Damage to silking corn plants can be considerable when corn rootworm beetles consume silks before potential kernels are fertilized. Additionally, particularly for continuous corn growers, eggs laid this season can be the source for larvae that feed on roots in a following corn crop.
Of several corn rootworm (CRW) species, the beetles or adults of the western and northern CRW are the most important for Corn Belt farmers. The western CRW beetle is identifiable by the striped (female) or nearly blackish (male) wings (Figure 1). Northern CRW beetles are green to yellowish green (Figure 1) with females being slightly longer and having larger abdomens.
Regardless of the management reason, CRW beetle counts are a key component in assessing the need for warranted foliar-applied insecticide applications. Scouting for CRW beetles should be conducted at least once each week, beginning at early tassel and continuing through early September. The scout needs to be able to accurately count beetles, identify males and females, and be able to determine if a female is laden with eggs.
All corn fields should be scouted, especially late planted fields, fields with volunteer corn, and corn fields in geographies where the extended diapause of the northern CRW is common. In other areas, the presence of the western CRW variant may necessitate the scouting of soybean fields for egg-laying females. Late planted corn can act as a trap crop for CRW beetles, which may result in high adult counts and a high number of egg-laying females. Prevented plant fields (fields that were unable to be planted before the last insurance defined planting date) can be populated with volunteer corn plants that can support larvae and attract CRW beetles. Fields with certain grassy weed species, such as the foxtails, may support or attract CRW beetles from neighboring fields and should be scouted.1
The eggs of northern CRW diapause variants hatch after two winter seasons; therefore, crop rotation may not provide sufficient management. Western CRW female variants have the ability to feed and lay eggs in soybean fields, which can result in the following corn crop becoming infested with CRW larvae.
Scouting guidelines and information to collect:
In general, treatment with foliar insecticides to control beetles during pollination is warranted when:
Thresholds vary by state and planting density. In general, if adult beetle populations exceed an average of 0.75 to 1.0 beetle/plant, the potential next season for significant yield loss in continuous corn may exist if no control tactics are instituted.3 Insecticide applications should be timed when the proportion of gravid females reaches 10% of the females collected. If the number of gravid females exceeds 25%, then it is likely that significant egg laying has already occurred. Under this scenario, the opportunity to control adults to reduce the potential number of eggs laid that can hatch the following season is limited.
If a foliar insecticide application is warranted, always read and follow label directions. If compatible, the use of two insecticides may improve beetle control due to temperature effects on insecticide activity. Activity of pyrethroid insecticides (such as lambda-cyhalothrin) typically decrease with increasing temperature, and toxicity of organophosphate insecticides (such as chlorpyrifos) can increase with increasing temperature.4 Length of residual activity is also affected by rain or irrigation. Some insect populations that are resistant to certain insecticides may be managed by using combinations of insecticides with different modes of action.
Following an adult control tactic and after application reentry restrictions have expired, field scouting should resume to monitor rebounding CRW adult population levels. Eggs can continue to hatch and beetles can immigrate from surrounding fields to replenish the population. If sequential insecticide applications are necessary, care should be taken to utilize products with different modes of action to lower the risk of insects developing resistance to insecticides.
When correctly timed, CRW beetle suppression can be an effective means for helping to protect the yield potential of the current crop and potentially reduce the overall larvae population the following season. An intensive scouting program forms the basis of integrated strategies to manage CRW larvae and beetles through the use of crop rotation, B.t. traits, and warranted soil-applied and foliar insecticides.
1Murphy, A.F., Rondon, S.l., Wohleb, C.H., and Hines, S. 2014. Western corn rootworm in eastern Oregon, Idaho, and eastern Washington. Universities of Oregon State, Idaho, and Washington State. PNW 662.
2 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.
3 Wright, B. 2009. Use of corn rootworm scouting numbers as basis for 2010 production decisions. CropWatch. University of Nebraska-Lincoln. http://cropwatch.unl.edu.
4Ostlie, K. 2006. Soybean aphids, drought and heat. University of Minnesota. www.extension.umn.edu.
Additional sources: Steffey, K.L. 1999. Handbook of corn insects. Entomological Society of America. Larson, E. 2006. Corn/wheat planting pitfalls. March 2006 Agronomy Notes Newsletter. Mississippi State University Extension Service. Ratcliffe, S.T., Gray, M.E., and Steffey, K.L. 2004. Corn rootworm Diabrotica spp. University of Illinois Extension. Integrated Pest Management. http://ipm.illinois.edu. Websites verified 7/26/2018. 130621060801