Catnip, Nepeta cataria (Lamiales: Lamiaceae)--a closer look: seasonal occurrence of nepetalactone isomers and comparative repellency of three terpenoids to insects

Evidence of repellent properties in catnip, Nepeta cataria L., to flies and cockroaches was observed in preliminary studies. This study compared catnip essential oil from steam distillation and elemol, a major constituent of osage orange essential oil, to current commercial repellents. These comparative studies found both the catnip steam distillate and elemol to be as good, and in some cases better, at repelling house flies, Musca domestica L., and American cockroaches, Periplaneta americana L., than N,N-diethyl-m-toluamide (DEET) or citronellal. Both short-term and long-term repellency bioassays were used to assess repellency. Catnip essential oil showed greater repellency than DEET and citronellal in the short term. Extended repellency bioassays showed elemol to be more repellent than catnip steam distillate, citronellal, and DEET. Nepetalactone, the major constituent of catnip essential oil, is present as two isomers, and previous studies have shown the E,Z-nepetalactone [2-(2-hydroxy-1-methylethenyl)-5-methyl-cyclopentanecarboxylic acid delta lactone] isomer to be even more repellent to cockroaches than the dominant isomer, Z,E-nepetalactone. This study examined the seasonal variation of the two isomers, Z,E- and E,Z-nepetalactone, in catnip. Samples of fresh catnip mature leaves, immature leaves, and stems were steam-distilled separately, and isomer composition was analyzed using high-performance liquid chromatography and gas chromatography (GC). An analysis of variance (ANOVA) showed significant differences by week. The mature leaf essential oil samples were tested in a repellency bioassay and exhibited significant repellency to German cockroaches, Blattella germanica L. The catnip floral volatiles were sampled using solid phase microextraction, and analysis with GC/mass spectrometry showed the presence of Z,E-nepetalactone, E,Z-nepetalactone, and beta-caryophyllene as the major constituents. Phytophagous insects and potential pollinators present on sampling dates were recorded.