April 19, 2021
Essential oils restore insecticide effectiveness against bed bugs
WEST LAFAYETTE, Ind. —Bed bugs tuck themselves away into dark, unseen spaces and multiply rapidly, making them difficult to control. That job has gotten even harder in recent years as the pests have developed resistance to the insecticides long used to eradicate them from homes, hotel rooms and other spaces.
Plant-based essential oils are generally lethal to bed bugs, but it’s been unclear how to use them most effectively. Now, Purdue University entomologist Ameya Gondhalekar and his former Ph.D. student, Sudip Gaire, have discovered how essential oil compounds act on bed bug physiology and shown how they can improve the lethality of pyrethroids, a class of commercial and household insecticides.
“We’ve seen that we can kill resistant bed bugs with traditional pyrethroid insecticides, but we have to use increasingly larger amounts. Applying them at those levels is a problem,” said Gondhalekar, a research associate professor in entomology. “Our findings show that essential oils can kill bed bugs, but the combination of essential oils and pyrethroid insecticides has a synergistic effect.”
Gaire and Gondhalekar first tested the pyrethroid insecticide deltamethrin and a series of essential oil compounds on non-resistant bed bugs and a resistant Knoxville strain of bed bugs. A single dose of deltamethrin meant to kill 25% of bugs killed that many non-resistant bed bugs, but it took 70,000 times more to kill 25% of the Knoxville strain.
“Deltamethrin is so ineffective in the Knoxville strain of bed bugs that if you’re using it in the field even in large doses, you’ll get almost no control,” Gaire said.
The active ingredients in essential oils — thymol from thyme, carvacrol from oregano and thyme, eugenol from clove, and others — worked equally against resistant and non-resistant bugs. A dose meant to kill 25% killed that many of each type.
Gondhalekar said bugs’ nervous systems normally open and close sodium channels to pass signals through neurons. Deltamethrin binds to those sodium channels and keeps them open so that neurons cannot stop firing. That repeated firing quickly uses up the bug’s energy and kills it.
But resistant bed bugs possess multiple mechanisms to resist pyrethroids, including overactive levels of an enzyme called cytochrome P450, which degrades deltamethrin. The essential oil compounds, Gaire and Gondhalekar reported, bind to and deactivate that enzyme and allow deltamethrin to do its job on the bed bug’s nervous system.
Gaire and Gondhalekar combined a single dose of deltamethrin with a single dose of essential oil compounds that would be expected to kill 25 percent to 50 percent of the resistant bed bugs. Instead, it killed more than 90 percent of the resistant bed bugs.
“When we treated the resistant Knoxville bed bugs with different essential oils and tested for cytochrome P450, we found these enzymes were inhibited,” Gaire said. “The essential oil compounds were able to neutralize those enzymes, allowing the deltamethrin to do its job.”
Gondhalekar’s lab will continue researching potential formulations of essential oils with pyrethroid insecticides and test them in the lab and the field to maximize pest control. The Purdue University AgSEED program, the Center for Urban and Industrial Pest Management and the Bilsland Dissertation Fellowship supported this research as a part of Gaire’s dissertation.
Writer: Brian Wallheimer; 765-532-0233; firstname.lastname@example.org
Sources: Ameya Gondhalekar; 765-494-3839
Sudip Gaire; email@example.com
Bed bugs, Cimex lectularius L., exhibiting metabolic and target site deltamethrin resistance are susceptible to plant essential oils
Sudip Gaire, Cari D. Lewis, Warren Booth, Michael E. Scharf, Wei Zheng, Matthew D.Ginzel, Ameya D.Gondhalekar
Pyrethroid resistance has been a major hurdle limiting the effective control of bed bugs (Cimex lectularius L.). Alternative approaches that include the use of plant essential oils (EOs) have been proposed for effective management of bed bugs. However, EO resistance level comparisons between pyrethroid susceptible and resistant bed bug populations have not been previously conducted. The goal of this study was twofold: (i) determine deltamethrin resistance levels and associated resistance mechanisms in the field-collected Knoxville strain and (ii) quantify resistance levels of the Knoxville strain to five EOs (thyme, oregano, clove, geranium and coriander), their major insecticidal constituents (thymol, carvacrol, eugenol, geraniol and linalool) and an EO-based product (EcoRaider®). First, we found that the Knoxville strain was 72,893 and 291,626 fold more resistant to topically applied deltamethrin in comparison to the susceptible Harlan strain at the LD25 and LD50 lethal dose levels, respectively. Synergist bioassays and detoxification enzyme assays revealed significantly higher activity of cytochrome P450 and esterase enzymes in the resistant Knoxville strain. Further, Sanger sequencing revealed the presence of the L925I mutation in the voltage-sensitive sodium channel α subunit gene. The Knoxville strain that possesses both enzymatic and target site deltamethrin resistance, however, did not show any resistance to EOs, their major insecticidal constituents and EcoRaider® in topical bioassays (resistance ratio of ~1). In conclusion, this study demonstrated that a deltamethrin-resistant strain of bed bugs is susceptible to EOs and their insecticidal constituents.
Plant essential oil constituents enhance deltamethrin toxicity in a resistant population of bed bugs (Cimex lectularius L.) by inhibiting cytochrome P450 enzymes
Sudip Gaire, Wei Zheng, Michael E. Scharf, Ameya D. Gondhalekar
Plant essential oils (EOs) are secondary metabolites derived from aromatic plants that are composed of complex mixtures of chemical constituents. EOs have been proposed as one of the alternative methods for bed bug (Cimex lectularius L.) control. In insecticide resistant mosquitoes and tobacco cutworm, EOs synergize pyrethroid toxicity by inhibiting detoxification enzymes. However, whether EOs and their constituents enhance pyrethroid toxicity in C. lectularius has remained unknown. Therefore, this study was designed to (i) determine the effects of binary mixtures of deltamethrin (a pyrethroid insecticide) with EOs or EO constituents or EcoRaider® (an EO-based product) on mortality of insecticide resistant and susceptible bed bugs, and (ii) evaluate the effects of EO constituent pre-treatment on detoxification enzyme activities of resistant and susceptible populations. Topical bioassays with binary mixtures of deltamethrin and individual EOs (e.g., thyme, oregano, clove, geranium or coriander oils) or their major constituents (e.g., thymol, carvacrol, eugenol, geraniol or linalool) or EcoRaider® at doses that kill approximately 25% of bed bugs caused significant increases in mortality of resistant bed bugs. However, in the susceptible population, only coriander oil, EcoRaider®, thymol, and carvacrol significantly increased the toxicity of deltamethrin. Detoxification enzyme assays with protein extracts from bed bugs pre-treated with EO constituents suggested selective inhibition of cytochrome P450 activity in the resistant population, but no impacts were observed on esterase and glutathione transferase activities in either population. Inhibition of P450 activity by EO constituents thus appears to be one of the mechanisms of deltamethrin toxicity enhancement in resistant bed bugs.
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