Corn Insects to Monitor

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There are numerous corn insects in our area that have the potential to cause economic injury. Listed below are some brief comments concerning identifying injury and management considerations for some common corn insects. For further, more detailed information concerning corn insect management consider going to https://www.aganytime.com and typing a specific insect species in the Search Agronomic Library feature.

Corn Rootworm (CRW).

CRW eggs have already begun hatching in Kansas.1 CRW larvae are slender, cream-colored and have brown heads and a dark plate on the top side of the tail. Mature third instar larvae are about 1/2 inch long. Newly hatched or first instar CRW larvae are very small (less than 1/8 inch long) and may go unnoticed. Larvae feed on root hairs and inside the roots, interfering with water and nutrient uptake (Figure 1). Heavy feeding can reduce plant stability resulting in lodging.


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Figure 1. Pruned roots from CRW larvae feeding.


Sample for larvae by digging up corn plants and washing the roots in a bucket of water; larvae should float to the top of the water. Sample corn plants in different areas of the field to estimate infestation levels. There is no established economic threshold for CRW larvae; however, agronomists have advised applying rescue treatments if there are 2 or 3 CRW larvae per plant.2 CRW larvae are best controlled with preventative measures but rescue treatments do exist. Insecticides can be applied through an irrigation pivot (preferred) or by making lay-by cultivator applications (least effective).2


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Figure 2. CRW beetles clipping corn silks.


Adult CRW Beetle Scouting and Thresholds.Monitoring adult beetle populations is key to assessing whether or not postemergence insecticide applications are warranted. Weekly scouting for beetles should begin at early tassel and continue through early September.

Action thresholds for ear protection. In general, treatment with foliar insecticides to control beetles during pollination is warranted when: beetle counts of 5 or more per plant are found, beetles are clipping silks to within ½ inch of ear tips, and pollination is not complete (Figure 2).3

Action thresholds for suppressing egg laying. Thresholds vary by region, planting density and crop rotation. In general, if adult beetle populations exceed 0.5 to 1 beetle per plant, potential for significant yield loss the next season may exist if no control tactics are instituted.3,4 Insecticide applications should be timed when the proportion of gravid females (females with eggs) reaches 10% of the females collected. If the number of gravid females exceeds 25% then it is likely that significant egg laying has occurred and reduces the chance that adult control will have much, if any, affect on larval pressure and subsequent root damage levels the following season.

Best Management Practices for Root Protection

  • Rotate to a non-host crop such as soybeans to break the CRW life cycle. Periodic rotation provides a number of benefits in addition to effective CRW control.
  • Plant SmartStax® RIB Complete® Corn Blend products that provide dual mode-of-action (pyramided) B.t. traits and deliver excellent CRW protection.
  • If rotation or SmartStax® RIB Complete® Corn Blend products are not acceptable options, consider using soil-applied insecticides in combination with seed products that do not provide B.t. protection from CRW larvae.
  • Due to resistance concerns, planting single-mode-of-action technologies such as Genuity® VT Triple PRO® products or Genuity® VT Triple PRO® RIB Complete® Corn Blend products is not recommended when less than satisfactory control of CRW larvae has been previously observed.

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Figure 3. WBC egg masses (left) and newly hatched WBC larvae (right).



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Figure 4. WBC 6th instar larvae. Note the two black rectangles behind the head capsule.


Corn Earworm (CEW)

CEW larvae can feed on leaves, tassels, and silks but are primarily found on kernels at the tip of the ear. Feeding on the ear can destroy developing kernels and predispose the ear to secondary pests, including other insects and pathogens that may cause mycotoxins.

Larvae vary in color, ranging from yellow, pink, green or sometimes nearly black. Larva are easily confused with other larvae such as armyworm, fall armyworm, or western bean cutworm. However, CEW larvae are usually marked with alternate light and dark stripes and when examined closely with a hand lens, small triangular spines can be observed on the cuticle (skin).

Control is rarely justified in field corn, but seed production fields may require insecticide treatment.3,5

Western Bean Cutworm (WBC)

WBC larvae feed on kernels with damage similar to corn earworm. WBC moths prefer to lay eggs on late-whorl stage corn that is near pollination. Eggs are laid in masses on the upper surface of leaves and are pin-head in size. Eggs are pearly white when first laid and within several days they turn tan, then dark purple shortly before hatching (Figure 3). Young WBC larvae are dark brown with faint diamond-shaped markings on their back. Older larvae change to a lighter color, but by maturity they are gray to pinkish brown with two black rectangles behind the head capsule (Figure 4).

Consider an insecticide treatment if 4 to 8% of the plants are infested with newly hatched larvae and/or eggs on leaves and corn is at least 95% tasseled. If corn is at milk (R3) growth stage before WBC eggs are laid, generally no treatment is needed.5 Treatment timing is critical, as application should be timed when most of the WBC eggs are expected to hatch and before larvae move into the silks and ear tip to feed.

Sources:

1 www.insectforecast.com.  
2 Wright, B. 2013. Scouting for corn rootworhttps://cropwatch.unl.edu/. 
3 Bissonnette, S.M., Pataky, N.R., Nafziger, E.D., et al. 2010. Field crop scouting manual. University of Illinois Extension. 
4 Wright. B. 2009. Use of corn rootworm scouting numbers as basis for 2010 production decisions. Crop Watch. University of Nebraska-Lincoln. http://cropwatch.unl.edu/. 
5 Wright, R.J. 2013. Corn insects - quick reference guide. University of Nebraska - Lincoln.  http://extensionpublications.unl.​edu/. 
6 Steffey, K.L., Rice, M.E., All, J., Andow, D.A., Gray, M.E., and Van Duyn, J.W. 1999. Handbook od corn insects. Entomological Society of America .
7 Hodgson, E. 210. Predicted 2010 corn rootworm hatch. Integrated Crop Management. Iowa State University. https://crops.extension.iastate.edu/. Web sources verified 06/02/18. 180604150449

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