Target site insensitivity mutations in the AChE enzyme confer resistance to organophosphorous insecticides in Leptinotarsa decemlineata (Say)

Leveraging RNA Interference to Impact Insecticide Resistance in the Colorado Potato Beetle, Leptinotarsa decemlineata

The Colorado potato beetle, Leptinotarsa decemlineata Say, is a potato pest that can cause important economic losses to the potato industry worldwide. Diverse strategies have been deployed to target this insect such as biological control, crop rotation, and a variety of insecticides. Regarding the latter, this pest has demonstrated impressive abilities to develop resistance against the compounds used to regulate its spread. Substantial work has been conducted to better characterize the molecular signatures underlying this resistance, with the overarching objective of leveraging this information for the development of novel approaches, including RNAi-based techniques, to limit the damage associated with this insect. This review first describes the various strategies utilized to control L. decemlineata and highlights different examples of reported cases of resistances against insecticides for this insect. The molecular leads identified as potential players modulating insecticide resistance as well as the growing interest towards the use of RNAi aimed at these leads as part of novel means to control the impact of L. decemlineata are described subsequently. Finally, select advantages and limitations of RNAi are addressed to better assess the potential of this technology in the broader context of insecticide resistance for pest management.

Insights into the Use of Eco-Friendly Synergists in Resistance Management of Leptinotarsa decemlineata (Coleoptera: Chrysomelidae)

The Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say), is the most notorious insect pest of potato globally. Injudicious use of insecticides for management of this pest has resulted in resistance to all major groups of insecticides along with many human, animal health, and environmental concerns. Additionally, the input cost of insecticide development/discovery is markedly increasing because each year thousands of chemicals are produced and tested for their insecticidal properties, requiring billions of dollars. For the management of resistance in insect pests, synergists can play a pivotal role by reducing the application dose of most insecticides. These eco-friendly synergists can be classified into two types: plant-based synergists and RNAi-based synergists. The use of plant-based and RNAi-based synergists in resistance management of insect pests can give promising results with lesser environmental side effects. This review summarizes the resistance status of CPB and discusses the potential advantage of plant-based and RNAi-based synergists for CPB resistance management. It will motivate researchers to further investigate the techniques of using plant- and RNAi-based synergists in combination with insecticides.

Role of Insect Gut Microbiota in Pesticide Degradation: A Review

Insect pests cause significant agricultural and economic losses to crops worldwide due to their destructive activities. Pesticides are designed to be poisonous and are intentionally released into the environment to combat the menace caused by these noxious pests. To survive, these insects can resist toxic substances introduced by humans in the form of pesticides. According to recent findings, microbes that live in insect as symbionts have recently been found to protect their hosts against toxins. Symbioses that have been formed are between the pests and various microbes, a defensive mechanism against pathogens and pesticides. Insects' guts provide unique conditions for microbial colonization, and resident bacteria can deliver numerous benefits to their hosts. Insects vary significantly in their reliance on gut microbes for basic functions. Insect digestive tracts are very different in shape and chemical properties, which have a big impact on the structure and composition of the microbial community. Insect gut microbiota has been found to contribute to feeding, parasite and pathogen protection, immune response modulation, and pesticide breakdown. The current review will examine the roles of gut microbiota in pesticide detoxification and the mechanisms behind the development of resistance in insects to various pesticides. To better understand the detoxifying microbiota in agriculturally significant pest insects, we provided comprehensive information regarding the role of gut microbiota in the detoxification of pesticides.

Metabolic-based insecticide resistance mechanism and ecofriendly approaches for controlling of beet armyworm Spodoptera exigua: a review

The beet army worm, Spodoptera exigua, is a widely distributed polyphagous pest of economically important crops worldwide. The management of this pest insect continues to face many challenges. Despite synthetic chemicals posing a serious threat to the environment, these remain the conventional approach for controlling S. exigua in the field. An over-reliance on chemical control has not only led to selection for resistance to insecticides and to a reduction of natural enemies, but has also polluted various components of ecosystem. Given these increasing pressures on the ecosystem, there is a need to implement integrated pest management (IPM) approaches exploiting a wider range of tools (biotechnological approaches, microbial control, biological control, cultural control, and use of host plant resistance) for an alternative to chemical control. The IPM approach can not only reduce the hazard of chemical residues in the environment and associated health problems, but may also provide best strategies to control insect pests. This review synthesizes published information on insecticide resistance of S. exigua and explores alternative IPM approaches to control S. exigua.

Sequence variation and regulatory variation in acetylcholinesterase genes contribute to insecticide resistance in different populations of Leptinotarsa decemlineata

Although insect herbivores are known to evolve resistance to insecticides through multiple genetic mechanisms, resistance in individual species has been assumed to follow the same mechanism. While both mutations in the target site insensitivity and increased amplification are known to contribute to insecticide resistance, little is known about the degree to which geographic populations of the same species differ at the target site in a response to insecticides. We tested structural (e.g., mutation profiles) and regulatory (e.g., the gene expression of Ldace1 and Ldace2, AChE activity) differences between two populations (Vermont, USA and Belchow, Poland) of the Colorado potato beetle, Leptinotarsa decemlineata in their resistance to two commonly used groups of insecticides, organophosphates, and carbamates. We established that Vermont beetles were more resistant to azinphos-methyl and carbaryl insecticides than Belchow beetles, despite a similar frequency of resistance-associated alleles (i.e., S291G) in the Ldace2 gene. However, the Vermont population had two additional amino acid replacements (G192S and F402Y) in the Ldace1 gene, which were absent in the Belchow population. Moreover, the Vermont population showed higher expression of Ldace1 and was less sensitive to AChE inhibition by azinphos-methyl oxon than the Belchow population. Therefore, the two populations have evolved different genetic mechanisms to adapt to organophosphate and carbamate insecticides.

