The frequent and extensive use of insecticides leads to the evolution of insecticide resistance. It is well recognized that over-expression of cytochrome P450s involved in insecticide detoxification is one common and major mechanism of resistance to
various classes of insecticides in insects. However, the molecular mechanisms for overexpression of insecticide resistance-related P450 genes in insects have not been well identified until now. In the recent decade, with the development of biological science and technology, many significant advances have been achieved in this direction. In this article, an overview of the current knowledge with respect to expression regulation of insect cytochrome P450 genes was provided. In addition to gene duplication and amplification, up-regulation of P450s at the transcriptional level can be ascribed to the interaction between the cis-regulatory elements and the trans-acting factors. Transcription factors such as CncC, CREB and AhR directly regulate the expression of P450s. The G-protein-coupled receptors and their downstream effectors play indirect roles in the transcriptional regulation of P450 genes. The universality of the role of CncC:Maf/Keap1 in the expression regulation of P450s is supported by studies on a variety of insect species. Increasing reports confirm the involvement of microRNAs in the regulation of P450 expression. Current findings reveal the diversity of regulatory factors and signal transduction pathways, and the complexity of underlying mechanisms in the regulation of P450 expression.

Symbiotic microorganisms can account for 1%-10% of insect biomass, including acteria, fungi, archaea and viruses. Insects and symbiotic microorganisms co-evolve to form holobionts. Symbiotic microorganisms play an important role in the biological characteristics, diversity formation, ecological adaptability and stress resistance of insects. Crop pest insects seriously affect agricultural production. In this article, the research advances in the diversity, research methods, and functional mechanisms of insect symbiotic microorganisms, the interaction between symbiotic microorganisms and their application in pest control since 2000 were reviewed and prospected. With the continuous development and application of advanced research methods such as molecular microbial ecology and metagenomic sequencing, breakthroughs have been made in the research of symbiotic microorganisms of agricultural pest insects. It was found that symbiotic microorganisms mainly affect host insects in the following ways: (1) Synthesis of nutrients or production of digestive enzymes to promote host growth and development and to expand host ecological niche; (2) Production of protective metabolites to directly protect the host against stress or indirectly protect the host by regulating the defense response of host plants; and (3) Production of active substances to regulate host propagation, mating, aggregation and movement. The abundance and community composition of insect symbiotic microorganisms maintain dynamic changes in a certain spatial-temporal range and have an important impact on host phenotype, which is the result of the benefit trade-off among host, environment, and interactive microorganisms. We suggest that future research should focus on clarifying the molecular mechanisms underlying the formation and maintenance of symbionts, ascertaining the complex interactions among symbiotic microorganisms, host insects, plants, natural enemies and the environment in more spatial-temporal dimensions, and designing green and efficient pest control strategies through targeted regulation of pest insect symbionts.