17 natural insect repellants in home made soap making: As proven by science

The past few weeks I’ve had a new problem which I’ve never had to deal with in my life before. I don’t know if it’s because I’m heavily pregnant (do the hormones change the way I smell?) or if the wasps in Donegal are just more persistent than the rest of the world, but I keep getting them showing way too much interest in me.

I actually got stung by a bee for the first time in my life last week! I was stuck at some roadworks and a bee flew between my dress and the car seat. I had no idea it was there, so when I leaned back to wait for the line to move, it stung me! Usually, insects avoid me. So I started wondering about natural insect repellants.

Being a scientist at heart, I couldn’t just buy any random essential oil rumored to work as a natural insect repellant, so in this article I’m going to give you an overview of the scientific evidence with links back to the original research so you can investigate for yourself which essential oils make the best insect repellants for soapmaking.

The factors affecting how effective an essential oil is as an insect repellant:

One of my favourite articles on this topic is a really detailed meta-analysis done by Maia and Moore in 2011, where they compared the results of a huge number of studies done on essential oils including citronella, neem, and the pine/cedar and mint families of essential oils. They found varying effectiveness. The main factor affecting how well an essential oil worked as an insect-repellant was the type of insect. Even different sub-species of the same insect could react differently to the same oil.

For example, two different types of mosquitoes are An. Arabiensis and An. gambiae. Studies have shown that citronella oil gives 90% protection from An. Arabiensis for 6 hours, and 100% protection from An. Gambiae for 6-7 hours.

Another example is thyme (variety: thymus vulgaris). This was found to offer 100% protection against An. albimanus for up to 105 minutes while it only offered 91% protection against C. Pipiens sallens for 65 minutes in a different study.

The concentration of the various natural compounds in an essential oil also makes a difference to the effectiveness. Using the above example again, thymus vulgaris offered 91% protection against C. pipiens sallens for 65 minutes when it was applied topically (directly on the skin) as linalool. When it was applied topically as thymol, it offered 91% protection for 70 minutes against the same insect. And when it was applied topically as carvacrol, it offered 95% protection for 80 minutes against the same insect.

This shows that different compounds in the essential oil can make it more or less effective. For best results, you need to ensure your essential oils are top quality and not diluted with any other compounds before you add them to your soap.

So if you’re looking to repel a specific type of insect, such as headlice, wasps or mosquitoes, it’s worth reading through this article to find out what will work best.

Which common essential oils work best as general insect repellants?

Citronella:

Citronella is widely known to be an excellent insect repellant. And now studies have been done to support this. The Israel Medical Journal published a double-blind study showing that, when citronella was used on school-age children, 12% got headlice, compared to 50% of the control group (who didn’t use citronella). That’s a reduction of 76%! Article here. It was also studied extensively in the article I discussed above, by Maia and Moore.

Neem:

Neem oil is well-known as an insect repellant. It has been shown to repel mosquitoes effectively by Sharma et al (1993) who found it provided 100% protection for 12 hours against mosquitoes. They mixed the neem oil with coconut oil then applied it directly to the skin. The results were published in the Journal of the American Mosquito Control Association. Article here.

In a second study, researchers looked at how effective neem oil was for repelling headlice. The study can be criticized as it had a complicated design measuring multiple factors at the same time (making it impossible to control variables), and a very small sample size (only 47 participants in total). The results showed combing with conditioner alone was 25% effective in removing lice while combing with conditioner and using neem oil was 35% effective in removing lice. The results were published in Advances in Pediatric Research. Article here.

Potentially confounding factors that made this research not very scientific include: The age of participants. Anyone from 6 months to elderly could participate. The participants were recruited from the local area and had to have at least “one headlice” to participate. Obviously, the treatment for someone with a mild headlice infestation or “one headlice” is going to be significantly easier than dealing with a severe infestation that has affected someone’s entire family for months. The home situation was not considered: It wasn’t considered whether pillows, bedding, towels etc were causing re-infestation before the person had been assessed as “cured”. In a home where multiple participants all have lice, the whole family should have been treated together and this was not done because they excluded anyone with specific hair treatments (e.g. coloured hair) and they didn’t control for cleanliness of the house, or sharing of hairbrushes, hats etc, all of which would cause re-infestation. So overall, I’m not happy with the lack of rigour of this study but it’s a great example of why “proven” results don’t always work the same way in real life.

In another multi-study review, Rossini et al (2008) found that neem oil had documented anti-lice activity. Link here. And in an analysis comparing evidence for neem oil and other natural oils for headlice, Heukelbach et al (2007) found that neem oil had an effectiveness of over 98%, repeated across two different studies. Unfortunately, the natural oils they looked at in this study were often mixed into other products so it’s not clear if it was the essential oils or other ingredients in those products that got results.

Tea Tree Oil

Tea tree oil is sold almost everywhere in the UK and Ireland and I think a lot of people use it to try and repel head lice. Di Campli et al (2012) found a 1% concentration of tea tree oil killed 100% of headlice within 30 minutes, and a 2% concentration also killed 50% of lice eggs (full article here).

In other parts of the world, it’s used against other insects. In a study in Indonesia, researchers showed tea tree oil repels and even kills T. castaneum (commonly known locally as the red flour beetle). Research results here (this will download a PDF file from the researchers, as the researchers haven’t put this on a web page for some reason).

Meanwhile Fonesca-Santos et al (2016) researched whether a commercial mosquito repellant could be made from tea tree oil and found it was very effective against the A. aegypti breed of mosquito. You can read about it here.

Great, but what about using essential oil to repel wasps?