Downregulation of imidacloprid resistant genes alters the biological parameters in Colorado potato beetle, Leptinotarsa decemlineata Say (chrysomelidae: Coleoptera)

Colorado potato beetle, Leptinotarsa decemlineata Say (coleoptera: chrysomelidae), is the important pest of potato all over the world. This insect pest is resistant to more than 50 active compounds belonging to various chemical groups. Potential of RNA interference (RNAi) was explored to knock down transcript levels of imidacloprid resistant genes in Colorado potato beetle (CPB) under laboratory conditions. Three important genes belonging to cuticular protein (CP), cytochrome P450 monoxygenases (P450) and glutathione synthetase (GSS) families encoding imidacloprid resistance were targeted. Feeding bio-assays were conducted on various stages of imidacloprid resistant CPB lab population by applying HT115 expressing dsRNA on potato leaflets. Survival rate of insects exposed to CP-dsRNA decreased to 4.23%, 15.32% and 47.35% in 2nd, 3rd and 4th instar larvae respectively. Larval weight and pre-adult duration were also affected due to dsRNAs feeding. Synergism of RNAi with imidacloprid conducted on the 2nd instar larvae, exhibited 100% mortality of larvae when subjected to reduced doses of GSS and CP dsRNAs along with imidacloprid. Utilization of three different dsRNAs against imidacloprid resistant CPB population reveal that dsRNAs targeting CP, P450 and GSS enzymes could be useful tool in management of imidacloprid resistant CPB populations.

Characterization of the Fifth Putative Acetylcholinesterase in the Wolf Spider, Pardosa pseudoannulata

Background: Acetylcholinesterase (AChE) is an important neurotransmitter hydrolase in invertebrate and vertebrate nervous systems. The number of AChEs is various among invertebrate species, with different functions including the 'classical' role in terminating synaptic transmission and other 'non-classical' roles. Methods: Using rapid amplification of cDNA ends (RACE) technology, a new putative AChE-encoding gene was cloned from Pardosa pseudoannulata, an important predatory natural enemy. Sequence analysis and in vitro expression were employed to determine the structural features and biochemical properties of this putative AChE. Results: The cloned AChE contained the most conserved motifs of AChEs family and was clearly clustered with Arachnida AChEs. Determination of biochemical properties revealed that the recombinant enzyme had the obvious preference for the substrate ATC (acetylthiocholine iodide) versus BTC (butyrylthiocholine iodide). The AChE was highly sensitive to AChE-specific inhibitor BW284C51, but not butyrylcholinesterase-specific inhibitor tetraisopropyl pyrophosphoramide (ISO-OMPA). Based on these results, we concluded that a new AChE was identified from P. pseudoannulata and denoted as PpAChE5. Conclusion: Here we report the identification of a new AChE from P. pseudoannulata and increased the AChE number to five in this species. Although PpAChE5 had the biggest Vmax value among five identified AChEs, it showed relatively low affinity with ATC. Similar sensitivity to test insecticides indicated that this AChE might serve as the target for both organophosphorus and carbamate insecticides.

Overexpression of a cytochrome P450 and a UDP-glycosyltransferase is associated with imidacloprid resistance in the Colorado potato beetle, Leptinotarsa decemlineata

Current control of insect pests relies on chemical insecticides, however, insecticide resistance development by pests is a growing concern in pest management. The main mechanisms for insecticide resistance typically involve elevated activity of detoxifying enzymes and xenobiotic transporters that break-down and excrete insecticide molecules. In this study, we investigated the molecular mechanisms of imidacloprid resistance in the Colorado potato beetle, Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae), an insect pest notorious for its capacity to develop insecticide resistance rapidly. We compared the transcriptome profiles of imidacloprid-resistant and sensitive beetle strains and identified 102 differentially expressed transcripts encoding detoxifying enzymes and xenobiotic transporters. Of these, 74 were up-regulated and 28 were down-regulated in the resistant strain. We then used RNA interference to knock down the transcript levels of seven up-regulated genes in the resistant beetles. Ingestion of double-stranded RNA successfully knocked down the expression of the genes for three cytochrome P450s (CYP6BQ15, CYP4Q3 and CYP4Q7), one ATP binding cassette (ABC) transporter (ABC-G), one esterase (EST1), and two UDP-glycosyltransferases (UGT1 and UGT2). Further, we demonstrated that silencing of CYP4Q3 and UGT2 significantly increased susceptibility of resistant beetles to imidacloprid, indicating that overexpression of these two genes contributes to imidacloprid resistance in this resistant strain.