Resource Guide for Organic Insect and Disease ... - Cornell University

144 Organic Resource Guide
use is the variability in oil composition and the absence of well-defined standards for pesticidal
usage (Sams & Deyton 2002).
Another category of products currently available includes mixtures of essential plant oils, such
as wintergreen, clove, and rosemary. These are generally pressed from leaves, stems, or flowers,
rather than seeds, and then separated by distillation. They may be formulated with mineral oil
in products labeled for insect, disease, and weed control. Some are exempt from EPA labeling
requirements (see Appendix F).
HOW THEY WORK:
Petroleum oils are widely used to control the egg stage of various mites and insects by
preventing the normal exchange of gases through the egg surface or interfering with the egg
structure. When used against other stages of insects and mites, oils can block the respiratory
system, causing suffocation or breakdown of the outside tissue (cuticle) of the insect or
mite. Secondary toxicity mechanisms include penetrating and degrading arthropod tissues
and fumigant effects of volatile oil components (Taverner 2002). Oils may also repel some
pests (Stansly et al. 2002). Whether plant and fish-derived oils have similar modes of action is
unclear. Oils derived from all sources may also alter the behavior of insects and mites, causing
them to avoid laying eggs or disrupting their feeding. Additional work is needed in this area
to determine which fractions may cause this behavior and to what extent such changes in
behavior may affect pest management.
Besides direct control of insects and mites, oils may also provide some control of insectvectored
plant viruses. Stylet oils are derived from petroleum and, when sprayed on plants,
inhibit the ability of aphids to acquire a non-persistently transmitted virus from infected plants
and transmit it to other plants (Davidson et al. 1991). Scientists believe that oils interfere with
the retention of virus organisms on insect stylets (Wang & Pirone 1996).
Both petroleum and plant oils suppress some fungal diseases, especially powdery mildew.
While the mechanism is not clear, it may involve disruption of fungal membranes or
interference with spore attachment or germination. Oils may also increase host plant resistance
response (Northover & Timmer 2002).
Oils are often added to other pesticide products to improve efficacy. In this sense, they are
considered spray adjuvants, even though they may have pesticidal activity on their own.
An application of fish oil can be used as a fruit thinner in organic fruit production, sometimes in
conjunction with lime sulfur. Evidently, it works by suppressing photosynthesis, indicating that
crop yield should be examined in studies of fish oil and other oil products. Mineral and soybean
oils have been shown to delay bloom and thin the crop in peaches, most likely by suppressing
respiration (Sams et al. 2002).
Essential oils have a different mode of action. They are volatile oils found in some plants that
have strong aromatic components and give distinctive flavor or scent to the plant. Typically,
they are liquid at room temperature and transform to a gaseous state at slightly higher
temperatures. Against arthropods (i.e., insects and mites), essential oils may be repellents
or induce other behavioral modifications. Research indicates that the rapid action of some
oils against certain pests is indicative of a neurotoxic mode of action (Koul et al. 2008). The
mechanism of action against fungal pathogens is unknown, but may be related to essential
oil’s general ability to disrupt the integrity of cell walls and membranes (Koul et al. 2008).
Essential oil products are generally mixtures of two or more essential oils, often including
rosemary, peppermint, clove, or thyme oils. Essential oils are also formulated as burn-down
herbicides, which will not be discussed here.
TYPES OF PESTS IT CONTROLS:
Oil products can control a wide range of soft-bodied insects, such as aphids, mites, thrips,
whiteflies, mealybugs, and psyllids. In the 1940’s, highly refined “white” oils were widely used