Submitted to: Journal of Medical Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 17, 2010
Publication Date: March 25, 2011
Citation: Weiler, L.F., Behle, R.W., Stafford, K. 2011. Susceptibility of four tick species Amblyomma americanum, Dermacentor variabilis, Ixodes scapularis, and Rhipicephalus sanguineus (Acari: Ixodidae) to nootkatone. Journal of Medical Entomology. 48(2):322-326. Interpretive Summary: Urban residents often do not practice tick control by applications of acaricides because of potential hazards and adverse environmental impacts related to the general toxicity of most of the available pesticides. We determined the ability of nootkatone, a relatively non-toxic essential oil from grapefruit, to kill four important tick species known to vector human disease pathogens. These results provide a baseline of data demonstrating acaricidal activity against these pests that can be used for development of formulations to further improve the potential for commercialization of nootkatone as a natural pest control product. Ultimately, the public will benefit from the successful development of a natural pest control product that kills disease vectoring ticks with no adverse environmental impacts.
Technical Abstract: The essential oil nootkatone has shown acaricidal activity on ticks. The toxicity of nootkatone was determined in laboratory assays using a vial coating technique against unfed nymphs of four Ixodid ticks: Amblyomma americanum L., Dermacentor variabilis (Say), Ixodes scapularis Say, and Rhipicephalus sanguineus Latreille. We determined the lethal concentration (LC50 and LC90 of nootkatone by recording tick mortality 24 hours after exposure in treated glass vials. Nymphs of these species were susceptible to nootkatone with LC50 values of 0.35 µg/cm**2, 0.22 µg/cm**2, 0.16 µg/cm**2, and 0.19 µg/cm**2 a.i. for A. americanum. D. variabilis, I. scapularis, and R. sanguineus, respectively. The LC90 values after 24-hour exposure period were 0.98 µg/cm**2, 0.63 µg/cm**2, 0.54 µg/cm**2, and 0.48 µg/cm**2 a.i. for A. americanum, D. variabilis, I. scapularis, and R. sanguineus, respectively. LC50 values of nootkatone among tick species were not significantly different from I. scapularis except for A. americanum, which was higher. Because nootkatone is volatile, we measured the amount of nootkatone recovered from duplicate treated vials before tick exposure (T=0) and from vials after tick exposure (T=24h). Nootkatone-coated vials had initial nootkatone recovery (T=0) ranging from 82-100% among various treatment concentrations. The nootkatone recovered after 24-hour exposure period (T=24) ranged from a high recovery of 88% of the expected amount from vials coated with higher concentrations of nootkatone, down to 30% of the expected nootkatone from vials coated with low nootkatone concentrations. The toxicity of nootkatone to ticks reported here provides a reference point for effective control of the highly mobile and most active questing stage of ticks. The susceptibility of ticks to nootkatone at low concentration and the determination of the nootkatone residue after vial coating served as a venue to fully exploit the potential of nootkatone as a safe, environmentally friendly alternative for tick control. This study also provides important information for future formulation development of the compound.