Effect of catnip (Nepeta cataria L.) essential oil on the blacklegged tick (Ixodes scapularis) chemosensory system

Natural products provide a large reservoir of active ingredients that can be used for pest management. Essential oils are an interesting alternative to synthetic pesticides, because they are environmentally friendly and are safer products that can protect humans and animals against pest vectors such as ticks. Because of global warming, the rise in tick population has resulted in an increased risk for the transmission of vector borne diseases, such as Lyme disease which is vectored by the blacklegged tick (Ixodes scapularis Say), and an increase in demand for effective yet safe repellent products. Repellent activity of catnip (Nepeta cataria L.) essential oils (EOs) and the main chemical components were evaluated against I. scapularis adult female ticks. Seven varieties of catnip EOs were analyzed using GC-MS to determine their chemical compositions. Nepetalactone isomers (76.6 ± 0.3% and 16.9 ± 0.4% relative overall abundance) were detected in only one EO variety. These compounds were linked to the highly repellent action (100% repellency up to 8 hours post-exposure) exerted by the oil in repellent bioassays. The isolation of the nepetalactone isomers was performed through liquid chromatography, and chemical identity of the isomers was confirmed by GC-MS and NMR. Quantification (using a calibration curve and the internal standard method) of catnip EO main components (i.e., nepetalactone, L-menthone, a-pinene, and b-caryophyllene) was performed through GC-MS. The concentration of the major isomer, (4aS, 7S, 7aS)-nepetalactone, was determined to be 80 ± 10 wt% in the New Directions Aromatics Inc. catnip EO. The exposure of repellents and their impact on the tick chemosensory system was also investigated through electrophysiology. The electrophysiological response of adult tick females to a known attractant and host volatile (i.e., butyric acid), pre- and post-exposure to catnip EO, was recorded. Exposure was performed by fumigation assay, where it was found that pre-exposure to catnip EO significantly reduced tick response to butyric acid. Results of this study will contribute to better understanding the response of tick chemosensory system to different essential oil components and better select effective active ingredients for the development of repellent products.