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The goal of adult corn rootworm (CRW) spray programs is to suppress corn rootworm beetle populations and reduce egg-laying so that larval populations the following season will not cause economic loss, particularly in continuous corn.
Several factors related to beetle emergence patterns should be considered to determine the optimal time for the initial insecticide application. Female beetles need to feed on high quality food (pollen, green silks, leaves) for at least 10-14 days before they are capable of laying eggs.1 Fields pollinating later than most of the surrounding corn fields will be most attractive to beetles. Cool weather can lengthen the beetle emergence period and egg-laying periods. Conversely, hot weather can speed up larval development, compress the emergence period and reducing egg-laying periods. During early beetle emergence, more males emerge than females. Try to avoid spraying during the first 2-3 weeks of the emergence period, even if beetle populations are high.
Figure 1. Male (left) and female (center) western corn rootworm beetles, northern corn rootworm beetle (right).
Scout corn fields starting in late June to early July to identify initial beetle emergence and which corn rootworm species are present (Figure 1). In a typical year, peak CRW beetle activity occurs in July-August (visit http://www.insectforecast.com for current monitoring information). Scouting should take place in the morning and late afternoon, when the beetles are most active. Continue scouting every 2-3 days until silks turn brown. Randomly select 10 to 25 locations within the field and count the total number of beetles on at least two plants within each location. Between beetle counts, collect beetles and determine the number of males and females in the sample. The proportion of gravid females, or females carrying eggs, in the sample should be determined. In general, if silks are clipped to one-half inch or less before pollination is 50% complete, control measures would be indicated.
Figure 2. Gravid females can be distinguished by abdomen contents; a finger rub through the contents will separate out eggs (left). The contents of non-gravid females appears gelatinous or ‘slimy’ (right). Photos courtesy of Kevin Black, Growmark.
Adult female beetles also begin to lay eggs approximately two weeks after they emerge. In general, treatment with foliar insecticides to control beetles during pollination is warranted when: beetle counts of 5 or more per plant are found, fewer than 75% of the plants have emerged silks, and clipped silks are present.1
After silks turn brown, fields should continue to be scouted for adult CRW to assess potential for larval infestation the following year. In general, if adult beetle populations exceed ¾ to 1 beetle per plant, the potential for significant yield loss the next season may exist if no control tactics are instituted.2
1 Bissonnette, S.M., Pataky, N.R., Nafziger, E.D., Davis, V., Estes, K.A., Jones, D., Gray, M.E., Bradley, C.A., Niblack, T., and Hager, A.G. 2010. Field crop scouting manual. X880b. University of Illinois. 2Wright, B. 2009. Use of corn rootworm scouting numbers as basis for 2010 production decisions. CropWatch. University of Nebraska-Lincoln. http://cropwatch.unl.edu