Essential Oils: An Untapped Resource for Managing Urban Insect Pests
By John P. Roche, Ph.D.
Urban pest insects such as ants, termites, cockroaches, and fleas cause economic losses and can spread allergic reactions and pathogens. Synthetic chemical pesticides are the primary mode of defense against most pest insects, but insecticides have several drawbacks, including effects on non-target organisms and development of insecticide resistance that makes them ineffective.
Essential oils, meanwhile, are attractive potential alternatives to synthetic insecticides. Some show efficacy in repelling or killing insect pests, with fewer environmental and health concerns. Seun Oladipupo, Ph.D., Xing Ping Hu, Ph.D., and Arthur Appel, Ph.D., of Auburn University have prepared a comprehensive review of studies on essential oils for control of urban pests, published last week in the Journal of Economic Entomology.
I am excited to share the second chapter from my PhD dissertation. We wondered why there is a forest of publications on essential oils (EOs) and a desert of pointers on how these essential oils could be commercialized.
So we wrote a review!
— Seun Oladipupo, Ph.D. (@iam_seunpop) July 6, 2022
Effectiveness of essential oils and their chemical components in insect control has been tested from a variety of plant families. Oladipupo and colleagues report that the most effective compounds to date have been from the myrtle, mint, laurel, ginger, and aster families. Examples of essential oils that have been tested are eugenol (found in clove, nutmeg, cinnamon, and basil), carvacrol (found in oregano, basil, mint, and marjoram), trans-cinnamaldehyde (found in cinnamon), and thymol (found in thyme). Essential oils have been found to have a range of effects on target insects, including inhibiting feeding, inhibiting egg hatching, and causing mortality to eggs, larvae, nymphs, and adults.
Oladipupo and collagues provide a comprehensive summary of research on essential oils on several urban insect pests: ants, termites, fleas, cockroaches, silverfish, brown marmorated stink bugs, and stored product moths. An example of the potential of essential oils in pest control is the use of plant extracts to control red imported fire ants (Solenopsis invicta). Mint oil has been found to repel fire ant workers, and extracts from camphor trees and indigenous cinnamon plants were discovered to be toxic to them. Most tests of essential oils on ants have used fumigation applications, which would work well for ant species that make colonies underground.
Another example of control is found in human fleas, rat fleas, and cat fleas, which can cause allergic reactions and pathogen infections in humans. Fleas are usually controlled with synthetic insecticides, but essential oils have shown promise as control tools. Extracts taken from leaves of indigenous cinnamon and Caribbean oregano have been found to repel cat fleas, and extracts from plants of the mint, grass, and ginger families have been found to be toxic to eggs, larvae, and adults of cat fleas.
Several physical properties effect the efficacy of essential oils, including water solubility, fat solubility, boiling point, volatility (the tendency to become a gas), and molecular weight (the sum of the atomic weights of atoms in a molecule). Each of these physical properties present tradeoffs. For example, essential oils have high volatility. “That property makes them the ideal fumigant,” Oladipupo says, “but high volatility could result in rapid evaporation, reducing efficacy. So, a repeated application might be required to achieve a satisfactory level of control.” One way out of this is to make formulations that increase dispersibility and persistence of essential oils.
Many previous studies have tested the effects of specific essential oils on specific pest insects, but there has been very little research on the mechanisms by which essential oils act on insects. Because of this, Oladipupo and colleagues suggest that future research should not focus on more tests of the effects of particular essential oils but rather should investigate the mechanisms of action of these essential oils. Some data indicate certain essential oils act by inhibiting acetylene esterase and cytochrome P450 enzymes. Because of the wide importance of these enzymes across taxa, these data suggest that essential oils could be effective against a broad spectrum of insect species.
After many decades of exposure, widespread resistance to synthetic insecticides has evolved in insects. Since essential oils are more novel, resistance has not evolved on a large scale. “Insects do not have the exact mechanisms to resist or counteract exposure to essential oils,” Oladipupo says. With repeated use, however, resistance to essential oils could develop. One way to prevent this, the authors conclude, is to create synergistic formulations combining multiple essential oils or combining essential oils with other compounds.
In their review, the investigators found that variability in essential oils across plant families, across seasons, and across different studies is a serious problem. Numerous studies don’t give complete information on the methods used, making comparisons among studies impossible. For example, researchers often list mortality caused by essential oils, but not the dose at which the essential oil caused mortality. Oladipupo and colleagues emphasize that future studies should provide all relevant information so that studies can be compared with each other and specific studies can be rigorously replicated. “Inherent variabilities exist in data across studies,” Oladipupo says. “If everyone uses at least one common methodology, we will have a better basis to compare the performance of essential oils.”
Differences between conditions in the lab and conditions in the field are another problem, the authors found. “In my opinion,” Oladipupo says, “the biggest challenge in the study of essential oils is the inability to replicate laboratory successes in the field (or in homes) and the lack of theoretical frameworks and hypotheses in studies.” The authors suggest that, in the future, investigators should design lab tests to parallel field conditions as much as possible.
Journal of Economic Entomology
John P. Roche, Ph.D., is an author, biologist, and educator dedicated to making rigorous science clear and accessible. He publishes, edits, and teaches widely on science and science communication.