Corn Insects to Monitor

Numerous corn insects have the potential to cause economic injury to corn during the growing season. Corn rootworm, corn earworm, western bean cutworm, and two-spotted spider mites are of particular importance.

Corn rootworm egg hatch throughout central and northern Illinois and Indiana began the last week of May or early June.¹ 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, Top). Heavy feeding can reduce plant stability resulting in lodging.

Sample for larvae by digging up corn plants and washing 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

Monitoring adult beetle populations is key to assessing if 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 1, Bottom).³

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, effect on larval pressure and subsequent root damage levels the following season.

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

Corn earworm larvae can feed on leaves, tassels, and silks but are primarily found on kernels at the tip of the ear (Figure 2). 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 larvae feed on kernels and cause damage similar to corn earworm. The 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, are pin-head in size, and pearly white when first laid (Figure 3, Top left). Within several days the eggs turn tan and then, shortly before the larvae hatch, the eggs turn dark purple. Young WBC larvae are dark brown with faint diamond-shaped markings on their back (Figure 3, Top right). 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 3, Bottom).

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.⁵ 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.

Spider mites feed on the plant sap on the undersides of leaves resulting in yellowish or whitish spots (stippling) across the upper leaf surface (Figure 4). Premature plant death, resulting in potential yield loss and poor grain quality, can occur if mite populations are high and hot, dry conditions exist. When drought conditions exist, TSM populations can explode.

Adults are yellow-green with two irregularly shaped dark spots on the abdomen. Fine silken webs produced by mites can be seen with the use of a hand lens. TSMs move from host plants along field margins into the edge of the field and continue deeper into the field when environmental conditions favor their spread and development. Infestations are generally more sporadic within corn fields and rarely seen on corn plants before flowering (VT growth stage). Treatments may be warranted if active mite colonies are found on 1/3 of the leaves of 50% of the plants.

¹ www.insectforecast.com.

² Wright, B. 2013. Scouting for corn rootworm larvae and treating postemergence. CropWatch. University of Nebraska - Lincoln. https://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. https://cropwatch.unl.edu/.

⁵ Wright, R.J. 2013. Corn insects - quick reference guide. EC1562. University of Nebraska - Lincoln. http://extensionpublications.unl.edu/assets/pdf/ec1562.pdf.

⁶ Steffey, K.L., Rice, M.E., All, J., Andow, D.A., Gray, M.E., and Van Duyn, J.W. 1999. Handbook of 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/cropnews/2010/05/predicted-2010-corn-rootworm-hatch.

⁸ Cullen, E. and Schramm, S. 2009. Two-spotted spider mite management in soybean and corn. University of Wisconsin Extension. A3890. http://corn.agronomy.wisc.edu/Management/pdfs/A3890.pdf.

Web sources verified 06/02/18.

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