What are Horticultural Oils?

Horticultural oils are petroleum-based or vegetable-based pesticide oils which are used to control insects and mites. Being non-toxic, they work by smothering the pests and suffocating them, and as such, they’re a direct contact pesticide, meaning that you have to spray the pest directly at the time of application for the pesticide to be effective. Horticultural oils are also non-selective, that is, they kill whatever insect or mite you spray them on, regardless of whether they’re pests or beneficials.

Both natural and synthetic horticultural oils are mixed with water for spraying, but as we are all well aware, oils don’t normally mix with water. To allow them to mix with water, horticultural oils also contain emulsifiers, soap-like substances which break up the oil into tiny droplets, allowing them to evenly disperse through the water, forming what us called an emulsion (basically oil mixed in water).

A Brief History of Horticultural Oils

The use of oils to control pests is not a new technology by any means. In 200 BC, the Roman senator and historian Marcus Porcius Cato (also known as Cato the Elder) advocated oil sprays for pest control. Before the era of fossil fuels and petroleum refinement, natural oils were historically used to control pests, and with industrialisation, there was a shift towards synthetic petroleum-based oils.

The early petroleum-based oils were called dormant oils, because they were heavy, unrefined oils which contained substances that were toxic to plants and damaged their leaves, and could therefore only be used on dormant deciduous plants and trees (which have dropped their leaves).

Manufacturers eventually refined the petroleum-based oils to remove toxic impurities, such as compounds containing sulphur, nitrogen or oxygen and aromatic compounds. By the additional processes of filtration, distillation and dewaxing, manufacturers were able to produce the very light and highly purified petroleum-based horticultural oils available today, which can be used in all seasons of the year and do not cause leaf burn.

With the trend back to natural pest control methods, we’ve seen increased interest in the use of vegetable-based oils for pest control in recent years. Some of the vegetable-based pest oils also contain additional plant oils such as neem oil, an extract from seeds of the neem tree (Azadirachta indica), which contains the compounds Azadirachtin A & B, which have insecticidal and fungicidal properties.

Which Pests and Diseases Do Horticultural Oils Control?

Traditionally horticultural oils were developed for hard-to-control pests on fruit trees that overwintered in crevices in the bark while the trees were dormant, such as mites, scale, aphids, mealybugs and the eggs of some caterpillars.

Modern horticultural oils can be used all year round and are effective in controlling scale, aphids, two-spotted mite, mealybug, whitefly and citrus leafminer. They can possibly be used on plant bugs, lace bugs and some caterpillars.

Horticultural oils may also be used to prevent certain fungal diseases such as powdery mildew and rose black spot.

Additionally, horticultural oils do have some deterrent effect, as pest insects don’t like laying their eggs on oily leaf surfaces, so spraying may deter early outbreaks of aphids, scale and mites on fruit trees.

When horticultural oils also contain other plant oils such as neem oil, they may be effective against a broad range of chewing and sucking insects, killing pests which ingest the oil after it has been sprayed.

Advantages of Horticultural Oils

Horticultural oils are relatively non-toxic, and are quite safe to humans and wildlife. They don’t have an objectionable smell, and can be used indoors as well as outdoors. In terms of cost, horticultural oils are fairly cheap compared to other pesticides. Additionally, they will not harm non-target beneficial insects such as bees, parasitic wasps and ladybirds if you don’t spray them directly! Horticultural oils evaporate quickly, and once they dry they have little residual toxic activity to beneficial insects.

Disadvantages of Horticultural Oils

The biggest problem with horticultural oils is that they have to contact and cover the pest to be effective. This means that you have to be very thorough when spraying, being sure to spray the whole plant or tree, including areas such as the undersides of leaves and cracks in the bark, as these are places where pests often hide. It also means that immobile, slow moving and very small pests are easier to control than larger, faster moving and flying ones.

How to Use Horticultural Oils

As horticultural oils work by direct contact, sprays must be evenly applied, covering all plant surfaces including both the tops and undersides of all leaves, as well as stems, branches and bark.

Horticultural oils are commonly sprayed in winter on deciduous fruit trees and roses to control pests such as scale, aphids, mites and mealy bugs. The first winter application is usually carried out after pruning and a second application just before bud burst occurs. Spraying can also be done at any other times when pests are observed.

To control citrus leaf miner on citrus trees, spraying is done when new flush growth is present, and application is repeated every 5 to 14 days.
With vegetables and ornamental plants, to control mites, aphids and greenhouse whitefly, spraying is done when pests first appear and then repeat applications are carried our as directed on the product label.

Horticultural oils can also be used on Indoor plants to control scale insects, mealy bugs and spider mites. Some indoor plants, such as maidenhair ferns, are more sensitive to leaf damage so it is recommended to test any horticultural oil on a small area of foliage before using to avoid yellowing or leaf burn.

The withholding period is the amount of time to wait before consuming any produce that has been sprayed. With petroleum oils, do not apply to edible crops later than 1 day before harvest. Plant based oils are safe for use on vegetables with no withholding period, so you can spray and eat on the same day, just wash thoroughly before eating as you normally do.

