Part 2, Section 2: Corn Pest Management
Corn Pest Management
Insect pest populations, greatly influenced by weather conditions and other natural phenomena, cause variable crop losses from year to year. Since losses from insects may be high one year and of little consequence the next, growers should use a system for monitoring fields and making control decisions.
Crop rotation provides effective control of corn rootworms and helps reduce several other corn pests. Although the Midwest has seen new variants of the western and northern corn rootworms that overcome defeat the corn and soybean rotation by laying its eggs in soybean fields or diapausing for two years, respectively, these variants have yet to reach Pennsylvania, and are unlikely to do so given our rotations are more complex than the two year cycle predominant in the Midwest. Minimum-till and no-till systems and poor weed control increase the incidence of true armyworm, black cutworm, and stalk borer. In most cases, eliminating grassy weeds at least 2 weeks before planting reduces the possibility of these problems. True armyworms also can be serious problems in fields where corn is planted into wheat stubble or rye mulch. It must be noted that the true armyworm population that arrived in Pennsylvania during spring 2012 was so large that many fields were overwhelmed even when growers used proper management strategies. These types of outbreak populations are only manageable via rescue treatments as they do not occur annually and are very difficult to predict. Insecticides can be applied to reduce pest populations, but unless the predicted losses are greater than the cost of application, such applications are not profitable.
Biologically engineered corn hybrids that produce an insecticidal toxin from the bacterium Bacillus thuringiensis (Bt) provide a modern method for managing European corn borer, some other caterpillar species, and corn rootworm. This technology offers yield protection that could not be obtained by using conventional insecticides. Because of its high degree of effectiveness, Bt corn is rapidly gaining popularity; however, this technology is not immune to the threat of insects developing resistance to the toxin, particularly if Bt varieties are overused—as evidenced by Midwestern populations of western corn rootworm that are resistant to some Bt varieties. Growers who choose Bt corn hybrids will be required to implement a resistance management program to minimize the rate of resistance development. For more information on using Bt corn hybrids, see the discussion on insect-resistant (Bt) corn varieties earlier in this chapter.
Another option for managing insect pests in corn uses seed treatments. Although hopperbox treatments have been available for a long time, commercially applied seed treatments using a newer class of chemical have become popular. This newer insecticide class, neonicotinoids, has three compounds that are available for corn. The first material available as a commercially applied seed treatment, imadicloprid (Gaucho), was the initial compound that growers could use. However, with the introduction of two newer compounds, thiamethoxam (Cruiser) and clothianidin (Poncho), imadicloprid has been relegated to a hopperbox treatment now known as Concur or Latitude. The two products now commercially applied, Cruiser and Poncho, are available each at two rates, 0.25 mg ai per kernel and 1.25 mg ai per kernel. All rates provide some protection against the soil insect pests. At the higher rate, both seed treatments add corn rootworm larvae and billbugs to their labels. However, studies have shown that they only work well against low to moderate populations of rootworm larvae; they do not work well at intense rootworm pressure. Also, it is important to know that these neonicotinoid seed treatments are soluble in water; therefore, excessive rain around planting and early season seedling growth has the potential to carry the insecticides away from the zone where it can be absorbed by the plants’ roots. Researchers have also raised some concerns about the influence of neonicotinoid seed treatments on pollinator health, which in some cases appears to be significant, particularly around corn planting; this issue remains to be resolved.
The first step in managing corn pests is to know when the pest species is present in your area. Figure 2.2-1 illustrates the approximate timing of corn pest occurrences in Pennsylvania. The presence of a pest can vary within the presented range, depending on the location in the state and on local weather conditions.
See Table 2.2-19 for a quick reference to insecticides recommended for corn insect control, and Table 2.2-20 for specific rates, restrictions, and other information. Table 2.2-21 lists herbicides that are compatible with selected corn insecticides.