Monitoring the Crop for Pest Damage
Part 2, Section 1: Pest Management
Activities Involved in an IPM Program
Monitoring the crop for pest damage
Reasons for monitoring
The first step in an IPM program is to monitor fields for signs of pest damage or potential pest problems. Proper weed management involves a thorough inspection of each field after crop harvest to identify major weed species in the field. When an annual crop is to be grown in the field the following year, this information is used to assess the importance of each weed species and to select the appropriate management strategy to be used for the coming crop. In annual crops, fields also are monitored after the crop has emerged to assess the effectiveness of the selected management alternative and whether additional management tactics are needed. For establishing perennial crops, such as alfalfa, an assessment of weed species composition is conducted after harvest of the previous crop (usually in the fall), to determine the appropriate management alternative to be used during establishment. In an established crop, fields are monitored to determine the need for additional tactics to manage perennial and biennial weeds.
Except for seed treatments and foliar applications in small grains, fungicides seldom are used for managing diseases in field and forage crops. However, fields should be scouted to monitor the presence of crop diseases. By identifying diseases in a field, the farm manager can implement management tactics such as crop rotation, tillage, or selection of resistant cultivars. Disease scouting should be done frequently during the growing season.
Because of their natural mobility and reproductive potential, insects are among the most difficult pests to manage. A field badly infested with an insect pest one year may have very little damage the following year. Conversely, a field that has been insect free for years suddenly may have severe insect damage. Figures 2.1-1 and 2.1-2 show how alfalfa weevil populations can vary between years and locations.
The unpredictable nature of insect damage has led many farmers to apply insurance insecticide treatments. Although insurance applications may help you sleep better at night, you may be using resources better employed in other farm enterprises. Routine field monitoring can prevent the unnecessary use of funds to manage pests that are economically insignificant and can help direct management efforts to those fields that can benefit.
For insect management, fields should be scouted every seven to ten days to adequately monitor changing pest populations. Longer periods between field monitoring can mean detecting pest damage only after significant losses have occurred. For pests such as the black cutworm in corn, significant damage can occur within three to five days if fields are not watched closely.
In corn, monitoring should begin about a week after the crop has been planted. Insect pests such as the seed corn maggot, seed corn beetle, wireworm, and white grub can feed on newly planted seed. Table 2.1-2 lists the key corn insect pests and the plant growth stages during which they can cause crop damage. Monitoring can be terminated after the pest management decision has been made for the corn rootworm. In situations where European corn borer is prevalent, however, continue monitoring to assess the degree of stalk lodging and ear droppage. This information is useful in assessing the population potential for the next growing season and in determining if another more resistant variety should be planted in the future.
In alfalfa, monitoring should begin when the crop begins to regrow in early spring. Early infestations of alfalfa weevil can be seen in March if conditions are warm. The potential for significant damage by this pest, however, does not occur until late April to mid-May. Monitoring from the beginning of spring growth to late April should involve only a quick visual inspection for signs of feeding. Table 2.1-3 lists the key pests of new seedings and established stands of alfalfa and cuttings where the pests occur. Monitoring can be terminated when potato leafhopper populations begin to decline around the middle of August.
Few economically important insect pests of soybeans exist in Pennsylvania; however, several insect pests can cause significant damage in a limited number of fields within a season. Table 2.1-4 lists the key pests of soybeans and the stages of plant development when they are important. Monitoring should begin about a week after planting and continue until late podfill.
Scouting for insects in small grains should begin after plant emergence and continue until harvest. In wheat, corn leaf and English grain aphids and Hessian fly can attack young seedlings in the fall and early spring. Armyworm can clip off wheat heads as the grain begins to reach maturity. Table 2.1-5 lists the key insects of small grains in Pennsylvania.
Importance of complete monitoring
Monitoring crops for pests should not be a series of independent processes, but part of an overall integrated monitoring plan. For instance, scouts do not walk into an alfalfa field and look only for insect pests. Instead, they are observing insect numbers, diseases, weeds, the general health of the crop, and the stage of crop growth, while assessing potential problems. This information provides a baseline for future evaluations of crop health and helps measure the economic significance of specific pests in the field. By routinely monitoring a field, the farm manager can note subtle changes in the crop and take corrective action before major problems occur.
