Important oils restore insecticide effectiveness towards mattress bugs

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WEST LAFAYETTE, Ind. – Bed bugs hide in dark, invisible rooms and multiply quickly, making them difficult to control. This work has become even more difficult in recent years as the pests have developed resistance to the insecticides that have long wiped them out from homes, hotel rooms and other spaces.

Herbal essential oils are generally deadly to bed bugs, but it’s unclear how they can be used most effectively. Now Purdue University entomologist Ameya Gondhalekar and his former Ph.D. Student Sudip Gaire discovered how essential oil compounds affect bed bug physiology and demonstrated how they can improve the lethality of pyrethroids, a class of commercial and household insecticides.

Their results were published in two articles in the journal Pesticide Biochemistry and Physiology – one last year and one in March.

“We have seen that we can kill resistant bed bugs with traditional pyrethroid insecticides, but we have to use ever larger amounts. Their application at these levels is a problem, ”said Gondhalekar, research fellow in entomology. “Our results 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 range of essential oil compounds on non-resistant bed bugs and a resistant Knoxville strain of bed bugs. A single dose of deltamethrin meant killing 25% of the insects, killing 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 variety of bed bugs that you get almost no control if you use it in the field in large doses,” Gaire said.

The active ingredients in essential oils – thymol from thyme, carvacrol from oregano and thyme, eugenol from cloves and others – worked equally against resistant and non-resistant insects. A dose that was supposed to kill 25% killed so many of each type.

Gondhalekar said that the beetles’ nervous system normally opens and closes sodium channels to pass signals through neurons. Deltamethrin binds to these sodium channels and keeps them open so neurons can’t stop firing. This repeated shooting quickly depletes the beetle’s energy and kills it.

However, resistant bed bugs have several mechanisms to resist pyrethroids, including overactive levels of an enzyme called cytochrome P450, which breaks down deltamethrin. The essential oil compounds, Gaire and Gondhalekar reported, bind to and deactivate this enzyme, allowing deltamethrin to do its job in the bed bug’s nervous system.

Gaire and Gondhalekar combined a single dose of deltamethrin with a single dose of essential oil compounds, which are expected to kill 25 to 50 percent of resistant bed bugs. Instead, more than 90 percent of the resistant bed bugs were killed.

“When we treated the resistant Knoxville bed bugs with various essential oils and tested them for cytochrome P450, we found that these enzymes were inhibited,” said Gaire. “The essential oil compounds were able to neutralize these enzymes and allow the deltamethrin to do its job.”

Gondhalekar’s laboratory will continue to research possible formulations of essential oils with pyrethroid insecticides and test them in the laboratory and in the field to maximize pest control. Purdue University’s AgSEED program, the Urban and Industrial Pest Control Center, and the Bilsland Dissertation Fellowship supported this research as part of Gaire’s dissertation.

Writer: Brian Wallheimer; 765-532-0233; bwallhei@purdue.edu

Swell: Ameya Gondhalekar; 765-494-3839

Sudip Gaire; sgaire@uky.edu

ABSTRACT

Bedbugs, Cimex lectularius L., which have deltamethrin resistance in the metabolism and at the target site, are susceptible to vegetable essential oils

Sudip Gaire, Cari D. Lewis, Warren Booth, Michael E. Scharf, Wei Zheng, Matthew D. Ginzel, Ameya D. Gondhalekar

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Pyrethroid resistance was a major hurdle that limited effective control of bed bugs (Cimex lectularius L.). For effective bed bug management, alternative approaches have been suggested which include the use of vegetable essential oils (EOs). However, no comparisons of EO resistance levels between pyrethroid sensitive and resistant bed bug populations have been made to date. The aim of this study was twofold: (i) to determine the deltamethrin resistance levels and associated resistance mechanisms in the locally collected Knoxville strain and (ii) to quantify the resistance levels of the Knoxville strain to five EOs (thyme, oregano, clove, geranium and Coriander)), its most important insecticidal components (thymol, carvacrol, eugenol, geraniol and linalool) and an EO-based product (EcoRaider®). Initially, we found that the Knoxville strain was 72,893 and 291,626 times more resistant to topically applied deltamethrin than the susceptible Harlan strain at the lethal LD25 and LD50 doses, respectively. Synergistic bioassays and detox enzyme tests showed significantly higher activity of cytochrome P450 and esterase enzymes in the resistant Knoxville strain. Sanger sequencing also revealed the presence of the L925I mutation in the voltage-sensitive sodium channel α-subunit gene. However, the Knoxville strain, which has both enzymatic and deltamethrin resistance at the target site, showed no resistance to EOs, their main insecticidal components and EcoRaider® (resistance ratio of ~ 1) in topical bioassays. In summary, this study showed that a deltamethrin-resistant strain of bed bugs is susceptible to EOs and their insecticidal components.

ABSTRACT

Herbal essential oil components increase deltamethrin toxicity in a resistant population of bed bugs (Cimex lectularius L.) by inhibiting the cytochrome P450 enzymes

Sudip Gaire, Wei Zheng, Michael E. Scharf, and Ameya D. Gondhalekar

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Plant essential oils (EOs) are secondary metabolites from aromatic plants that consist of complex mixtures of chemical components. EOs have been suggested as one of the alternative methods of controlling bed bugs (Cimex lectularius L.). In insecticide-resistant mosquitoes and tobacco worms, EOs act synergistically on pyrethroid toxicity by inhibiting detoxification enzymes. However, it is unknown whether EOs and their constituents increase pyrethroid toxicity in C. lectularius. Therefore, this study was designed to (i) determine the effects of binary mixtures of Deltamethrin (a pyrethroid insecticide) with EOs or EO ingredients or EcoRaider® (an EO-based product) on the mortality of insecticide-resistant and susceptible bedbugs and (ii) evaluating the effects of pretreatment with EO ingredients on the 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 main components (e.g. thymol, carvacrol, eugenol, geraniol or linalool) or EcoRaider® at doses that kill approximately 25% of bed bugs caused a significant increase in the mortality of resistant bed bugs. In the susceptible population, however, only coriander oil, EcoRaider®, thymol and carvacrol increased the toxicity of deltamethrin significantly. Detoxification enzyme tests on protein extracts from bed bugs pretreated with EO ingredients indicated selective inhibition of cytochrome P450 activity in the resistant population, but no effects on esterase and glutathione transferase activities were observed in either population. The inhibition of P450 activity by EO components therefore appears to be one of the mechanisms that increase deltamethrin toxicity in resistant bedbugs.

Agricultural communication: 765-494-8415;

Maureen Manier, Head of Department, mmanier@purdue.edu

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