Scouting for corn rootworm beetles takes a commitment

Bryan Jensen
UWEX and UW Madison

We are soon to be in that time period where female corn rootworm beetles will be laying the majority of their eggs. Scouting for adults during this time period can give us great insight regarding the potential for damage in continuous corn.

These western corn rootworm adult beetles emerge late June through
August and lay eggs almost exclusively in corn fields from early August through
early September.

Adult emergence occurs over a long period of time. Although some beetles have already emerged, more are yet to come. Once emerged, the adults need time to find a mate and for those eggs to mature.

Typically, we target mid-August to early September as being that time period where our adult beetle scouting efforts are the best predictor of larval damage. Here is a dilemma. Corn rootworm adult populations have been very low the past three years. Although it is early to tell for 2018, the latest issue of DATCP’s Pest Survey Bulletin shows this trend is continuing.

Furthermore, corn prices are very low as are profit margins. If a grower can’t rotate to a non-host crop next year, beetle scouting during the egg laying period can give us a lot of direction (and confidence!) in choosing cost effective methods to control larval feeding.

Scouting for corn rootworm beetles, in my opinion, needs to be thorough and done according to established guidelines. It is a commitment. Spot-checking occasional fields and/or field edges leaves a lot to be desired because of the potential for a wrong recommendation.

Scouting for corn rootworm beetles needs to be thorough and done according to established guidelines.

However, if done according to these guidelines there is an upside which includes considerable rootworm management cost savings.

How to scout

  • Count the number of CRW beetles on 50 corn plants. Visit 5 random areas of the field and count beetles on 10 plants in each area. Do not pick plants directly adjacent to each other.
  • Count the beetles found on the tassel, silk, top and bottom of leaves, and feeding on the ear tip.
  • First, trap beetles in the silk by firmly grabbing the silk end of the ear. Count beetles on the rest of the plant before slowly opening your hand to count beetles feeding on the silk and ear tip.
  • For pollination protection, scout fields before 70% of the field has silked.
  • For root protection, scout fields during the egg-laying period from early August to early September. Repeat this scouting procedure on 7-10 day intervals one or two more times during the egg-laying period.

The long established economic threshold is if you have an average of 0.75 beetles/plant during the egg laying period, a control method needs to be used in continuous corn. If you are over the threshold on the first scouting visit, you could stop scouting that field and be satisfied with a recommendation that control will be needed.

I do think that because of the range of control costs a second scouting visit 7-10 days later will provide additional useful information that will help select a more cost effective control method.

Beetles are mobile and can readily move around the landscape. That second visit will give additional confidence and reduce anxiety. For that same reason if you are under threshold during the first visit, a second visit would be required to make sure beetles have not moved into that field.

In a nutshell

  • Scout during the egg laying period (mid-Aug. to early-Sept.).
  • Count the number of adults on a total of 50 plants in 10 areas of the field.
  • When approaching a plant, cover the silks with one hand to trap those beetles before counting the adults on the rest of the plant. When finished remove your hand from the silk and count those additional beetles.
  • Walk several feet down the row before counting adults on the second plant.
  • Adults will fly or drop off the plant when disturbed.

Jensen is the Integrated Pest Management specialist for Cooperative Extension and UW Horticulture professor.