Michigan State University Extension
Ornamental Plants plus Version 3.0 - 00001734
11/12/99
Why Insecticides Sometimes Fail to Work
Correct timing and thorough application of sprays
You can use the best, most effective insecticide in the
world, but if you don't apply it to a plant thoroughly
and at the proper time, you can expect poor results. To
give its best performance, an insecticide must be applied
when pests are present and vulnerable, and at the proper
rate in sufficient gallonage to permit thorough coverage
of the upper and lower surfaces of the leaves and
branches. For best results, it must be repeated as often
as needed.
Correct timing of sprays is important. For example, the
eastern tent caterpillar, a pest of wild cherry and apple
trees, begins its feeding about the time the buds are
breaking on these trees and the leaves are starting to
unfold. This is very early in the spring. By early
summer, the caterpillars have completed their development
and disappeared from the tree. If you wait until early
summer to spray, the treatment would be of little value
because the damage to the trees would have already
occurred.
Another point to remember is that an insect is much easier
to control when it is young. The older the insect, the
harder it is to kill. An example is scale insects, which
are a problem on many ornamental plants. The most
effective time to control scale is immediately after the
larvae have emerged, while they're in the crawler stage.
During this stage, which may last only about a week, the
tiny scale is unprotected and easily killed by even the
less toxic insecticides. Once the larva secretes a
protective covering over itself, it is very difficult to
kill, even with toxic insecticides.
Wrong insecticide used
No insecticide will control every insect. If you use
the wrong one, even though you apply it properly, the
results will be disappointing. If you want the best
results, you need to use the proper insecticide for your
particular pest.
Weather effects
Most insecticides do not perform efficiently at
temperatures below 50 degrees F and will not give
satisfactory results if used at or below this temperature.
Rain will wash an insecticide off a plant so that it is
ineffective against insects. Wind will alter the coverage
by a spray, preventing the spray from reaching the target
and often carrying the spray into areas you did not intend
to spray. For the most part, it is best to apply an
insecticide when the temperature is above 50 degrees F but
below 95 degrees F, and when no rain is expected for at
least 12 hours.
This information is for educational purposes only. References
to commercial products or trade names does not imply
endorsement by MSU Extension or bias against those not
mentioned. This information becomes public property upon
publication and may be printed verbatim with credit to MSU
Extension. Reprinting cannot be used to endorse or advertise
a commercial product or company.
This file was generated from data base ZZ on 01/19/00.
Data base ZZ was last revised on 11/12/99.
For more information about this data base or its contents please contact
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