Petroleum oils

Petroleum oils, often referred to as horticultural or dormant oils, are highly refined hydrocarbon fractions. They act primarily as contact insecticides and acaricides through physical rather than chemical pathways.

The product functions by creating a thin oil film over the insect or mite. This film physically covers the spiracles and tracheae of the pests, causing them to suffocate and die within a short period.

Furthermore, these oils exhibit ovicidal activity. When applied to pest eggs, the oil penetrates the outer shell, disrupting the embryonic development and causing the eggs to desiccate or fail to hatch.

A key advantage of petroleum oil is the absence of resistance development. Because the mode of action is entirely mechanical and physical, pests cannot evolve defensive mechanisms to survive exposure.

Beyond pest control, these oils are frequently used as stickers or surfactants in tank mixes. They improve the retention and spreading of other active ingredients on the foliage, enhancing the efficacy of the whole mixture.

The primary use of petroleum oils is the control of overwintering stages of pests on woody crops. They are particularly effective against species protected by waxy coverings or shells.

Scale insects and mealybugs are major targets. The oil penetrates their protective layer, providing control where many conventional chemical insecticides fail due to the pest's physical shield.

Various mite species, including spider mites and eriophyid mites, are highly susceptible to oil treatments. Regular applications help manage their populations before the growing season fully begins.

Aphids and whiteflies are also common targets. By targeting eggs and nymphs on bark and branches during the dormant period, oils significantly reduce the initial pressure of these pests.

• Scale insects and mealybugs
• Spider mites
• Aphids and psyllids
• Whiteflies
• Overwintering eggs

Timing is critical for petroleum oil applications. The main dormant season treatment occurs in early spring, just before buds begin to swell, to target pests emerging from diapause.

Application rates vary by growth stage. During dormancy, concentrations of 2–3% are commonly used. During the growing season, these rates must be reduced to 0.5–1.5% to minimize phytotoxicity.

The temperature must be carefully monitored. Applications should not be performed when temperatures are below +4°C (due to oil viscosity) or above +25°C (due to the risk of leaf burn).

Thorough coverage is essential. The spray must reach all parts of the tree, including cracks, crevices in the bark, and the underside of leaves, where pests often hide.

In tank mixes, ensure the oil is compatible with other substances. Constant agitation of the spray tank is required to prevent the emulsion from separating during the application process.

Phytotoxicity is the main concern. Petroleum oils should never be applied to stressed plants, such as those suffering from drought, waterlogging, or excessive heat.

There is a strict incompatibility with sulfur-based products and Captan. Applying these products within 30 days of an oil treatment can lead to severe tissue damage and plant death.

Avoid applying oils during the flowering stage, as the thin film can interfere with gas exchange in delicate floral tissues and may negatively impact pollinating insects.

Check the sensitivity of specific cultivars before application, as some plant species are naturally more prone to oil-induced stress than others.

Storage must be maintained at stable positive temperatures. Freezing causes the emulsion to break down, resulting in an inconsistent mixture that may lack effectiveness or increase phytotoxicity risk.