With pyrethroid resistance now a well established problem, scientists are looking towards alternative sources of insecticides, with promising initial results.
It is well documented that bed bugs are becoming increasingly resistant to synthetic pesticides. Essential oils derived from plants have been identified as a potential alternative, however there has been little research into understanding how these natural oils work. They have been shown to be lethal to bed bugs, but it remains unclear as to how they can be used effectively for pest control.
This issue is something that researchers from Purdue University in the US have been looking into over the last two years. In 2020, the team led by entomologist Ameya Gondhalekar published a paper in Pesticide Biochemistry and Physiology that investigated the lethal dose of essential oils required to kill pyrethroid-resistant and non-resistant strains of bed bugs.
The researchers tested the pyrethroid insecticide deltamethrin and a series of essential oil compounds (taken from flowers and herbs) on non-resistant bed bugs and a resistant Knoxville strain of bed bug. 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.1
“Deltamethrin is so ineffective against 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,” said former Ph.D. undergraduate Sudip Gaire, who was part of the 2020 research team.
Meanwhile, the active ingredients in essential oils — thymol from thyme, carvacrol from oregano and thyme (pictured above), 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.
Deltamethrin was developed to target the insect nervous system. The way it works is that the bed 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. This overloads the bed bug’s nervous system, resulting in death.
But resistant bed bugs possess multiple mechanisms to resist pyrethroids, including overactive levels of an enzyme called cytochrome P450, which degrades deltamethrin. In a follow up study published in 2021, the researchers took their investigation one step further. They knew how deltamethrin and the various essential oils performed against bed bugs, but what if the two were used together, in combination?
The team combined a single dose of deltamethrin with a single dose of essential oil compounds that would be expected to kill 25-50% of the resistant bed bugs. Instead, it killed more than 90% of the resistant bed bugs.2
“When we treated the resistant Knoxville [pyrethroid-resistant] bed bugs with different essential oils and tested for cytochrome P450, we found these enzymes were inhibited,” commented Mr Gaire. “The essential oil compounds were able to neutralise those enzymes, allowing the deltamethrin to do its job.
“Our findings show that essential oils can kill bed bugs, but the combination of essential oils and pyrethroid insecticides has a synergistic effect.”
The Purdue University team will continue researching potential pest control formulations of essential oils with pyrethroid insecticides, testing them both in the lab and the field. Meanwhile, other research teams, such as those at New Mexico State University, have been investigating the non-repellent activity of essential oils and their constituents to gain further insight into which oil-based formulations may be best used in bed bug management programs.
While information on contact, residual, and fumigant toxicity of plant-derived essential oils against bed bugs has been recently published, little is known about the repellent activity of these products and their constituents. Researchers from New Mexico State University believe the identification of essential oil constituents with repellent activity will help develop potentially efficacious essential-oil-based formulations for use in bed bug management programs.
In one study, bed bugs were shown to avoid residues of geraniol, eugenol, citronellic acid, and carvacrol.3 However, the barriers of essential oil compounds did not prevent bed bugs from reaching a warmed blood source and acquiring blood meals. More work is required to understand how host-associated cues might interfere with the bed bugs’ responses to these essential oils.
In summary, essential oils appear to be an interesting prospect for inclusion in bed bug products, particularly in synergising pyrethroids, although more work is required to discover the most effective essential oil/pyrethroid combination.
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References:
1 Sudip Gaire, Wei Zheng, Michael E. Scharf, Ameya D. Gondhalekar, Plant essential oil constituents enhance deltamethrin toxicity in a resistant population of bed bugs (Cimex lectularius L.) by inhibiting cytochrome P450 enzymes, Pesticide Biochemistry and Physiology, Volume 175, 2021, 104829.
2 Sudip Gaire, Cari D. Lewis, Warren Booth, Michael E. Scharf, Wei Zheng, Matthew D. Ginzel, Ameya D. Gondhalekar, Bed bugs, Cimex lectularius L., exhibiting metabolic and target site deltamethrin resistance are susceptible to plant essential oils, Pesticide Biochemistry and Physiology, Volume 169, 2020, 104667.
3 González-Morales, María & Terán, Martín & Romero, Alvaro. (2021). Behavioral Responses of the Common Bed Bug to Essential Oil Constituents. Insects. 12. 184. 10.3390/ insects12020184.
