decades, fruit growers have routinely applied a refined
petroleum oil spray to their trees in the early spring. This
is done to kill the eggs and adults of certain
and other insects that spend the winter on or under the bark.
It is also effective against
scale insects. Petroleum oil
sprays have many advantages. They are different from other
insecticides in that they act mechanically rather than as a
true "poison". Oils act by covering the insect or its eggs and
smothering them. They are not persistent in the environment
long and have extremely low toxicity to non-target organisms.
These sprays are called "dormant" oils because
they must be applied before the plant
begins pushing new growth. If applied after the plants have
broken dormancy, oils can "burn" the new growth. This type of
injury is called phytotoxicity.
Proper timing is of great importance with
dormant oil sprays. To be effective, the temperature of the
air and the surface of the bark has to be warm enough to allow
the oil to spread uniformly. It must be able to flow into the
nooks and crannies where eggs and insects may be hiding.
However, too much warm weather will push tender new growth and
increase the risk of phytotoxicity.
The general rule for oil applications is to
spray after 2 or 3 consecutive days of temperatures over 40
degrees. A single day of adequate temperatures is not enough
since it usually takes several days for the wood of the tree
to warm properly. Avoid applying oils when rain is expected
within 24 hours.
Dormant oils can generally be used on fruit
trees, winter creeper (Euonymus),
Check the label for other species.
tree species are sensitive to dormant oils. Generally, avoid
using oils on
birch, blue spruce,
Douglas fir, hickory,
walnut, Japanese maple,
Norway maple and sugar maples. Blue spruces sprayed with oils
tend to lose their blue color for a year or more.
Oil sprays should not be used within two
weeks of any fungicide application containing
mixing oils with the insecticides Sevin or dimethoate. Always
check the label of the product for other restrictions.
In recent years, new refining methods have
produced oils that may be used during both the dormant and
growing seasons. These "horticultural oils" are used at a
different mixing rate depending on the season. The label will
indicate the dilution to use for each season. Normally, the
summer rate is half as strong (i.e. more dilute) as the
dormant season rate.