Western corn rootworm beetles

Western corn rootworm beetle

Editor’s note: The following was written by Robert Wright and Julie Peterson, Extension entomologists for the University of Nebraska CropWatch website July 16.

Western corn rootworm beetles began emerging in late June and early July in southeastern and south central Nebraska. Beetle emergence will be somewhat later in northeastern and western Nebraska. These beetles can cause potential injury from silk clipping.

During mid-July and August these beetles will lay eggs in cornfields. These eggs overwinter in the soil, hatch into rootworms in the spring, and feed on corn roots if continuous corn is grown. However, not all continuous corn fields have economic infestations of corn rootworms.

Weekly scouting of adult rootworm beetles in July and August will provide the information needed to decide whether rootworm control is needed next year.

Start scouting for corn rootworm beetles soon after beetle emergence begins and continue scouting weekly until threshold levels are exceeded or beetle activity stops. Examine 50 plants per field, taking samples from each quarter of the field.

Sampled plants should be several paces apart so examining one plant doesn’t drive beetles off the next plant to be sampled. The most reliable method is to examine the whole plant for beetles. Beetles may hide behind leaf sheaths or in the silks, so take care to observe all beetles present.

In continuous corn if beetle counts exceed the thresholds, damaging populations of corn rootworms are possible in that field next year. In first year corn, there is a higher proportion of female beetles, so the threshold is lowered compared to those in continuous corn. The number of beetles per plant to equal a threshold level should be adjusted for different plant populations.

Another scouting method for adult rootworm beetles is to use yellow sticky traps (unbaited). Traps should be placed on the plant at ear level. In this case the treatment threshold is an average of two beetles/trap/day.

In fields with insect levels over the threshold, next year consider:

  • rotating out of corn
  • planting a pyramided transgenic corn expressing the Cry34/35 protein and another protein active against rootworms, or
  • using an insecticide at planting on non-Bt corn to prevent economic damage.

Fields remaining below the threshold level throughout the beetle egg-laying period are not expected to have economic populations of rootworms next year.

For adult beetle control programs, decisions as to whether to treat and if so, when to spray, should be based on information from field scouting. Individuals using adult beetle control programs to reduce egg laying should begin treatments when the beetle threshold is exceeded and 10% of the female beetles are gravid (abdomen visibly distended with eggs).

This is an important point since the first beetles to emerge are mostly male, and females require at least 10-14 days of feeding before they can lay eggs. Treatments applied too early may be ineffective if large numbers of females emerge after the residual effectiveness of the treatment has dissipated.