Here’s the first thing to know about insect repellents: They don’t repel, they confuse.
“A more accurate term might be ‘bite-preventing substances’ ” is how UNH Cooperative Extension entomologist Alan Eaton puts it, proving that he has no future in advertising.
Specifically, repellents create molecular interactions with the internal receptors that bugs use to find targets. This keeps mosquitoes, ticks and other biting arthropods from realizing that we are a suitable meal, so they don’t bother biting.
This distinction is important because the word “repellent” creates mental images of some sort of magic force field that deflects all bugs. But the effectiveness of products depends on the active ingredient, which determines the type of molecules it releases to interact with bugs’ antenna or other sensors.
Different molecules work better against different species at different concentrations, which determines their effectiveness. No magic force field; sorry.
In particular, Eaton says no repellent – sorry, I mean no bite-preventing substance – works well against snipe
flies, deer flies, greenheads or horse flies. They’re too smart to be fooled by our tricky molecules, I guess.
As far as safety, most bug repellents are registered as pesticides by the EPA, although some natural repellents fall under a category of food additives called Generally Regarded As Safe (GRAS), which gets much less testing.
Repellents must be registered in New Hampshire to be sold in New Hampshire, so some products are available online that you can’t buy in stores; that isn’t necessarily a commentary on their safety or effectiveness.
There are so many brands, often selling different concentrations and different formulations of the same active ingredient, that it’s hard to keep track. Consumer Reports has tested dozens and dozens of brands; their top picks for serious bug repelling included either 20 percent picaridin or 25 percent DEET, two synthetic compounds.
Here’s a quick rundown on active ingredients courtesy of various sources but mostly Eaton, who has been the state’s go-to guy for insect information for years.
DEET (an acronym for N, N-diethyl-meta-toluamide – but you knew that) has been around for 60 years and is still the gold standard for keeping away insects and other biting thingamajigs. Eaton says it works against most species of mosquito and against fleas, black flies and biting midges (a.k.a. no-see-ums) as well as ticks and chiggers.
It also lasts a long time, as much as eight hours depending on the concentration. Eaton says you generally don’t need concentrations above 30 percent, since higher concentrations don’t last longer and aren’t much more effective.
DEET is synthetic, meaning it doesn’t exist in nature but was invented by people looking to improve on natural repellents. Being synthetic isn’t necessarily good or bad, although it does mean that the human species didn’t evolve around the substance, and therefore it can be harder to predict effects.
DEET has been linked to a small number of medical issues, and gives some people a skin rash. In extreme cases it can be a neurotoxin, but such cases are extremely rare, although they are the subject of concern from some researchers.
Fortunately, human skin is an excellent barrier. Keep DEET products away from mucous membranes (eyes, nose and mouth) and don’t apply them to young infants.
An annoying side effect of DEET is that it can melt plastic and some synthetic materials, which contributes to its scary reputation. Don’t spray it on your sunglasses. It’s also smelly and greasy.
Picaridin is also a synthetic drug, made using plant extracts as the base material. It was invented by Bayer in 1980 and is generally as effective as DEET against mosquitoes, although not always quite as long-lasting. It’s not quite as effective as DEET against ticks.
The general rule from studies is that 7 percent picaridin is equivalent to about 10 percent DEET for one to two hours of protection from flying bugs, while 20 percent picaridin and about the same DEET concentration offers four to five hours protection. Eaton says only the highest concentrations of picaridin are effective against ticks.
Picaridin has no known health issues, although it hasn’t been around as long as DEET or been studied as much. It isn’t very smelly or greasy and doesn’t hurt plastic.
Permethrin is an artificial chemical pesticides against ticks that is toxic on contact, so it does repel them or kill them. Permethrin does not affect flying insects; it’s just for ticks and chiggers.
Aside from permethrin sprays, you can buy clothing impregnated with the chemical as a defense against ticks grabbing onto you as you walk through tall grass. But be careful, because permethrin is very toxic to cats. If you’re wearing anti-tick trousers, don’t let the family cat sit in your lap.
A number of products exist whose active ingredient comes directly from something in nature, usually a plant. As a general rule, these are less effective than synthetic compounds or don’t last as long.
Oil of eucalyptus lemon. This is the most promising of the natural products, although Eaton says it falls far short of synthetic repellents.
The Centers for Disease Control and Prevention found that oil of lemon eucalyptus was effective against U.S. mosquitoes “similar to repellents with low concentrations of DEET.” Sounds great, except Eaton says this level only lasted five or 10 minutes; after that, the effectiveness went down quickly.
Consumer Reports is less pessimistic, saying that some formulations can last several hours, although it varies widely among brands.
Although it’s natural, oil of eucalyptus lemon (which is a species of eucalyptus tree) isn’t completely safe. The CDC says pure oil of eucalyptus lemon should not be used because it hasn’t been tested for health effects, as compared to products that have a concentration of the oil. Also, no eucalyptus lemon product is recommended for children younger than 3.
Many products contain the synthesized version, called PME or para-menthane-3,8-diol, which is the same active ingredient as the natural oil.
Citronella, which is derived from two Asian grasses, is one of the oldest commercially available insect repellents. It may be best known in candles that are said to help ward off mosquitoes.
A 2002 study in the Journal of the American Medical Association declared it to be largely ineffective as a repellent, while other studies have found it works against some common mosquito species (Aedes, the species that carries Zika virus), although it is nowhere near as effective as DEET. A meta-analysis of 11 studies found no good evidence whether it works against other mosquito species including Culex, the main transmitter of West Nile virus in New Hampshire.
Applications combined with vanilla oil are likely to last longer.
– A chemical derived from the wild tomato plant called 2-Undecanone, marketed under the trade name BioUD, has been shown in some studies to work as well as DEET against some mosquitoes and ticks, but it doesn’t appear to be registered for sale in New Hampshire at the moment, Eaton said. Consumer Reports does not recommend it.
(David Brooks can be reached at 369-3313 or dbrooks@cmonitor.com or on Twitter @GraniteGeek.)
