Mosquito repellents

When it comes to mozzie sprays, products with diethyltoluamide (DEET) are known as the gold standard. However, some people don’t like how it smells and feels on their skin. It can also damage some fabrics and plastic items such as sunglasses.

Some consumers are also concerned about the safety of DEET. A survey conducted by US consumer organisation Consumer Reports found 25 percent of Americans said they avoided using repellents with DEET. About a third of respondents said they thought DEET products weren’t as safe as the alternatives.

So how do the alternatives stack up? We tested four natural products containing plant oils, four picaridin-based products, and nine DEET products to find out.

What we found

The DEET-based products we tested contained between seven and 40 percent of the active ingredient. Some of these DEET products also contained other active ingredients (see “Mosquito repellents” table).

All tested products containing DEET performed similarly regardless of how much the product contained, and repelled mozzies for up to six hours (which is how long we tested effectiveness for).

It was a similar story with picaridin repellents. Products with picaridin were good at repelling mosquitos regardless of concentration. Off! Family Care (9.5 percent picaridin) was as effective at repelling mozzies after four hours as Off! Tropical Strength with nearly double the picaridin.

Bug Grrr Off Natural Personal Insect Repellent Jungle Strength was the only natural product to go the distance. It was as effective as the DEET and picaridin-products. With 36 percent lemon eucalyptus oil, it kept mozzies at bay for six hours. In comparison, Repel OLE! with the same active ingredient, lost effectiveness after one hour. The label didn’t specify the percentage of active ingredient.

Thursday Plantation Walkabout Insect Repellent Roll-on contains plant oils, including tea tree (melaleuca) and citronella. It provided 90 percent protection for two hours and kept the majority of mozzies away for six hours.

Goodbye Sandfly Repellent + Bite Soother, with a combination of essential oils, was very good at initially repelling mosquitoes, but its effectiveness wore off more quickly than other products.

John Sanderson, co-owner of Goodbye Sandfly said its product works differently to a chemical repellent and was mainly developed to deter sandflies.

“It can take up to 60 seconds for the heat of your skin to begin evaporating the oils. Once the oils are evaporated there is no more protection. To get the best results, Goodbye Sandfly should be applied frequently in small amounts. This might be as often as every 20 minutes, but usually every one to two hours. This will depend on the weather and what activity people are doing,” he said.

We also tested two insect repellent-sunscreen combos from Bushman and Rid. We’re not fans of these products – sunscreen should be reapplied every two hours, whereas repellents usually last for several hours. If you need to use repellent and sunscreen together, it’s best to apply the sunscreen first, let it dry, then put on the repellent.

What’s in them?

The Health Navigator Charitable Trust website lists DEET (diethyltoluamide), picaridin, IR3535 and oil of lemon eucalyptus (also known as OLE or PMD) as providing reasonably long-lasting protection.

DEET is referred to as the “gold standard” for repelling mozzies. The US organisation Centers for Disease Control and Prevention (CDC) states DEET is safe to use as long as you follow a product’s directions.

DEET isn’t recommended for babies younger than two months and products with more than 30 percent shouldn’t be used on children. Some people could experience skin problems, especially if used in high concentrations or in large quantities for several days.

DEET can damage some synthetic fabrics, as well as plastic and leather, and some people don’t like the smell. When it comes to the environment, it’s OK to use. DEET gets into the air when you spray, but the chemical is broken down by sunlight and air. In water, DEET is degraded by aerobic microorganisms.

Picaridin is based on a molecule found in the black pepper plant. It has little odour, doesn’t feel sticky or greasy, and is less likely to irritate the skin. Products containing 20-25 percent are recommended as being effective. Picaridin isn’t recommended for children younger than two years.

Oil of lemon eucalyptus is the most effective natural product. It’s not recommended for children younger than three years.

Some natural products contain plant oils with known insect-repelling capabilities such as citronella, melaleuca, lemongrass, eucalyptus or lavender. These oils have an initial repellent effect, but it diminishes quickly so these products need to be regularly reapplied.

Our test

To test repellency, each product was tested on four people in an independent laboratory. Each person had a repellent applied to their forearm according to the directions, before placing it in a cage containing approximately 40 unfed mosquitoes. Cages are specially constructed out of clear plastic for easy viewing and fine polyester netting for ventilation.

At set time intervals (up to the amount of time a product claimed to be effective with a maximum of six hours), volunteers exposed their arm for three minutes and the number of attempted and successful landings were recorded. An attempted landing was when a mosquito touched the forearm but didn’t remain there. A successful landing was when a mosquito landed and remained for more than one second, before being shaken off by the tester.

Before the test, each volunteer exposed an untreated arm to the mosquitoes. There was no significant difference in the number of attacks during the control trial.

Our test was tough – you’re unlikely to find that many mozzies in close proximity out and about. How long a repellent lasts also depends on the conditions and what you’re doing – sweating and swimming will shorten protection time. All our tested products also claim to protect against other annoying bugs, but we didn’t test this. Repellent effectiveness can vary between insect species, including different mosquito species.

Natural oil-based repellents compared

Tips for using repellents

• Apply repellents evenly to all areas of exposed skin. Reapply if you go swimming or sweat heavily. Don’t apply repellent to broken skin.
• Don’t apply aerosols or pump sprays directly on to your face. Spray them on your hands first and then rub on evenly, avoiding contact with your eyes and mouth.
• Don’t use near food, and wash hands after applying and before eating or drinking.
• Before using a repellent on your baby or child, check that it’s suitable for their age. Use netting on prams, cots and play areas to avoid bites.

Protection overseas

New Zealand mozzies don’t carry diseases, but it’s a different story in other countries. Malaria, dengue fever, yellow fever and zika are some of the illnesses spread by biting insects.

Dr Yvonne Partridge a travel medicine specialist strongly advises those travelling overseas to use insect repellent and other measures to avoid bites. Dr Partridge recommends a 30-40 percent DEET repellent or 20-25 percent picaridin product.

“These should provide protection up to six to 10 hours”, she said.

“Oil of lemon eucalyptus is an approved natural repellent alternative to DEET or picaridin. But repellents based on plant-based oils such as citronella shouldn’t be relied on where there’s a risk of insect-borne disease.” Dr Partridge said.

To minimise the amount of repellent required, travellers should cover up with long-sleeved shirts and trousers when outdoors and sleep under protective netting or in air-conditioned accommodation. For added protection, permethrin can be sprayed on clothing, mosquito nets or sleeping bags. Permethrin kills mosquitoes and ticks, is safe and doesn’t stain. The effect lasts for about 10 washes on clothes or six months on mosquito nets and sleeping bags. Some tramping equipment comes pre-treated with permethrin.

Dr Partridge says there’s no evidence you will repel mozzies by taking vitamin B, eating marmite, yeast or garlic, or wearing mosquito-repellent wristbands.

“Wristbands (even those containing DEET) are of limited benefit because they only protect the skin near the wrist,” she said.