To Table of Contents: Scouting Soybeans (North Carolina Agricultural Extension Service publication AG-385)
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 to consider all appropriate methods of lowering pest levels (rather than relying solely on chemicals), to use pesticides only according to need, and to help produce crops more profitably.
One way to improve profitability is to lower 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). Some knowledge about the advantages and disadvantages of specific pesticides can 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 for their investment.
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. The following chart gives the suggested number of stops for various field sizes.
Field Size Number of StopsSampling locations in a soybean field should be chosen in such a way that they accurately represent actual pest conditions throughout the field. Specific sample sites should be selected on the basis of a predetermined random pattern, not chosen because the crop looks better or worse at particular spots than in the rest of the field. There are several points to remember when scouting a field:(acres) Insects Weeds
up to 20 5 10
21 - 35 8 15
36 - 50 10 15
over 50 split into split into
two fields 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. Scan 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.
Many soybean 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, soybeans before they bloom can withstand 30 percent defoliation from insect feeding without a loss of yield. Similarly, a vigorous soybean crop may be largely unaffected by minor competition from weeds. Treating the crop to control a pest that will not reduce the 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.
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 talking to your chemical supplier. There are times when control may be achieved with substantially reduced rates.
Looking for Patterns in the Field
A logical starting point for diagnosing soybean problems is to look for patterns of occurrence within the field--areas where the problem occurs and those where it is absent. If the condition appears uniformly throughout the field, the cause of the problem must also have been distributed uniformly over the field. Problems that correspond to the topography of the field or to soil type (which is also often related to the topography) are more likely to be soil related than caused by pests or field operations. Problems that are worse on one side or edge of the field are likely to be related to spray drift or to the movement of insects into the field from one side. Some problems, such as root and stem rots and bacterial chlorosis, may occur on isolated plants throughout the field.
Problem areas with sharply defined boundaries are frequently related to some field operation--something that was done differently on one pass than on an adjacent pass (Figure 2) Nematodes, however, are immobile enough that the edge of a nematode-infested spot may also be very distinct (Figure 3) Problems that are concentrated in one row but do not appear in an adjacent row are usually equipment related. The distance between affected rows will provide some insight into how wide a piece of equipment was involved.
Plant Symptoms
The abnormal symptoms of individual plants are also useful in diagnosing problems. Since some symptoms are more useful than others, relying solely on plant symptoms is often unsatisfactory. If plants fail to emerge, the cause may be low seed quality, root rots, use of certain chemicals, planting too deep, or severe crusting. If seedlings emerge but do not survive, root rots and chemicals that inhibit photosynthesis (for example, metribuzin, linuron, or atrazine) are very possible causes. Misshapen leaves (wrinkled, cupped, or elongated) are most likely related to chemical use but may be a result of virus infection.
Some leaf discolorations are fairly distinctive. Potassium deficiency causes yellowing of the leaves, starting at the outer edges. Manganese deficiency also causes yellowing at the leaf edges, but the veins stay green noticeably longer than the interveinal areas of the leaf. Viruses often cause a bronze coloration of the leaves. Nitrogen and sulfur deficiencies are typically indicated by lighter-colored to yellowish plants, but nitrogen deficiency can be created by several different problems. An uncommon but fairly distinctive leaf symptom is caused by bacterial chlorosis. The top leaves on isolated plants look like the plant has a combination of manganese and potassium deficiencies, but the discoloration develops almost immediately on normal-sized plants, and nodulation is usually poor.
Holes in leaves suggest damage by foliage-feeding insects or hail, although hail damage looks more like a tear than a spot where the foliage was removed. Spots or lesions on the leaves (or perhaps stems) suggest the presence of diseases or tiny insects such as thrips. Twisted plants are mostly commonly a result of chemical damage (frequently by chemicals in the phenoxy family). Dead leaves or other plant parts help pinpoint where to look for causes of the problem--somewhere on or below the dead plant part.
Generally poor growth suggests poor growing conditions. Problems of this type often involve limited root activity or an insufficient supply of one or more of the inputs that roots supply to the plant. Abnormally shortened plants with shortened internodes (stunted plants) are often caused by viruses or chemicals. Absence of normal pod or seed development late in the season is often an indication that growing conditions were poor earlier in the year.
Roots are not nearly as easy to see as the upper parts of the plant, but they can provide very useful information. Stunted roots are often the result of chemicals or nematodes. Rotting of the roots--whether it occurs throughout the root, on the surface, or only on the interior--is usually caused by the root and stem rot diseases. Small cysts on smaller roots are typically caused by soybean cyst nematodes, while galls on the roots are usually a sign of one of the root-knot nematodes. Limited or unhealthy nodules may be caused by lack of adequate rhizobia, low pH, molybdenum deficiency, or insect injury. The condition may also be caused by factors such as nematodes or inadequate fertility that limit root activity, or by something that limits the overall supply of photosynthate available to the roots.
The later a problem is noticed after it first develops, the more difficult it is to diagnose the cause, partly because soybeans have a relatively great ability to compensate for problems. If plants are missing from a "normal" stand, adjacent soybean plants tend to produce extra branches, and they produce about as much extra yield as would have been produced by the missing plant. If the growing point of a plant is killed or removed (for example, by hail, deer, or rabbits), one or more new stems tend to emerge from a secondary bud at a lower leaf axil.
The better the growing conditions and general plant vigor at flowering, the more pods are set. If no or few pods are set early in the flowering period, more pods tend to be set later. If no or few pods are set anytime during the normal flowering period, the flowering period tends to be extended, thus allowing for later-than-normal pod set. If pods are set markedly later than normal, they tend to develop and mature seeds in fewer than the normal 55 to 60 days (as much as 10 to 15 fewer days).
The number of pods per foot of row is much more constant than the number of pods per plant. The number of seeds per pod tends to reflect growing conditions during the seed set period as well as the number of pods set. (Setting more pods leads to fewer seeds per pod, and vice versa.) Thus the number of seeds per foot of row is more constant than the number of seeds per pod. Similarly, seed size tends to reflect growing conditions during the seed- filling period and the number of seeds set per foot of row.
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