The conditions are set for a bumper crop of mosquitoes this summer in Michigan. Mosquito populations come in waves, and as the spring mosquitoes are dying off, the summer mosquitoes are set to emerge followed by another peak in the population early July. Summer mosquitoes thrive in warm weather and breed in stagnant waters. Following a rainy month of May that left water tables high, it could be perfect conditions for an especially large population of those nasty pests that leave us all itching for relief.
Mosquitoes have a great impact on our quality of life as we try to enjoy the great outdoors, but they also harbor disease organisms and pass them on to humans and other animals. Examples of this are Eastern Equine Encephalitis (EEE) and West Nile Virus (WNV), mosquito-borne viruses that can be found in Michigan. The best way to reduce the risk of infection by mosquito-transmitted diseases is to reduce exposure. The Michigan Mosquito Control Association has a few recommendations to reduce exposure to the hungry mosquitoes: Wear long sleeved shirts and long pants, try not to go outdoors during hours of peak mosquito activity (dusk and dawn), keep all window and door screens in good repair, and wear mosquito repellents containing DEET, picaridin, or oil of lemon eucalyptus, making sure to read repellent labels before use.
The most common and effective chemical used in commercial repellents is N, N-diethyl-3-methylbenzamide (DEET). DEET has over a 50 year history of use, first becoming commercially available in 1957. Despite the numerous lay press reports questioning the safety of DEET, this repellent has been subjected to more scientific and toxicologic scrutiny than any other insect repellent. DEET has a remarkable safety profile spanning the last half century with over 8 billion human applications. Still, there have been significant efforts in academia, government and private sector to identify new insect repellents. This has largely been driven by reports of DEET toxicity, minimal efficacy towards certain subspecies of insects, high incidences of insect-borne diseases, decreasing consumer acceptance and the potential for insects to develop resistances.
In the search for alternatives, thousand of plants have been screened as potential insect repellents from botanical sources. Most plant-based insect repellents on the market contain essential oils from one or more of the following plants: geranium, citronella, cedar, eucalyptus, peppermint, lemongrass, and soybean. Products made from oils-of-eucalyptus perform the best out of these examples.
Catnip is another example historically used as an insect repellent and as a folk lore remedy. Although not native to North America, catnip now grows throughout Michigan and is generally considered a weed. Nepeta cataria (also known as catnip, catswort, or catmint) is a plant in the Lamiaceae family. The common names can also be used to refer to the Nepeta genus as a whole. The main chemicals in oils of catnip were identified to be nepetalactones, consisting primarily of two isoforms. Here I review some of the recent studies on catnip for its ability to repel mosquitoes.
Field tests were conducted using a hydrogenated form of catnip oil in Florida and Maine. First, the essential oil of catnip was catalytically hydrogenated to yield dihydronepetalactones (DHN). Strictly speaking, hydrogenated catnip oil (HCO) is not something the average person can make without a palladium catalyst, hydrogen gas and a pressure vessel. DHN was previously detected in the defensive secretions of certain insects and it had been reported that DHN had the ability to repel ants. HCO was formulated into a lotion or alcohol-based spray. All HCO formulas exhibited some degree of extended protection with the 15% by weight HCO lotion providing complete protection during the eight hour tests. The authors suggest that formulations of HCO can be effective alternative to existing repellents such as DEET.
In Australia, a commercial product containing 5% catnip essential oil was tested as repellent against four different species of mosquitoes. Significant variation was observed for protection afforded against different mosquito species ranging from no protect to four hours on average. In contrast, a 7% DEET spray provided complete protection over a six hour period. Overall, the authors concluded that catnip does provide limited protection against some mosquito species in Australia, and may be more effective than other products containing natural plant extracts, but it was not as effective as DEET.
A study from China compared catnip essential oil along with other plant essential oils and DEET. Catnip essential oil (composed of 36%, 45%, 18% isomer 1, isomer 2, and caryophyllene) provided the best protection against mosquitoes and the only oil to provide complete protection for over six hours. When testing the major ingredients of catnip oil, their tests showed that a blend containing the nepetalactone isomers at a 3:1 ratio has the highest and longest repellent activity.
The most recent study published in 2011 on the use of catnip essential oils was performed on Afro-topical mosquitoes originally cultivated from Tanzania. They compared two different batches of catnip and found that the isomeric composition of nepatalactone varied considerable (batch A: 92% isomer 1 and 8% caryophyllene, and batch B: 17% isomer 1, 70% isomer 2, and 13% caryophyllene). Upon testing, batch A was not as effective at repelling mosquitoes as compared to batch B. Purified isomers provided inferior protection to either batches of essential oils. Testing of binary mixtures confirmed the synergistic effect between the two isomers. Lower activity was seen with purified isomers and, surprisingly, with equivalent or near equivalent binary mixtures. Highest activity was afforded when the isomers were mixed in 3:1 ratios. Furthermore, a ratio mixture equivalent to batch B did not perform as well compared to either batch of essential oils. A three component blend containing caryophyllene at the levels found in batch B had the same activity as the essential oil.
Typical of plant extracts, the concentration of active ingredients various from batch to batch and the variation is dependent upon things like supply location, seasonal variations, age of the plant, and extraction procedure. Indeed, the ratio of isomers within a catnip plant was shown to vary weekly and the effectiveness of the essential oils to repel insects varied greatly.
Overall, the research on catnip essential oil has proven it to be an effective repellent of mosquitoes. Some variation on the species of mosquitoes repelled and the duration of effectiveness was found. The data suggests that catnip can be used as an effective insect repellent when used as an unfractionated essential oil due to the presence of both nepetalactone isomers and other components such as caryophyllene. However, for practical use of these plant essential oils, further studies on their safety to human health are necessary.
Many of the articles cited are behind paywalls. Copies of specific articles will be provided upon request.