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Great Information about pesticides containing OILS!

7 years ago
last modified: 7 years ago

I thought this would be very helpful to those who wonder how oils work on pests and their trees...Enjoy

In the late nineteenth century, mixtures
of kerosene and soap were recommended
for treating insect-infested crops.
While quite effective in this regard, these
early horticultural oils also caused collateral
damage with their phytotoxicity;
leaves in particular were heavily damaged.
In the mid-1900s, refining technology
improved to produce lighter petroleumbased
oils with fewer impurities, thus
reducing the damage to plants.
Today’s horticultural oils include
vegetable as well as mineral oil products.
Recent studies have suggested that
plant-based oils, such as soybean, are often
just as effective as mineral oils and may
be less phytotoxic. Regardless of their
origin, horticultural oils are attractive
alternatives to many conventional
pesticides, as they are just as effective in
controlling pests while simultaneously
conserving nontarget organisms and
decomposing quickly. Moreover, they
can be used as adjuvants, or enhancers,
for other alternative pesticides, including
Bt (Bacillus thuringiensis) and NPV
(nucleopolyhedrovirus) insecticide.
What are the differences?
The first commercially available
products were dormant oils, or those
meant for winter application while trees
and shrubs are resting. These oils have a
high distillation temperature, meaning
they evaporate more slowly and stay on
the plant longer than those with lower
distillation temperatures.
Dormant oils are usually more
viscous, enhancing their residency on
sprayed surfaces. In contrast, summer oils,
sometimes called superior oils, are lighter
weight and more volatile than winter oils
(i.e., have lower distillation temperatures).
These summer oils have less residency
time on sprayed plants, and they are also
less likely to cause damage to leaves.
The distillation temperature is one of
the two most important numbers on the
label; the second is the UR designation,
or percent unsulfonated residues in the
product. Sulfonated residues contribute
to the phytotoxicity of oils, so purer
oils will have a higher percentage of
unsulfonated residues and cause less foliar
damage (discussed below): this number
should be no lower than 92 percent.
It may take some trial and error to
select the best oil for a particular pest on
a given plant: insecticidal activity must
be balanced with potential damage to
the plant and other nontarget species.
While the rate of evaporation is part of
this balance, horticultural oils may also
contain detergents or other proprietary
additives, which improve application and
enhance effectiveness of the product.
How do oils work?
Horticultural oil application
immediately creates a physical barrier
to respiration by clogging the spiracles,
or breathing pores, along the sides of
adult and larvae abdomens. Similarly,
oils applied to egg masses inhibit oxygen
uptake and decrease hatching success.
Oxygen demands of all life stages decrease
with decreasing temperature, however, so
winter applications must be at a higher
rate than those of summer oils to ensure
sustained coverage.
Though contact mortality is
considered to be the primary mode of
action, horticultural oils also interact
with cell membranes, interfering with
their function and possibly creating
toxins. Other research also points to a
preventative role in deterring some insects
from laying eggs, especially if plants are
sprayed while females are actively seeking
egg-laying territory. Furthermore, several
studies have found horticultural oils to be
feeding deterrents.
myth, miracle or marketing?
Horticultural oils
Linda Chalker-Scott, Ph.D.
MasterGardener WSU editor
Extension Urban Horticulturist
and Associate Professor,
Puyallup Research and Extension Center,
Washington State University
Puyallup, Washington
Conflicting information is given
regarding the residual effectiveness of
horticultural oils. One research group
reported that oil effectiveness lasted only
24 hours after application in the field,
compared to two weeks postapplication
in laboratory or greenhouse settings.
However, oils are more quickly degraded
and/or evaporated in the field than under
more controlled conditions, so these
differences in residual effectiveness are
not surprising.
There are some conditions under
which horticultural oils are not as
effective. Wet leaves, or those washed
by irrigation or rainfall too soon after
application will not retain oils well.
Likewise, oils sprayed onto plants with
vertical leaves (including ornamental
grasses) require a higher application
rate and/or a greater viscosity, since
the leaf angle enhances oil runoff. And
don’t waste time and resources using
horticultural oils as soil drenches: they
only succeed when they contact the
targeted pest.
Are all insects controlled?
Unlike many gardening products that
show promise in laboratory experiments
but fail under real-world conditions,
horticultural oils have been extensively
tested in the lab, greenhouse, nursery, and
field on a variety of insect pests found on
many species of herbaceous and woody
plants. Though not effective on all garden
pests, horticultural oils can successfully
combat common nuisances including
aphids, scale, whiteflies (insects), and
mites (arachnids). All are controlled
by relatively low concentrations of oil
(usually 1-2%) that generally are not
phytotoxic. In comparison, slightly
higher concentrations (3-5%) are
needed for control of members of the
orders Coleoptera, Heteroptera, and
Lepidoptera. Still, these concentrations
are usually not enough to cause leaf
A handful of pests are apparently
unaffected by horticultural oil
application; for some insects, such as
the Eastern tent caterpillar, researchers
suspect that their dense hairs may prevent
oils from reaching the spicules. Others,
such as calico scale or adult beetles, have
thick waxy plates or heavy carapaces
that inhibit oil penetration. Though
technically they do not induce genetic
resistance, repeated use of horticultural
oils can select for more resistant pest
populations, such as scale insects with
thicker, less permeable plates.
Choosing which horticultural oil to
use is dependent on the life history of the
targeted pest. If the insect overwinters
on affected plants, then dormant oils can
be used; likewise, species that arrive in
the spring or summer should be treated
with oils intended for application during
the growing season. In any case, the pest
population must be completely covered
for the oil to be effective. This means
spraying both sides of leaves, growing tips,
or anywhere else control is needed.
Other plant problems
Given their effectiveness on many
insect pests, horticultural oils could
logically be used to decrease the
transmission of insect-borne viruses,
and in fact this property has been
demonstrated in at least one study.
Several researchers have found
widespread success in reducing the
incidence of powdery mildew species
on susceptible plants including apple,
crapemyrtle, lilac, pepper, and rose.
Antimicrobial activity against other
disease organisms is more sporadic: for
instance, one research group found oils
to be ineffective against rose blackspot
unless inoculum levels were quite low;
yet another group reported success in
controlling the same disease.
Mixed results have also been reported
for treatment of fungi responsible for
flyspeck and sooty blotch on apple and
bacterial leaf spot on hydrangea and
leaf blight on pepper. Still, researchers
remain positive that use of horticultural
oils and other biopesticides are effective
alternatives to synthetics in suppressing
foliar diseases.
What are the drawbacks?
Injury to plants
Early formulations of horticultural
oils tended to contain impurities that
caused damage to the foliage of many
plant species. Modern formulations are
highly purified and bear little resemblance
to those first toxic mixtures of kerosene
and other volatile chemicals, and
professionals who routinely apply oils
report little damage to plant material.
Yet the fear of phytotoxicity lingers, so
it’s worth exploring the circumstances
Horticultural oils are easy to apply and
are relatively safe even for new growth,
provided mixing and application
directions are carefully followed.
under which plants might be damaged by
horticultural oils.
While horticultural oils are intended
to remain on leaf surfaces to have their
pesticidal effects, they often end up inside
the leaves as well. Using a variety of
imaging techniques, researchers have been
able to visualize movement of oils through
the stomata and across the cuticle. Once
inside, oils are able to move throughout
treated leaves and into adjacent, untreated
tissues. In some cases, this foliar uptake
can result in phytotoxicity, manifesting
itself as chlorotic or yellowing leaves,
which might then develop brown
stippling, necrotic leaf tips and margins,
and/or a water-soaked appearance before
eventually dying.
Happily, horticultural oils do not
cause phytotoxicity to the vast majority
of landscape trees and shrubs tested so far,
including several “sensitive” species. Even
herbaceous species in the greenhouse
or landscape are relatively immune to
damage. That being said, there are some
species that react negatively to correctly
applied horticultural oils:
• Junipers and spruces may lose their
bluish bloom, as these foliar waxes are oil
• Roses, though others refute this
• Grapevines may show reduced fruit
set with repeated use of horticultural oils,
though no foliar injury
In addition, phytoxicity can occur
even in tolerant species when:
• Oils are applied at too high a rate
• Oil suspension is not constantly
agitated, resulting in separation of oil and
• Leaves are wilted at application time
• Humidity is too high, resulting in
reduced oil evaporation
Most actively growing plants are not
affected by horticultural oils, especially if
they are not under environmental stress.
The exception may be seedlings and
other tender tissues, which can be injured
by lengthy or repeated exposure to oil,
though others have found no damage
under similar conditions. Trees, shrubs,
and vines that are actively used for fruit
production may show a decrease in fruit
quality parameters if leaves are repeatedly
treated with oil; this is most likely due to
the decreased photosynthetic ability of
leaves with clogged stomata.
Dormant plants are even less likely to
have a negative reaction to horticultural
oils, tolerating up to four times the
recommended spray application (8%)
without damage. But it is important to
ensure plants are completely dormant
before applying winter oils, as they
otherwise may be damaging.
Other factors that reportedly will
lower the risk of oil phytoxicity include
formulations with high percentages
of UR (unsulfonated residues), UV
radiation absorbers, or vegetable oils.
High ambient temperatures apparently
are not problematic as long as plants are
well hydrated and relative humidity is low.
Paradoxically, phytotoxicity can be a
desirable outcome, particularly when the
pest in question is a weed. Horticultural
oils can be used as carriers for fungal
spores or other bioherbicides, and their
ability to penetrate waxy leaf surfaces
undoubtedly enhances the activity of the control agent...


