Essential oil pesticides

herbs in a mortar and pestle

Once you start enjoying the fruits; and, vegetables in your organic garden, it’s not long before you start receiving visitors. Meet the arthropods. Those freeloading vegetarians who come swinging their many limbs and whose intent it is to share the bounties of your hard labour.

Organic gardeners have a few options. Ignore them — pray for some leftovers. Physically remove them, or, murder. If you choose the last option, why not try a lethal whiff or contact with some killer herbs and spices.

The effectiveness of plant essential oils as botanical pesticides continues and is being confirmed. Several products have been formulated and commercialised.

These essential oil products bypass the regulatory control of scheduled poisons or registered pesticides in the US and they are considered food safe products.

What are they and how do they work?

The process of producing essential oils is generally via steam distillation. The end result is a volatile oil containing 100’s of compounds, some identifiable, many not.

Terpenoids are one of the many compounds and they play a major role in repelling insects says Canadian entomologist and toxicologist, Dr Murray Isman.

Isman has been investigating the development of pesticides for 30 years, and has a particular interest is in discovering how the compounds in essential oils affect insects and their fate in the environment.

He says that while some of the pure essential oil compounds are slightly toxic (to humans) ie carvacrol and pulegone, a proprietary mixture of essential oil constituents fed to rats at a high dose were not lethal.

Essential oils have several modes of action against insects and mites including repellent and anti-feedant deterrence, inhibition of moulting and respiration, reduction in growth and reproduction, and cuticle disruption.

Many of these effects result from interference on the invertebrate octopamine pathway. Octopamine is a neurotransmitter unique to invertebrates. The advantages of these many modes of action is that they may delay resistance development in the target pest.

“Essential oils may have minimal direct and/or indirect effects on pest enemies, although bees appear to be sensitive,” says Isman.

Any plant essential oils containing eugenol or thymol, ie thyme (Thymus vulgaris) rosemary (Rosmarinus officinalis) and clove (syzygium aromaticum) are effective pesticides. They can be applied as a contact or fumigant.



The Greenhouse whitefly Trialeurodes vaporariorum, is a major pest of greenhouse vegetables, especially tomatoes, cherry tomatoes, cucumbers, and ornamentals. This insect has also developed immunity to many conventional insecticides. Several essential oils have been found to be effective against all insect stages, including eggs, nymphs and adults.

• Bay
• Caraway seed
• Clove leaf
• Lemon eucalyptus
• Lime
• Pennyroyal
• Peppermint
• Rosewood
• Spearmint, and
• Tea tree.

Spider Mites

A big eater of greens are the small spider mites (Tetranychus sp). Their feeding on chlorophyll in plants cells interferes with the plant’s ability to grow. When bronzing occurs under leaves, you can guarantee numbers are high. So voracious are their appetites they can kill the plant.

Did you know some mites live in the gills of edible mushrooms?

Spray the oil of oregano, (Origanum vulgare) or thyme (Thymus vulgaris) or mint (Mentha spicata) where the Carmine spider mites, (Tetranychus cinnabarinus) are feeding. These minute acarides will go on a hunger strike, and stop growing and procreating.

Isman and a colleague, studied the effect of rosemary oil (Rosmarinus officinalis) against the two spotted spider mite (Tetranychus urticae Koch) on tomato plants. The oil was effective on contact and even high doses did not harm the tomato plants. Other researchers have also found it useful as a fumigant.

The problem with any form of lethal control is the effect on pest predators. In the study above it was found that when both spider mites and predatory mites (commonly used for biological control) were sprayed with different pesticides containing rosemary oil, no mortality was found among predators, but up to 60% mortality was observed in the two spotted mites.

The researchers believe this might be due to the differences in the way rosemary oil is metabolised by predatory and phytophagous mites.

Moths and aphids

Through Isman’s research with rosemary essential oil, it was illustrated that camphor was the most toxic compound to larvae of Pseudaletia unipuncta, a noctuid moth larva, followed by d-limonene and p-cymene. Against the larvae of Trichopulsia ni, (cabbage looper) α-terpineol was the most toxic followed by p-cymene and β-pinene. Rosemary oil also had good toxicity to aphids.

Many of the Eucalyptus species are potent for insect control (E. alba, E. camaldulensis, E. citriodora, E. deglupta, E. globulus, E. Rob) (See Appendix 1.) Eucalyptus globulas- Blue Gum, offers very good control of many insect pests.

The oldest tree on earth, Ginkgo biloba has been investigated as a pest deterrent with lab tests showing snails were repelled from eating lettuce leaves to which Gingko extract had been applied.


Fungi can be a friend or destroyer of plants, as Anton de Bary called the genus Phytophthora; a serious plant pathogen. This single organism caused more than one million people to die of starvation and initiated one of the largest human migrations on the planet. Collectively, fungi and fungal-like organisms cause more plant diseases than any other group of plant pest with over 8,000 species shown to cause disease.

But the cycle of life is not complete without fungi. Fungi participate in recycling decaying dead animal and plant materials converting them into nutrients for the benefit of plants.

Beneficial fungi, mychorrhizae, also grow in harmony with the roots of plants providing a beneficial relationship.

Some of the fungal pathogens and their remedies include:

White mould

Essential oils can help control some fungal pathogens. White mould (Sclerotinia) is among the world’s most dangerous plant pathogen, especially in areas of high humidity. When it occurs on tomatoes it can be controlled using oregano and fennel oils as a bio-fumigant.

