July 15, 2008

Western corn rootworm beetle

Western corn rootworm beetles are emerging in south central Nebraska. Beetles typically emerge somewhat later in northeastern and western Nebraska. Beetles emerging before silk emergence may feed on corn leaves. They feed by scraping the surface tissue, leaving a white parchment-like appearance. Once silks emerge, they become the favored food. The earliest silking fields in an area often are most heavily damaged because beetles will move to it in search of green silks.

Determining Treatment Needs

There are no thresholds for silk-clipping damage based on beetle numbers because damage levels are not correlated well with beetle densities. Usually an average of at least 10 beetles per ear is required to seriously affect pollination. Severe silk feeding at 25-50% pollen shed may indicate a need to apply insecticide, especially in seed production fields. Visit the UNL Department of Entomology Web site at http://entomology.unl.edu for a list of insecticides labeled for adult rootworm control.

Traditionally we have talked about the value of rootworm beetle scouting to determine the need for insecticides the next year if a field is to be planted back to corn. With the registration of Bt corns active against corn rootworm larvae, beetle scouting also can be used to determine where it would be most profitable to use this technology.

During late July and August these beetles will lay eggs in corn fields. 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. With adult beetle control programs, decisions as to whether to treat and if so, when to spray, should be based on information from field scouting.

Scouting Recommendations

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 that examining one plant doesn't drive beetles off of 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. An alternative method is to check for beetles only in the ear zone (the area including the upper surface of the leaf below the primary ear and the under surface of the leaf above the primary ear).

Threshold Counts for Predicting Future Damage

In continuous corn if beetle counts exceed 0.75 beetle per plant, 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 to 0.56 beetle per plant. These thresholds are based on a 24,000 plant population per acre. The number of beetles per plant to equal a threshold level should be adjusted for different plant populations (see Table 1). If the ear zone method is used for scouting, divide the above thresholds in half, since on average only 50% of the beetles on a plant are counted using this method.

In fields with insect levels over the threshold, next year consider rotating out of corn, planting a transgenic corn active against rootworms or using an insecticide at planting or cultivation 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.

Adult Beetle Control

Individuals using adult beetle control programs 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.

Continue to monitor fields weekly after treatment for rootworm beetles. If beetle numbers exceed 0.5 beetles per plant, retreatment is warranted. Late maturing fields are particularly susceptible to corn rootworms moving into them from nearby earlier maturing fields.

Bob Wright
Extension Entomologist