What’s the bottom line for using
horticultural oils as pesticides? DO
• Know the life history of a pest and choose the correct seasonal oil.
• Choose oils with the highest percent UR (unsulfonated residues).
• Follow label directions to the letter.
• Use the lowest possible concentration (e.g., 1-2% in summer; 3-4% in winter).
• Keep your oil mixture agitated at all times to prevent separation.
• Apply summer oils to plants with fully hydrated, but dry-surfaced leaves.
• Only apply oils when target pests are present.
• Be sure to cover the target insect completely; the oil will only work if the insect is completely enveloped.
• Be aware of beneficial insects and avoid spraying when they are actively feeding on pests.
• Test spray only a portion of a plant or groupings of plants if concerned about phytotoxicity.
• Overuse oils. Like any other pesticide, natural or synthetic, overuse can damage nontarget species and ecosystems.
• Use oils on unregistered insects, arachnids, or diseases.
• Use unregistered vegetable or petroleum-based oils for pest control.
• Use oils as soil drenches.
• Mix with sulfur-containing pesticides. This effectively decreases the UR number and can cause phytotoxicity.
• Apply oils to wilted plants.
• Use oils on seedlings or other tender plant tissues.
• Use oils on conifers or other species with a waxy, bluish cast to their foliage.
• Apply when rain is expected, or when humidity is high (over 90% RH).
• Let oil drift onto water surfaces; it will inhibit oxygen transfer and possibly harm aquatic organisms.

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