Petroleum oils are highly refined, paraffinic oils that are used to manage pests and diseases of plants. Petroleum oils may be referred to by many names, including horticultural oil, spray oil or white mineral oil.
Oils are most effective against soft-bodied insects and mites. They are commonly used against mites, aphids, whiteflies, thrips and mealybugs.
Advantages of oils in pest control include safety, effectiveness and limited effects on beneficial insects.
Mode of action of oils
Oils have different effects on pest insects:
- Oils block the air holes (spiracles) through which insects breath, causing them to die from asphyxiation.
- Oils may also act as poisons by interacting with the fatty acids in the insect and/or interfering with normal metabolism.
- Oils can disrupt how an insect feeds, a feature that is particularly important in the transmission of some plant viruses by aphids.
- Their toxic action is more physical than chemical and is short-lived.
Stylet oils are highly refined oils and may be used to manage insect-vectored plant viruses. These oils reduce the ability of aphids to acquire the virus from an infected plant and transmit it to healthy plants. Stylet oils may interfere with the virus’s ability to remain in aphid mouthparts (stylets).
Using mineral oils
- Always read and follow label instructions.
- Cover all plant surfaces, especially the undersides of leaves and stems where pests can hide.
- Avoid large spray droplet sizes by using the right equipment and spray pressure.
- Oil-based pesticides have low residual activity and must be sprayed directly on the insect or mite.
- To minimize the risk of plant injury, avoid treating when temperatures are below 5°C or above 28°C degrees or when the relative humidity is above 90 percent.
- Not all pests/diseases are susceptible to oils. When using any pesticide, proper pest identification is critical.
- Avoid treating drought-stressed plants.
Although generally considered safe, oils can injure susceptible plant species. Symptoms of plant injury (phytotoxicity) may be acute or chronic. They can include leaf scorching and browning, defoliation and stunted growth.
Phytotoxicity may be associated with plant stress, ambient temperature and humidity and application rate. It can vary among plant species and cultivars. To reduce the risk of phytotoxicity, do not treat stressed plants. Applications during the summer season are best in the morning or late evening. The longer wet oil sprays remain on foliage, the greater the chance of phytotoxicity.