Gardening Articles: Care :: Pests & Problems

Horticultural Oil

by Whitney Cranshaw

Gardeners have long used oils to control plant-damaging pests, but for many years their use was limited to the dormant season. Today, new horticultural oils on the market are more versatile and safer to use on more plants. Horticultural oils are now one of the best ways to control a wide variety of plant pests during the growing season.

How Horticultural Oils Work

These oils (except neem oil) kill insects by suffocating them. Oils also kill insect eggs by penetrating the shells and interfering with metabolic and respiratory processes. In addition, oils disrupt feeding by insects such as flea beetles, whiteflies, and aphids without necessarily killing them.

The fact that oils kill insects by smothering is a key virtue. Many other pesticides kill them by interfering with biochemical processes that are similar to those in other animals, including people. What kills a tiny insect can make us sick, too.

Also, oils have few residual effects, and so their impact on beneficial or benign insects is minimal.

To Control Diseases

Horticultural oils prevent the spread of viruses by aphids, including watermelon mosaic, squash mosaic, and potato virus Y. Oils also curb the spread of viruses that humans transmit by hands or tools (for example, tobacco mosaic virus). Additionally, oils control powdery mildew. Diluted horticultural oils, mixed with baking soda, control this common fungus.

Effects on Beneficial Insects

Most beneficial insects, such as green lacewings and ladybird beetles, scatter before the spray comes and aren't bothered by the residue when they return. However, small, soft-bodied beneficial insects such as predatory mites can't move out of the way fast enough and are killed. If you rely on beneficial mites to deter other pests, think twice before using oil (or any other pesticide). Better still, release beneficial mites several days after you treat with the oil spray.

Types of Oils: Petroleum, Vegetable, and Neem

Horticultural oil is the preferred general term for the various oils gardeners use on plants. Most horticultural oils are refined from crude oil. They have several common characteristics but different names (see below). The exceptions are vegetable and neem oils, which share some petroleum-oil features but not all.

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