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Mild winters and early planting generally contribute to higher Japanese beetle populations.1 Areas heavily infested with the larval stage white grub is not an indicator of severe injury from adult beetles in the same area. Adults can reduce yield potential by interfering with pollination in corn and damaging leaf tissue and pods on soybean plants.
Figure 1. Japanese beetle.
Adult Japanese beetles are nearly a ½-inch long, have a metallic green head and neck region, reddish to bronze wing covers, rows of six white bristle bunches along each side of their abdomen, and live 30 to 60 days (Figure 1).1 Adults emerge from the soil starting in late May and early June, with peak emergence occurring 4 to 5 weeks later. Mating occurs soon after emergence causing the females to burrow 2 to 4 inches into the soil and lay 1 to 4 eggs every 3 to 4 days for several weeks.2
The grubs emerge from the eggs in about 10 days and grow quickly to full size, about 1-inch long. The grubs feed on the roots of living plants and then overwinter. When soil temperature climbs above 50 °F in the spring, the grubs begin to move toward the soil surface to feed and pupate prior to emerging as adults.2
Figure 2. Japanese beetles clipping silks.
The adults feed on leaves, tassels, silks, and pollen. Corn leaves may appear lacy or skeletonized, but leaf feeding is rarely of economic importance. Economic damage can occur when beetles clip silks during pollination (Figure 2), which can result in partially pollinated ears (Figure 3). Silk clipping after pollination does not affect yield potential.
Figure 3. Silks still attached to unfertilized ovules.
When scouting corn for Japanese beetles, a representative portion of the entire field should be evaluated. If sampling is only conducted near field edges, where populations of Japanese beetles are usually clumped together, populations across the field could be overestimated. An insecticidal treatment should be considered during corn silking stage if:
Figure 4. Soybean leaf defoliation caused by Japanese beetles.
Although Japanese beetles can cause extensive defoliation, soybean plants have the capability to compensate for the damage, and defoliation seldom affects yield potential (Figure 4). Flowering soybean fields should be scouted for the presence of Japanese beetles and the extent of defoliation. The percent defoliation should be estimated on randomly selected leaves in at least five different areas of the field. Insecticide applications should be considered if:
Individual state insecticide recommendations for the control of Japanese beetles can differ and must be followed. Thresholds and single active ingredient insecticide recommendations for controlling Japanese beetles in corn and soybean crops in Indiana can be found in Table 1. Combinations of these active ingredients are also available. Damage from Japanese beetles can add to other stresses the crop is experiencing, and economic thresholds may need to be adjusted if plants are under moisture stress.1 This, along with commodity prices, should be taken into consideration when using thresholds to determine if insecticide treatment is needed. Insecticides may initially control or knock-down a population; however, poor residual activity and the mobility of the insect could lead to the need for a second application if populations resurge later. With subsequent treatments, consider an insecticide with multiple or different modes of action (MOA). The first column in Table 1 lists the Group Number assigned by the Insecticide Resistance Action Committee, which represents an MOA classification.3 Sub-groups are assigned to insecticide compounds within an MOA when the structure differs, and metabolism is believed to be by another enzyme. For example, Group 3 insecticides are sodium channel modulators, and the 3A Sub-group represents pyrethroids which have a specific structural component within the Group 3 MOA.
1 Cook, K.A. and Gray, M.E. 2003. Japanese Beetle (Popillia japonica Newman). University of Illinois Integrated Pest Management. http://ipm.illinois.edu. 2 Townsend, L. Japanese Beetles. University of Kentucky Cooperative Extension. ENTFACT- 409 http://www.ca.uky.edu. 3 Poster Version 2. 2009 Insecticide Resistance Action Committee. Based on the Mode of Action Classification—Version 6.3. 4 Hodgson, E. 2009. Japanese beetles expanding range in Iowa. Integrated Crop Management. Iowa State University Extension. http:// www.extension.iastate.edu. 5 Hodgson, E. 2014. Japanese beetles emerge in Iowa. Integrated Crop Management News. Iowa State University. 6 Krupke, C.H., Obermeyer, J.L., and Bledsoe, L.W. 2015. Soybean insect control recommendations. Purdue Extension E-77-W. 7 Krupke, C.H., Obermeyer, J.L., and Bledsoe, L.W. 2015. Corn insect control recommendations. Purdue Extension E-219-W. 8 Product labels. Web sources verified 06/27/16.