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Identifying Corn Rootworm Egg Hatch
December 11, 2023
Egg Hatching Signs
It is a commonly held belief that the presence of fireflies in the evening signals that corn rootworm (CRW) eggs (Figure 1) might be hatching. However, the presence of these insects does not always mean that CRW egg hatch is occurring.2,3
The accumulation of growing degree days (GDDs) is a more accurate determination of CRW egg hatch. About 50% of CRW eggs have been found to hatch from 684 to 767 accumulated GDDs (base 52 °F).3 While some Northern corn rootworm eggs exhibit a trait called extended diapause and will not hatch until the following year, Western corn rootworms do not have this trait. Local GDD information can generally be accessed from university extension weather specialists and/or websites. The website http://www.insectforecast.com/ provides useful information on the timing of potential CRW damage as well as that of other insects.
Larval Identification
Corn rootworm larvae (Figure 2) have three developmental stages, or instars, before pupating to become adult CRW beetles. Each instar period lasts from 7 to 10 days.
The larvae are slender, cream colored, with brown heads and a dark anal plate. Newly hatched or first-instar CRW larvae are very small (less than 1/8 inch long). Second instar larvae are usually the earliest stage to be noticed when field sampling. The third instar are about ½-inch long. After the third instar, the pupa forms in a cell in the soil. The pupa is white, somewhat translucent, sessile, and dormant (Figure 3).
Scouting Procedure for Larvae and Root Injury Evaluation
Scouting for CRW larvae should occur from late May through mid-June, depending on the region, planting date, and crop stage. Scouting procedures for CRW larvae include digging plants from several areas of the field and either placing them into a bucket of water for washing or placing the soil samples on a white drop cloth to examine the soil in the field. When placed in water, the majority of any larvae present will float to the top. Though when using the water bucket method, note that some larvae may be inside the roots and air drying the root mass over a container of water will be necessary to extract them.
When evaluating root injury, digging roots too early can underestimate the amount of root injury that can be present while roots dug too late can be difficult to wash and rate due to root regrowth. Root injury from CRW is greatest about the time when most larvae have completed the third instar, which is often around the tasseling stage of corn in July or August. The third instar can occur as early as June with above-average temperatures in spring and early summer. There is usually a 2- to 3-week window that is optimum for conducting root digs.
To assess root injury by CRWs, walk through a field in a “W” formation and dig two root balls at the start of the W and at the end of each leg of the W, for a total of 10 corn root balls. Examine the 10 root balls using the Iowa State University 0-3 Node Injury Scale (NIS), a useful method which helps quantify the amount of larval feeding injury to corn roots.1 Fields without any CRW control tactics should be a priority for scouting.
Planting-time Protection
In most situations, growers should be proactive in developing their CRW management plans. Because of the small size of CRW larvae, the time from detection to peak injury is often so small that insecticide control options are usually unsuccessful. Therefore, the best protection against damage from CRW larvae feeding is planting B.t.-traited corn products with multiple modes of action, such as SmartStax® technology. Using cultivation or irrigation to apply labelled insecticides to the soil at planting or, if practical, closer to egg hatch could be considered if B.t.-traited products are not used and there is potential for CRW feeding.4,5
If CRW larvae are discovered before corn plants are too tall to cultivate, a granular or liquid insecticide applied ahead of the cultivator shovels during layby may provide some control. There is no established economic threshold for CRW larvae, although some agronomists have advised applying a rescue treatment if there are two or three CRW larvae per plant.4,5
Farmers with pivot irrigation systems may gain some control by applying a labeled insecticide as a chemigation treatment. Ample water (irrigation or rain) is needed to move the insecticide into the root system. If practical, a delayed insecticide application may provide better control because the insecticide is being applied after egg hatch compared to an at-planting application that can lose efficacy before the insecticide is most needed for control.
Sources
1Oleson, J.D., Park, Y.-L., Nowatzki, T.M., and Tollefson, J.J. 2005. Node-injury scale to evaluate root injury by corn rootworms (Coleoptera: Chrysomelidae). Journal of Economic Entomology 98(1):1-8. https://doi.org/10.1093/jee/98.1.1
2Bechman, T. 2015. Are corn rootworms here, or was it a false alarm? Indiana PrairieFarmer. https://www.farmprogress.com/story-are-corn-rootworms-here-was-false-alarm-9-128386
3Hodgson, E. 2010. Predicted 2010 corn rootworm hatch. ICM News. Integrated Crop Management. Iowa State University. https://crops.extension.iastate.edu/cropnews/2010/05/predicted-2010-corn-rootworm-hatch
4Wright, B. 2013. Scouting for corn rootworm larvae and treating postemergence. CropWatch. University of Nebraska–Lincoln. https://cropwatch.unl.edu/scouting-corn-rootworm-larvae-and-treating-postemergence-unl-cropwatch-june-27-2013
5Hodgson, E. and Gassmann, A.J. 2015. Corn rootworm management update. ICM News. Integrated Crop Management. Iowa State University. https://crops.extension.iastate.edu/cropnews/2015/08/corn-rootworm-management-update
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