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Adult corn rootworm (CRW) beetles (Figure 1) generally begin emerging in early July in Nebraska. After emergence, the beetles feed on leaf tissue, tassels, and pollen, but prefer to feed on silks. Silking is the most critical time to protect corn from adult CRW feeding. CRW beetles usually mate and lay eggs from August through early September.
Begin scouting at least once a week at early tassel. Increase the frequency of scouting to twice a week during silking. After silking has completed, continue scouting once a week until early September when egg laying has finished. Scout in mornings or late afternoons when the beetles are most active. Inspect 25 to 50 plants in a field (5-10 plants in 5 different locations).
To protect the silks, insecticides may be warranted during pollination when 5 or more beetles are found per plant, fewer than 25-50% of the plants have shed their pollen, and severe silk clipping (silks chewed to within 1/2 inch or less of the ear) is noted. To prevent egg laying after pollination, treat if an average of 0.75 beetles per plant are observed.
Western bean cutworm moths generally begin to emerge in early July. The moths mate after emergence and quickly begin laying eggs, with peak egg laying by mid to late July. Egg laying often occurs in the whorl of the corn plant. As the larvae grow they will spread about the plant feeding on tassels, then silks, and eventually move into the ear to feed on the developing kernels.
Figure 1. Western CRW adult (left) and northern CRW adult (right). Western CRW beetles are yellow to green colored with a black stripe along the sides of the wing covers, and the male wing covers are typically darker colored than those of the females. Northern CRW beetles are tan to pale green and males and females are similarly colored, but females are longer than males.
Emergence of adult moths can be first detected with traps. Once moths are observed in the traps, begin inspecting individual plants for egg masses. Inspect the upper surface of new leaves and inside the whorls for egg masses. Also check the tassels for young larvae. If you are scouting after pollen shed, you may need to inspect silks and ears for older larvae.
Figure 2. Adult western bean cutworm moth (top left), older larvae clipping silks with three characteristic dark stripes immediately behind the head (top right), egg masses are white at first and turn purple just before hatch (bottom left), and hatching larvae (bottom right).
The University of Nebraska-Lincoln (UNL) recommends scouting 10 consecutive plants in about 10-15 randomly selected locations of each field. Be sure to inspect each different corn product in the field, especially if maturities vary. According to UNL, consider an insecticide application if 5 to 8% of the plants have egg masses and/or small larvae. Insecticide applications should be made after the majority of eggs have hatched. Insecticides are most effective while the larvae are young and when they are actively moving about the plant feeding on tassels. Control is more difficult after the tassels have dried and the larvae move to the silks and ear tips to feed, where they are more protected from the insecticide.
Growers can track corn rootworm hatch and western bean cutworm moth catches at http://www.insectforecast.com. For more information on western bean cutworm, including scouting and management, see the AgKnowledge Spotlight Western Bean Cutworm in Corn or visit http://extensionpublications.unl.edu/assets/html/g2013/build/g2013.htm for the UNL NebGuide Western Bean Cutworm in Corn and Dry Beans.
Sources: Seymour, R.C., Hein, G.L., and Wright, R.J. 2010. Western bean cutworm in corn and dry beans. NebGuide G2013. University of Nebraska-Lincoln Extension. Corn rootworms. 2009. Purdue University Field Crops IPM. https://extension.entm.purdue.edu/fieldcropsipm/insects/corn-rootworms.php 180525130206