By John P. Roche, Ph.D.
Urban insect pests 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 insect pests, but insecticides have several drawbacks, including effects on non-target organisms and the development of insecticide resistance that renders them ineffective.
Essential oils, on the other hand, are interesting potential alternatives to synthetic insecticides. Some show effectiveness 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 urban pest control , published last week in the Journal of Economic Entomology.
I am delighted to share the second chapter of my doctoral thesis. 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 marketed.
So we wrote a review!
— Seun Oladipupo, Ph.D. (@iam_seunpop) July 6, 2022
The effectiveness of essential oils and their chemical constituents in the fight against insects has been tested on various families of plants. Oladipupo and colleagues report that the most effective compounds to date belong to the myrtle, mint, bay, 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 inhibition of feeding, inhibition of egg hatch, and mortality of eggs, larvae, nymphs, and hatchlings. adults.
Oladipupo and colleagues provide a comprehensive summary of essential oil research on several urban insect pests: ants, termites, fleas, cockroaches, silverfish, 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 worker fire ants, and extracts from native camphor trees and cinnamon have been shown to be toxic to them. Most essential oil tests 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 pathogenic infections in humans. Fleas are usually controlled with synthetic insecticides, but essential oils have shown promise as control tools. Native cinnamon leaf and Caribbean oregano extracts repel cat fleas, and plant extracts from the mint, grass and ginger families have been shown to be toxic to eggs, larvae and adults of cat fleas.
Several physical properties affect the effectiveness of essential oils, including water solubility, fat solubility, boiling point, volatility (the tendency to become a gas), and molecular weight (the sum of the weights atomic numbers of atoms in a molecule). Each of these physical properties has trade-offs. For example, essential oils have high volatility. “This property makes it the ideal fumigant,” says Oladipupo, “but high volatility could lead to rapid evaporation, which would reduce efficacy. Thus, repeated application might be required to achieve a satisfactory level of control. One solution is to manufacture formulations that increase the dispersibility and persistence of essential oils.
Many previous studies have tested the effects of specific essential oils on specific insect pests, but there has been very little research on the mechanisms by which essential oils act on insects. For this reason, Oladipupo and his colleagues suggest that future research should not focus on more testing the effects of particular essential oils, but rather should investigate the mechanisms of action of these essential oils. There is some evidence that some essential oils work by inhibiting acetylene esterase and cytochrome P450 enzymes. Due to the high importance of these enzymes in all taxa, these data suggest that essential oils may be effective against a wide range of insect species.
After several decades of exposure, widespread resistance to synthetic insecticides has developed in insects. As essential oils are newer, resistance has not evolved on a large scale. “Insects don’t have the exact mechanisms to resist or counter exposure to essential oils,” says Oladipupo. However, with repeated use, resistance to essential oils may develop. One way to avoid this, the authors conclude, is to create synergistic formulations that combine multiple essential oils or combine essential oils with other compounds.
In their review, the researchers found that the variability of essential oils from plant family to plant family, season to season, and study to study is a serious problem. Many studies do not give complete information about the methods used, which makes comparisons between studies impossible. For example, researchers often list the mortality caused by essential oils, but not the dose at which the essential oil caused the mortality. Oladipupo and colleagues point out that future studies should provide everything relevant information so that studies can be compared with each other and specific studies can be rigorously replicated. “Inherent variabilities exist in the data from study to study,” says Oladipupo. “If everyone uses at least a common methodology, we will have a better basis to compare the performance of essential oils.”
Differences between lab and field conditions are another issue, the authors found. “In my opinion,” says Oladipupo, “the biggest challenge in studying essential oils is the inability to replicate laboratory successes in the field (or at home) and the lack of theoretical frameworks and hypotheses in the studies”. The authors suggest that in the future, researchers should design laboratory tests parallel to field conditions as much as possible.
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.