Fir (Abies balsamea) (Pinales: Pinaceae) needle essential oil enhances the the knockdown activity of select insecticides

Authors: Norris, Edmund; BLOOMQUIST, JEFFRER - University Of Florida

Submitted to: Journal of Medical Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/4/2023
Publication Date: 8/10/2023
Citation: Norris, E.J., Bloomquist, J.R. 2023. Fir (Abies balsamea) (Pinales: Pinaceae) needle essential oil enhances the the knockdown activity of select insecticides. Journal of Medical Entomology. 60(6):1350-1356. https://doi.org/10.1093/jme/tjad101.
DOI: https://doi.org/10.1093/jme/tjad101

Interpretive Summary: The generally pleasant aroma and perceived salubrious qualities of plant essential oils (PEO) have given rise to a shift in consumer preference from synthetic insecticides to these natural formulations for both home pest control and personal bite protection. In addition to their overall safety to mammals and non-target organisms, they are effective at controlling pest insects, making them ideal candidates in future insecticidal formulations/repellents. We have characterized the ability of fir needle oil to synergize the knockdown of 9 different synthetic insecticides. For some insecticides screened, fir needle oil was capable of synergizing knockdown by almost 20-fold, indicating its potential to drastically enhance the efficacy of commercial insecticides on the market. This study attempts to elucidate the mechanism behind this synergism, and we report that the ability of fir needle oil to enhance the speed of action of select insecticides is mediated by another mechanism than direct action on the insect nervous system.

Technical Abstract: The generally pleasant aroma and perceived salubrious qualities of plant essential oils (PEO) have given rise to a shift in consumer preference from synthetic insecticides to these natural formulations for both home pest control and personal bite protection. In addition to their overall safety to mammals and non-target organisms, they are effective at controlling pest insects, making them ideal candidates in future insecticidal formulations/repellents. We have characterized the ability of fir needle oil to synergize the knockdown of 9 different synthetic insecticides. Thus far, fir needle oil has strongly synergized the knockdown of select neonicotinoids, clothiandin and thiamethoxam (between 16 and 24-fold), and natural pyrethrins (12-fold). Furthermore, we have analyzed the individual chemical constituents within this sample to identify those most responsible for its activity. Thus far, delta-3-carene is the most bioactive constituent, producing synergism similar to that of the whole oil. In fact, this constituent synergizes the 24-hr mortality of clothianidin to a higher degree than fir needle oil itself (4.9-fold vs. 2.4-fold). These effects are not mediated by synergism on the nervous system, as fir needle oil produces no excitation or block of mosquito central nervous system firing at a concentration (100 ppm) which produces block or excitation by other plant oils. Moreover, fir needle oil at high concentrations (100 ppm) does not synergize the nerve blocking potential of an inactive concentration of natural pyrethrins (10 nM). To better understand why fir needle oil increases the knockdown of select insecticides, but not toxicity, we evaluated the ability of this oil to increase or decrease the ability of Aedes aegypti monooxygenases to degrade a model substrate, 7-ethoxycoumarin. Interestingly, fir needle oil caused a significant increase in metabolic degradation of 7-ethoxycoumarin. This finding could indicate that fir needle oil upregulates metabolic processes that allow for degradation of certain insecticides. This result may explain why fir needle oil enhances knockdown but not 24-hr mortality.