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.
Plant-based 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.
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