been reported that plasmids that produce insecticidal dsRNA are transferred to Bt, so that Bt can act as a dsRNA vector during the infection of pests. After feeding Bt, the pests not only have the intestines attacked by toxins, but also the key genes in the cell are silenced by RNAi, which can lead to higher mortality and malformation rates (Peng et al., 2010). 6.2 Synergistic effects with symbiotic bacteria or plant endophytes In nature, the growth and development of pests are often affected by symbiotic microorganisms and host plant endophytes in their bodies. Using these factors to cooperate with Bt strains is also an idea to improve prevention and treatment effectiveness. Certain insect gut symbionts can produce enzymes or molecules that contribute to the action of Bt toxins. Research has found that some bacteria that coexist in the intestines of mosquito larvae can secrete chitinase, destroying the intestinal barrier and enhancing the mosquito killing effect of the Israeli subspecies Bt (Bti). Therefore, in practical applications, the combination of Bt with such symbiotic bacteria can improve the lethality rate of mosquito larvae (Wakil et al., 2020). Many plant endophytes bacteria or fungi have the ability to produce plant defense compounds. If it can be applied in conjunction with Bt preparation, it may have the effect of "killing two insects" in one insect. For example, studies have shown that the endophytic corn fungus Beauveria bassiana can parasitize in corn borer larvae, while Bt attacks from the intestine, and the combined increase the mortality rate of corn borer by about 20% compared with the single use (Yasin et al., 2024). The composition of the intestinal microbial community of pests will affect the Bt action, and some symbiotic bacteria can even decompose Bt toxins or help the pests detoxify. Changes in the intestinal flora of the pest by applying probiotics or regulators to facilitate Bt infection, which will have the potential to become a new efficiency strategy. 6.3 Complementary application with chemical pesticides or other biological control measures Although Bt strains have unique advantages in biological control, a single measure is often insufficient to fully control the diverse pest threat. Combining Bt preparation with other prevention and treatment methods can learn from each other and improve overall prevention and efficacy. First, the combination of Bt and low-dose chemical pesticides: Studies have shown that some chemical insecticides such as avermectin and pyrethroids can increase insect intestinal wall permeability or weaken their immunity at low doses, thereby synergizing the effects of Bt
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