Bryan Jensen, Department of Entomology and IPM Program
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. 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 through 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. However, if done according to these guidelines there is an upside which includes considerable rootworm management cost savings.
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.
There are several resources listed below which can help describe or demonstrate rootworm scouting methods. In a nutshell. Scout during the egg laying period (mid-August to early-September). 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.