Agriculture Reference
In-Depth Information
Rarely does any pesticide kill all the target pests. Each time a pesticide is
used, it selectively kills the most susceptible pests. Some pests avoid the
pesticide. Others are able to withstand its effects. Pests that are not destroyed
may pass along to their offspring the trait that allowed them to survive.
When we use one pesticide repeatedly in the same place, against the same
pes t, the survivin g pes t pop ulati on may sho w greater res istan ce to the
pes ticide than did the original population. Some pests have become partially
resistant to the action of the pesticide.
Not every pesticide failure is caused by pest resistance. Make sure that
you have used the correct pesticide and the correct dosage, and have applied the
pesticide correctly. Also, remember that the pests that are present may be part
of a new infestation that occurred after the chemical was applied.
b.
Factors Affecting Pesticide Use Outdoors
Soil Factors
—Organic matter in soils may "tie up" pesticides, limiting
their activity. Soils with high organic matter content may need higher appli-
cation rates of some pesticides for best control.
Soil texture also affects the way pesticides work. Soils with fine particles
(silts and clays) have the most surface area. They may need higher rates for
total coverage. Coarser soils (sands) have less surface area and, therefore, they
may require lower rates.
Surface Moisture
—Pesticides work best with moderate surface mois-
ture. Wetness may keep the pesticide from adequately contacting the protected
surface. Dryness may prevent the pesticide from spreading evenly over the
surface and contacting the target pest.
Rain may interfere with pest control by causing pesticides to run off or to
leach down through the soil. Rain during or soon after over-the-top or foliar
applications may wash pesticides off the plant. However, some protectant
fungicides are sometimes purposely applied just before periods of expected
high humidity and light rain. When preemergence pesticides are applied to the
surface, moderate rainfall aids in carrying them down through the soil to the
pests. Rain may also release pesticide action after some granular applications.
Humidity and Temperature
—Humidity also affects the way pesticides
work. Herbicides often work best when weeds are growing fast—usually in
high humidity and optimum temperature. However, these same conditions
may make the protected plant more susceptible to pesticide injuries.
High temperature and sunlight will cause some pesticides to break down
when they are left exposed on top of the soil or on other surfaces. Low tem-
peratures may slow down or stop the activity of some pesticides.
