Oils

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Introduction

There is a lot of buzz in the organic world regarding essential oils. I should point out that essential oils are quite different from synthetic oils like mineral oil. Essential oils are plant derived natural oils having the characteristic fragrance of the plant from which they are extracted by pressing or distillation. Examples are garlic oil, cinnamon oil, and eucalyptus oil.

Essential oils have been used for centuries in the fragrance market, but in recent decades they have found a place in commercial agriculture. With the move away from traditional, hard, synthetic chemistry due to residues on fruits and vegetables, consumers are demanding “softer” chemistries be used to control weeds, insects, and diseases on crop plants. For this reason essential oils have become routinely used in crop protection products for use on organic crops. And essential oils are generally more sustainable and renewable than conventional agricultural sprays.

Due to their reduced toxicity the Environmental Protection Agency (U.S. EPA) has categorized them as Generally Recognized As Safe (GRAS). This designation greatly reduces the cost and time associated with registering essential oils as organic insecticides, fungicides, and herbicides. This reduction in cost and regulatory burden is attractive to crop protection product manufacturers and it is the reason why we have seen an increase in the number of these products on the market.

Fungicides

Essential oils are used as soft fungicides, particularly for surface pathogens like powdery mildew. Corn oil, soybean oil, cottonseed oil, and other crop oils (not necessarily essential oils) suffocate fungal mycelium and spores. However, once a fungus has penetrated a plant surface you can remove the fungus from the surface but still have tissue yellowing or death underneath. So, when using oil-based fungicides be aware there is a point-of-no return you have to watch out for.

Insecticides

Essential oils are becoming more common in commercial organic insecticides. They are particularly effective on crawling insects or insect stages than cannot fly or adequately clean themselves. As with the oil-based fungicides the oil-based insecticides primarily suffocate insects by coating and plugging the spiracles or breathing holes in the insect abdomen. For this reason oil-based insecticides are used as “contact kill” materials. They do not work well if sprayed on a plant then hoping an insect will contact the material. 

In the insecticide category some essential oils do have a niche role as “repellents”. It is well known that garlic oil is a natural insect repellent and this characteristic is often used in organic insecticide preparations. In fact, one usage pattern is for growers to spray on a contact kill organic oil-based product then follow it up with a garlic oil spray around the field perimeter to keep other insects from entering the area thereby giving a longer residual action.

Herbicides

Oil-based herbicides are also regarded as contact kill materials. However, contact kill does not mean instant kill. Essential oils dissolve the waxy layer on leaves so that the plant dehydrates to death. As you can imagine these materials work better when the weather is hot and dry. They are not as effective in cooler climates. One shortcoming of this approach is that killing the top of the plant leaves the roots intact. The roots can store a lot of energy reserves to push up another plant. For this reason organic herbicides are applied several times to keep killing the new growth coming up until the energy reserves are exhausted. Because they work this way essential oil herbicides have no residual activity. In fact, soil microorganisms will feed on essential oils contacting the ground making them biodegradable.