Disease Control Using Horticultural Oils
By Whitney Cranshaw, Colorado State University Cooperative Extension
Gardeners have long used oils to control
plant-damaging pests, but for many years their use was limited to the dormant season.
Today, however, the 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
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.
Most horticultural oils contain naphthene and
paraffin compounds. Paraffins are valuable to gardeners because they're more toxic to
insects and less toxic to plants than other oil compounds. In contrast, oils containing
naphthene are less pesticidal and more likely to injure plants than paraffinic types. Oils
high in naphthene also contain more impurities such as phytotoxic aromatic and unsaturated
hydrocarbons. However, the newest horticultural oils contain only tiny amounts of those
Another plant-damaging compound in oil is sulfur, and oils sometimes have a "UR"
(unsulfonated residue) rating that indicates sulfur content. The higher the rating, the
lower the sulfur content. Most horticultural oils have a UR rating of 90 or above.
Viscosity or thickness is another labeled measure of an oil's effectiveness and safety.
(Oil viscosity is measured by how long it takes a given amount of oil to pass through a
hole or ring.) For example, a lighter oil that takes 60 seconds to pass through a ring is
a 6E oil; thicker oil that takes 80 seconds is an 8E oil. Lighter or thinner oils are more
desirable. The UR rating and evaporation range are more reliable plant-safety predictors.
Over the years, horticulturists have coined several, sometimes confusing, terms for
Dormant oil is used on woody plants, especially fruit trees, during their dormant seasons.
New, refined, lightweight oils have replaced older heavy dormant oils. Today, the name
refers to the time and rate of application.
Mineral oil is a light, petroleum-derived oil gardeners can use to control corn earworm.
Narrow-range oil is a light oil graded according to the range of temperatures over which
it evaporates. Lighter oils evaporate over a narrower range of temperatures than other
oils, and thus this term is synonymous with superior or supreme oil. If an oil evaporates
quickly, as light oils do, plants have a greater margin of safety.
Spray oil includes soaplike emulsifiers that allow water and oil to mix for spraying.
Summer oil is used on leafy plants during the growing season. Generally, it's the same as
narrow-range, superior, and supreme oils.
Superior oil describes new, more refined oils that can be applied safely-at lower rates-to
green leaves. Today, all horticultural oils are superior-type oils, and label directions
specify varying application rates for use during dormancy or the growing season.
Supreme oil is one brand name for a superior or narrow-range oil.
Neem oil comes from the seeds of the neem tree ( Azadirachta indica ) and is used
as both an insecticide and a fungicide. Neem oils such as Rose Defense or Trilogy
(formerly NeemGuard) are effective at killing insect eggs and immature insects, notably
small soft-bodied pests such as whiteflies and aphids. It has been shown that neem oil
kills certain mite eggs, too. Neem oil also prevents powdery mildew and black spot. Use it
on roses, fruit trees, and vegetables.
Neem-derived insecticides contain azadirachtin. Sold under trade names such as Azatin,
Bioneem, Margosan-O, and Neemazad, they control whiteflies, aphids, and other soft-bodied
insects. Neem oils are largely azadirachtin-free.
Other neem-seed compounds inhibit insect feeding, repel pests, disrupt insect growth, and
Vegetable oil is the term for any oil that's derived from oilseed crops such as soybean,
rapeseed (canola), or cottonseed. Stoller's Natur'l Oil is cottonseed oil (the most
insecticidal vegetable-seed oil) with an emulsifier. Soybean oil provides good control.
Canola and sunflower oils are less effective, and corn oil shows mixed results intests.
How to Apply Oils
Most often, gardeners apply oils with a hose-end
sprayer. Apply mineral oil with an eyedropper in the ear tips of corn to kill corn
Label directions on most oils prohibit their use within 30 days before or after a sulfur
application. Other oil-incompatible pesticides exist. Be sure to check the label.
Labels of most horticultural oils warn against
applying them to plants when temperatures are below 40 o F or above 90 o
F. Labels continue to include this advice despite increasing evidence that this
temperature range is conservatively narrow. It's more important that the plant shouldn't
dry out, and humidity should be low enough (45 to 65 percent) for oil to evaporate
Actively growing insects or mites are more
susceptible than dormant ones. The best time to apply dormant-season oils is after insect
dormancy ends in late winter or early spring when insects resume growth.
Read and follow labeled application instructions
and precautions. Although the recommended temperatures may be moderate, don't assume
safety. Irrigate the day before spraying to be sure plants do not lack water. Likewise,
spray in the early morning on cloudy days of low humidity (to speed evaporation). Don't
apply oils when shoots emerge in spring.
Several plants are susceptible to oils: maples,
particularly Japanese and red maple; hickories and black walnut; plume cedar (
Cryptomeria japonica ) and smoke tree ( Cotinus coggygria ). Injury to these
plants can occur from either dormant or summer oil applications. Several plants are also
somewhat sensitive: redbud, junipers, cedars, spruce, and Douglas firs.
Light yellowing indicates that a summer oil application has burned the foliage. Later,
these sites become water-soaked, darken, and die. Terminal or branch dieback indicates
damage from a dormant application.
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