The occurrence of multiple pests in a field can influence the potential damage from pests. For instance, a field with a heavy weed infestation will experience greater yield losses if an insect or disease problem also is present. In fact, weedy fields often attract insects because of the high humidity. Certain weeds also are attractive to insects. Because common chickweed is a preferred host of the black cutworm, fields with heavy chickweed infestations have a significantly higher probability of developing a black cutworm problem. Therefore, fields with heavy chickweed infestations should be monitored closely for black cutworm infestations. Producers can reduce the potential for this pest by eliminating chickweed from fields at least 10 days before planting.
Besides the interaction of multiple pests, plant growth stage also can influence pest damage. Because some diseases and insect problems occur only during specific growth stages of the crop, it is important to know what growth stages are susceptible to which pests.
By knowing the pest history of the field and what problems exist in it during a growing season, farm managers can develop a crop production strategy to reduce pest damage. This strategy should be integrated along with other management decisions, such as fertility management, tillage method, and conservation management.
Many pest problems can be identified simply by observing them in the field. Weeds and diseases typically fall into this category, as do some insect and mammalian pests. Others, however, require specialized equipment to estimate their numbers. For example, monitoring populations of corn soil insects such as the wireworm, white grub, seed corn maggot, seed corn beetle, black cutworm, and rootworm larvae requires the use of a shovel or trowel in addition to a visual damage inspection. These insects all feed below ground level and must be located by digging.
One method for monitoring wireworm populations involves burying corn or other grain in a mesh material. Because carbon dioxide is given off by the seed as it begins to sprout, larvae of the wireworm are attracted to the grain and can be found feeding in the corn contained in the mesh material. This is an effective method of monitoring old sod fields before planting a corn crop. Economic threshold values have been established for this method.
Pheromone traps can be used to monitor the population of black cutworm adults that migrate into Pennsylvania during early spring. A pheromone trap contains the scent of the female moth, and this attracts the male. By calculating and summing 300 degree days from the time eight moths are collected, the period when larva feeding occurs can be calculated. This is useful in determining when to begin scouting fields.
Blacklight traps can be used to monitor the flight periods of a number of moths. Important insects that can be monitored this way include European corn borer, true armyworm, fall armyworm, sod webworm, and corn earworm. Egg deposition usually is correlated with moth flight. Although light traps are useful monitoring tools, considerable experience is needed to sort out the economically important insects from the hundreds of other insect species collected in the trap.
Evaluating a field for European corn borer infestations requires a sharp knife to split the corn stalk. Larvae of this insect feed within the corn stem. This procedure, however, is used only to determine the magnitude of the infestation. By the time larvae move into the stem, it is too late to implement a management strategy for the present crop. This information, however, is useful in assessing whether a management strategy should be used the following year.
Monitoring for the alfalfa weevil requires a bucket and a pocket knife. When sampling for this pest, randomly select alfalfa stems, cut off the stem, and place the stem in a bucket. Beat the stems against the side of the bucket to dislodge and count alfalfa weevil larvae. This number then can be compared with established economic thresholds.
Evaluating potato leafhopper pressure in a field requires a sweep (butterfly) net. Potato leafhopper numbers are determined by sweeping the field, which dislodges the leafhoppers from the alfalfa plant and deposits them in the net. Counting the number of leafhoppers captured allows the farm manager to make a management decision. Scouting procedures and economic thresholds for alfalfa pests are discussed in Extension Circular 284, A Pest Management Program for Alfalfa in Pennsylvania.
Although numerous methods have been developed to scout crop pests, the best pest management tool is observation. If you take the time to scout fields during critical periods, you can avoid most significant pest damage. Information and organization are keys to successful pest management. In addition to scouting fields, an experienced crop manager or consultant can use field characteristics to help identify fields with a high probability of developing a pest problem. Table 2.1-6 shows field characteristics for corn that increase the likelihood of infestation by a given insect pest. Both weeds and diseases are affected by field conditions. For instance, weeds such as yellow nutsedge prefer moist environments and tend to be a problem in areas that are wet. Nutsedge, in turn, serves as a host for billbug infestations. Many fungal diseases are more common in wet fields. Knowledge of the properties and history of a field can be a very powerful tool in making variety selections and deciding on crop production techniques.