Wasps are my main concern right now. We had two more in the house while I was researching and writing this article, today, and I’m so tired of ejecting them.

A study was done by Boevé et al (2014) to assess whether essential oils worked to repel wasps. They tested many different essential oils alongside conventional chemicals, and repeated their tests several times with different wasps, which makes the study more reliable. They found winterberry oil (galutheria procumbens), marjoram oil (o. marjorana), anything from the artemisia genus (over 400 species of plant, including tarragon and mugwort) and wild mint (m. arvensis) were all highly effective at repelling wasps (more effective than DEET, in fact). As far as chemical compounds go, they tested linalool (a natural chemical found in a lot of citrus plants, including citronella) with good results too. You can read about it here.

In another study by Zhang et al (2012), 21 different essential oils were tested to find out how effectively they repelled wasps (if at all). 17 of the essential oils were found to be highly effective, including clove, pennyroyal, lemongrass, ylang ylang, spearmint, wintergreen, sage, rosemary, lavender, geranium, patchouli, citronella, Roman chamomile, thyme, fennel seed, anise and peppermint. Read the full study here.

And finally…

As you can see, there is a huge amount of rigorous, repeatable, reputable scientific evidence proving that essential oils can make excellent insect repellants. For soapmaking, this gives you tons of options for making soaps that are insect repellent but which also smell nice. From my own experiments in this area, I recommend combining no more than three essential oils in one soap. Don’t try to make one soap that repels everything.

I suggest you make a test batch and try it on yourself in the shower before making a bigger batch to give to friends and family, as some of the strong insect repellant essential oils can also be irritants in soap or shampoo bars. For the same reason, you may want to reduce the amount of essential oil in your soaps to avoid ending up with itchy skin. Lastly, be extra-careful using any potent essential oil or other insecticide on children’s sensitive skin.


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How to calculate the yield from a soap recipe

When you’re making a soap recipe, one thing you might wonder is how to calculate the yield from your soap recipe.

Yield noun – the amount or quantity produced or returned.

There is a really easy way to calculate the yield from any recipe. However, some real world variables will affect the calculation, and in practice, you will find that the yield from any given soap recipe is a bit less than it ought to be.

Why don’t soap calculations produce an exact weight?

A viscous liquid is thick. As a liquid gets more viscous, it pours less readily, until eventually a liquid can be so viscous it doesn’t pour at all (like whipped cream). Soap is a liquid which is viscous, whether it’s melt and pour or a cold process soap that has reached trace. A light trace is less viscous than a heavy trace, but both are more viscous than water.

When you try to pour a viscous liquid, some of it will stick to the sides of the bowl or jug. It will also stick to your stick blender or stirring spoon. This results in a loss of about 10-20g of soap batter per batch. If you are making a small batch such as a single bar of soap, you can lose a significant amount.

The best way to compensate for this loss is to make a bigger batch, because you will not lose much more batter from a larger batch than from a smaller one.

With that in mind, here is how you calculate soap yield:

Add together the weight of all the solid ingredients. Convert water into grams (1ml of water weighs 1g). Other liquids don’t convert 1:1, because a ml is a measure of capacity while a gram is a measure of mass, and the mass of a given capacity is dependent on the density of its molecules. Oils have long chains of molecules, where water has small molecules made of only three atoms, so more water molecules fit into the same space as any oil, so water will always be heavier and more dense than oil for the same capacity.

Confused? Here’s how it works on a practical level. With liquid oils, such as avocado oil, you will need to weigh them separately. Do this before you mix your ingredients together. With electric scales (recommended for soaping), you can do this by turning on your scales and putting an empty container on top, then pressing “tare”. This will set the scales to ignore the mass of the container and just weigh what you put inside it. Next, pour your oil into the container. This will tell you how much it weighs. Add this to the mass of all the other ingredients and this will tell you the total yield of your soap recipe.

Lye dissolves in water, so do I need to weigh it?

Yes. This is because, when you add the lye to the water, even though it dissolves, it is still in the container. The mass of the water increases by the mass of the lye. Any time something soluble (like salt, sugar, or sodium hydroxide lye) is dissolved in water, the mass doesn’t go anywhere.

You can put this to the test if you want to do some at-home chemistry by getting a cup of 100ml water, stir in 50g of salt, and weigh the total mixture. You’ll see the liquid will now weigh 150g and it will have noticeably increased in volume, too. This is because of one of the laws of physics which explain how the universe works.

Example:

Melt and pour recipe (taken from my Easy AHA exfoliating melt and pour soap recipe which you can find here). These are the ingredients:

10 ml Cherry kernel oil

90g Melt and pour soap base

1 ml Cherry blossom fragrance

A pinch of sliced up loofah

The cherry kernel oil needs to be weighed. It weighs 8 grams. Add this to the melt and pour soap base and we get 98 grams. It wouldn’t work to try to weigh 1ml of fragrance so we will round it up to 1 gram although it’s more like 0.9g.

In a higher yield recipe (e.g. making a kilo of soap) we would use a lot more fragrance so we would be able to weigh it but here it will not make much difference. So our total is 99g. In a 100g soap mold, this leaves a tiny bit of room for the sliced up loofah to go slightly under the surface of the soap without it spilling over the mold.

90g + 8g + 1g = 99g

So that’s how to calculate the yield for a soap or cosmetics recipe!

PS I’m super excited that my lye just arrived, so I’ll be trying out some cold process soapmaking as soon as my new stick blender gets here (mine died last year before the first lockdown), and I’ll be sure to share my makes (and fails… part of the learning process) as I move into this awesome new world of handmade soapmaking!