Grey Mould

Another pathogen of tomatoes and ornamentals is grey mould, caused by (Botrytis cinerea Pers ex.Fr). This fungus infects leaves, stems, flowers and fruits, either by direct penetration or through wounds. Infection is encouraged by high humidity and low temperatures, particularly if moisture is present on the plant.

Research by Turkish scientist Soner Soylu found that essential oils obtained from oregano (Origanum syriacum L. var. bevanii), lavender (Lavandula stoechas L. var. stoechas) and rosemary (Rosmarinus officinalis L.), were effective against Botrytis cinerea, a parasitic fungus that affects many plants.

Contact and volatile phase effects of different concentrations of the essential oils were found to inhibit the growth of Botrytis. Volatile phase effects of essential oils were consistently found to be more effective on fungal growth than contact phase effect.

This volatile phase was achieved in vitro and can be achieved in a closed environment, but is going to be less effective in the field because of the volatility of essential oil vapour.

The vapour of origanum oil (at an appropriate level) was found to completely inhibit the growth of B. cinerea. Complete growth inhibition of pathogen by essential oil of lavender and rosemary (at an appropriate level) was, also observed.

Contact effects of the tested essential oils, (at an appropriate level) found that oregano oil inhibited the growth of B. cinerea completely. Essential oils of rosemary and lavender were inhibitory at relatively higher concentrations. Spore germination and germ tube elongation were also inhibited by the essential oils tested.

Oregano essential oil used under greenhouse conditions, on tomato plants resulted in good protection against grey mould severity especially as a curative treatment.

Pythium and Phytophthora

Further studies by the Turkish researchers on Curry plant (Helichrysum sp) found different (chemo) types had different and varying constituents. The highest level of oils were found in the flowering tops.

They did find one type that had good anti-fungal action against Pythium ultimum (damping off) and Sclerotium rolfsii (soilborne fungus), a moderate action against Phytophthora capsici (major fungus disease of peppers) and Septoria tritici (septoria leaf blotch of wheat)


Growers may already be familiar with neem oil an insecticide made from the seeds of Azadirachta indica.

It was teamed up with fish emulsion and for both years in a two-year study weekly foliar sprays reduced severity of bacterial leaf spot (Xanthomonas campestris pv. vesicatoria) on the foliage of inoculated field-grown tomato and pepper plants.

The study showed no phyto-toxic effect on tomato or pepper foliage in the field.

Anecdotally an infusion of peppermint leaves also offers mild protection against this bacterial disease.


The term ‘weed’ to the organic gardener is always under review. For the herb gardener the reference to weeds is even broader and dandelions, milk thistle and purslane may inhabit garden borders or the veggie garden.

There are some plants that can inhibit the growth of seedlings around them. This chemical interaction has been coined allelopathy. Researchers reported that in the vicinity of aromatic shrubs, such as whiteleaf sage (Salvia leucophylla Greene) or California sagebrush (Artemisia californica Less.), there were no annual plants within a diameter of 90 cm, and presence of annuals was very limited within 2–6 m.

It was found that volatile foliage compounds were the active ingredients causing the repression of growth in the vicinity of the Salvia and Artemisia species. This ecological phenomenon provides a competitive advantage to aromatic plants in their natural environments. These reports suggest that allelochemicals could be used for weed control in agriculture.

Most of the germination and growth inhibitors produced by perennial angiosperms identified by Rice (1984) were phenolic compounds or derivatives of cinnamic acid. Other authors also found coumarins, flavonoids, alkaloids, cyanoglycosides, proteins, and amino acids among the inhibitory compounds

To this list the terpenoids must be added, including the volatile terpenes that are the main components of essential oils.

Isman mentions the problem with the use of essential oils as a herbicide is that if the weed has a waxy cuticle the oil may not penetrate and will be ineffective.

Herbicides for the organic gardeners can include: clove and pine oil Obviously any strong essential oil is going to either burn plants or completely annihilate them.

How to make an organic pesticide


Isman recommends the ratio of any of the essential oils to water is 1:100, unless you are using them to kill weeds then you’re only restricted by financial considerations. Add just one or two drops of surfactant, ie dishwashing liquid and use a phosphate free product.

A crude extract can be made using plant material which is often referred to as an infusion or tissane in herbal circles. So for a peppermint tissane collect a bunch of peppermint leaves and soak. Strain the leftover liquid into a spray bottle.


  1. Don’t overdo the measurements. “One more drop won’t hurt.” It will! You can burn plants.
  2. Before spraying make sure pests are present and correctly identify them.
  3. You will need to spray regularly as the oils will dissipate quickly.

To start

You will need a selection of spray bottles, marker for labelling bottles, and biodegradable dishwashing liquid (phosphate free).

Clove herbicide

30 ml Clove oil*
2 or 3 drops of phosphate free dishwashing liquid
750 ml water

Put the oil and soap into your spray bottle and add water. Give it a good shake to emulsify, then shoot.

*This amount of clove oil could be overkill, but I tried the standard formula recommended and I had limited results, so I would say experiment with this. This amount of clove oil works very quickly.

Rosemary insecticide

1ml rosemary essential oil
100ml water
2 drops phosphate free dishwashing liquid.

Put the oil and soap into your spray bottle and add water. Give it a good shake to emulsify, then shoot.

In summary: the arsenal for the home gardener is a bottle of clove oil, The downside is the cost, as some oils are expensive.

Isman summed it up perfectly when he said you rarely need 100% kill. Most plants (and even some humans) can tolerate small amounts of damage from insects and two sprays, three to seven days apart, will give good control.

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