1 Corn Pest Management
Crop scouting has been used for many years to help identify pest problems
and determine what action, if any, should be taken. However, scouting is
only one part of an overall approach known as integrated pest management
(IPM). The objectives of integrated pest management are (1) to consider
all appropriate methods of lowering pest levels (rather than relying solely
on chemicals), (2) to use pesticides only according to need, and (3) to
help produce crops more profitably.
One way to improve profitability is to lower pesticide
costs. Pesticide costs may be reduced by applying chemicals only when necessary
and using only the amount needed to control the pests. To know precisely
when to take action against crop pests, it is necessary to scout for pests
regularly and systematically and to know how many pests must be present
before they will cause economic damage to a crop. This level is called
the economic threshold. Knowledge about the advantages and disadvantages
of specific pesticides can also be very helpful in selecting the best product
and minimum application rate needed for control.
Growers are quick to recognize the profit-robbing
potential of pests, but it is just as important to realize that using a
pesticide when it is not needed can also cut profits. The use of proper
scouting procedures and knowledge of economic thresholds can ensure that
growers use pesticides properly and realize maximum returns from their
investments.
Scouting
The purpose of field scouting is to obtain an objective summary of the
pest situation. Some of the information obtained will be useful in making
immediate pest control decisions. Other observations will help in knowing
what to expect at a comparable time next year.
Scouting is basically the process of walking through
a field and stopping at certain places to look for pests or pest damage.
The number of stops to make depends on the size of the field. Table 1 gives
the suggested number of stops for various field sizes.
| Field Size (acres) | Number of Stops | |
| Insects | Weeds | |
| up to 20 | 5 | 10 |
| 21 - 35 | 8 | 15 |
| 36- 50 | 10 | 15 |
| more than 50 | Split into two fields | Split into two fields |
1. Select stops on a representative basis. Walk a random pattern to
ensure that the field is thoroughly covered (Figure 1). Do not fall into
the habit of entering a field at the same place each week.
2. Be aware that pests can invade from field borders. Walk to within
50 feet of every border. If a pest problem is detected, try to determine
whether it is isolated to the border area or is more widespread and uniform.
3. Randomly select the places where you stop to sample for pests. For
example, you might select your next site by stopping in a spot that is
20 steps ahead (or to the left or right) of your present position.
4. As you take samples, observe the plants surrounding the sample site.
Look about the field constantly while walking from one area to another.
5. After obtaining counts and estimates, refer to the pest descriptions
in this guide to determine whether action is necessary.
6. If you find pests you do not recognize, take samples to your local
county Extension agent for identification.
Economic Thresholds
Many corn growers watch their crop closely and check regularly for pests.
When they find a potentially damaging pest, they must decide whether it
will pay to apply a treatment to control the pest. The number of pests
or level of pest infestation it takes to justify spending money on a pesticide
treatment is known as the economic threshold (or action threshold)
for that pest. The threshold can help determine whether control measures
will pay and when they should be applied.
One might think that if pests are present they must
be damaging the crop or interfering with its growth, thus causing a loss.
Actually, pest populations can sometimes build up to relatively large numbers
before it pays to control them. For example, 50 percent of the corn plants
in a field can be infested by whorl-feeding insects without a loss in yield.
Similarly, a vigorous corn crop may be largely unaffected by minor competition
from weeds. Treating the crop to control a pest that will not reduce quality
or yield is a waste of money.
Another point to consider is that there are almost
always natural control agents working against an increase in pest populations.
In other words, even though a certain pest is found in a field, there is
no guarantee that its population will ever reach damaging levels. There
is no point in spending money to control a pest that may never build to
damaging levels. Waiting to see if there will be enough pests to warrant
treating the crop is one of the ways growers can save money using integrated
pest management.
Keep in mind that although the thresholds given
in this manual have worked very well, they were developed as guidelines
to be used under average conditions. In unusual situations, such as during
periods of drought stress or when multiple pests are present, lower thresholds
should be used.
Deciding About Pesticide Applications
Once a pest has been found to exceed the economic threshold, the grower
must determine the best and least expensive way to prevent unacceptable
losses. If a pesticide treatment is needed, the pesticide and its rate
and method of application must be customized to the pest. This is probably
the surest way to save money. High pesticide rates are not needed if the
correct pesticide is chosen and the method of application delivers the
required amount of chemical to the pest.
To apply pesticides properly, sprayer pressure and volume must be correct;
the right nozzle type, arrangement, and number must be used; and the sprayer
must be accurately calibrated. When deciding on a pesticide, talk to your
local Extension agent and consult the North Carolina Agricultural Chemicals
Manual before contacting your chemical supplier. There are times when
control may be achieved with substantially reduced rates.