Precautions

  • Do not spray when temperatures are near or above 35°C because heat increases risk of leaf injury through drought-stress.
  • Only use on healthy, unstressed plants, do not apply to plants suffering from heat or moisture stress.
  • Do not use on citrus trees in late autumn/winter, as this can sometimes cause increased susceptibility to winter injury, wait until after winter hardening has occurred before spraying.
  • Do not spray directly on to sensitive flower heads to prevent oil spotting.
  • Do not combine with sulphur or sulphur-containing sprays such as lime sulphur as they can react with oils to form phytotoxic compounds (substances toxic to plants). Also, do not mix with fungicides such as Captan (which contains sulphur) or pesticides such as carbaryl.
  • Do not use horticultural oils within 30 days of a sulphur application because elemental sulphur can persist for long periods, check label directions as most oils prohibit their use.
  • Do not use on plants that tend to be sensitive to oils.

Environmental Impact of Synthetic and Natural Horticultural Oils

Synthetic horticultural oils such as ‘White Oil’ list their active ingredient as petroleum oil, paraffin oil, or sometimes as naphtha, it’s all pretty much the same thing, just a refined petroleum oil mixed with an emulsifier to make it mixable with water. By comparison, natural horticultural oils are emulsified plant oils.

Environmental impact of any garden product can be broken down into the categories of toxicity, soil mobility, persistence and bioaccumulation potential.

If we compare the toxicity of the natural and synthetic petroleum oils, we see some differences. In general, they’re both classed as low in toxicity to animals, birds, bees and fish, but moderately toxic to earthworms and some other aquatic organisms such as molluscs.

If we have a closer look at human toxicity, we see that the petroleum oils do present a higher toxicity hazard in terms of chronic toxicity hazard in the areas of reproductive and developmental toxicity.

According to the review by Thurston County Health Department in the US, “When evaluating petroleum oils for potential for reproductive and developmental toxicity, there were some effects that suggest that fetal toxicity can occur at doses without maternal toxicity. The EPA concluded that the very high doses used in the toxicity tests were so much higher than those expected from pesticidal uses that there are no concerns for potential sensitivity of infant and children to mineral oils and aliphatic petroleum hydrocarbons” (Reference: USEPA. Office of Pesticide Programs, Special Review and Reregistration Division. Revised Reregistration Eligibility Decision, Exposure and Risk Assessment on Lower Risk Pesticide Chemicals. CASE: Aliphatic Solvents (3004). Active Ingredients: Mineral Oil (063502) & Aliphatic Petroleum Hydrocarbons (063503). Revised: November 29, 2007.)

Thurston County Health Department qualify their statement by explaining that they lowered the risk rating from high to moderate in light of the US EPA assessment:  “Thurston County pesticide review system typically rates reproductive or developmental toxicity without maternal toxicity as high in hazard but since the EPA has determined that the concentrations that toxicity was observed was much greater than those expected from pesticidal use that the hazard is rated moderate.”

So, overall, petroleum oils are moderately toxic to humans, as compared to natural plant oil formulations which are often used in foods, and are considered low in toxicity.

The routes of exposure are typically through inhalation or skin contact. According to the safety data sheets for petroleum-based horticultural oils, to minimize risk:

  • Avoid skin and eye contact and inhalation of vapour, mist or aerosols.
  • Use personal protection equipment: Overalls, Safety Shoes, Safety Glasses, Gloves.
  • If risk of inhalation of exists, wear organic vapour/particulate respirator meeting the requirements of AS/NZS 1715 and AS/NZS 1716.
  • Consumer Use: Wear gloves. Wash hands after use.
  • Hygiene measures: Keep away from food, drink and animal feeding stuffs. When using do not eat, drink or smoke. Wash hands prior to eating, drinking or smoking.

While some of these measures sound overstated (overalls and safety shoes for spraying horticultural oil?), exercise common sense and take the necessary measures required to avoid breathing in petroleum oil or getting it on your skin!

Both natural and synthetic oils have low soil mobility, being poorly soluble in water they are expected to bind well to soil and vegetation, so there is little to no risk of run-off and contamination of groundwater and waterways.

If we look at environmental persistence, how long it takes for the product to break down, we find that natural oils have a low persistence, they break down very quickly into natural substances being plant-based, while synthetic oils have a high persistence, and tend to remain in the soil for a long time. Most synthetic petroleum oils aren’t broken down by sunlight or hydrolysis (breakdown through interaction with water), and are likely to take over 60 days to biodegrade to half of the applied concentration.

The synthetic petroleum oils are poorly absorbed when ingested, inhaled, or through skin contact, and are quickly eliminated from the body, unchanged and not metabolized, so their potential to bioaccumulate is low – that is, the levels of pesticide do not build up in plants, animals and other living things.

By comparison, natural plant oils, being harmless food products, if ingested, are likely to be absorbed and metabolized by humans, animals and fish, but exposures from uses in pest control are unlikely to accumulate to a toxic level, so the risk of bioaccumulation is also rated low.

In SGA’s Garden Product Guide – Safe for You ‘n’ Nature, which rates garden products on a 6-star rating (where 6 stars indicates a very low environmental impact and 1 star indicates a very harmful product environmentally and for humans), natural horticultural oils typically rate well with 5 out of 6 stars, while synthetic petroleum-based horticultural oils rate less favourably at 4 out of 6 stars.

 

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