Bryan Jensen, UW Extension and IPM Program
Useful corn rootworm management data can, and should be gathered during the month of August. Digging and washing roots will provide insight on efficacy of rootworm management tactics. Scouting for beetles will give you a perspective regarding forthcoming pressure in continuous corn fields.
The window is closing for the best time to observe rootworm feeding. Most larvae have now completed feeding. Root regeneration has started and will only continue to get worse. Especially when soil moisture is adequate. Although root regeneration is good, it does mask larval feeding and may not give you a true picture of efficacy. I don’t hear or see enough people digging roots to validate their management choices. You wouldn’t consider using herbicides in a weed management program without ever checking to see if it worked. Do the same for corn rootworms. Just because the field has not lodged doesn’t mean there isn’t economic damage. Conversely, lodging is not always a definitive sign of rootworm feeding. Dig and wash roots starting in late July through August. This information will validate your management choice but can also provide important information regarding Bt resistance and presence/absence of first-year western corn rootworms.
To quantify root damage, use the Nodal Injury Scale developed by J. Oleson, Y. Park, T. Nowatzki and J. Tollefson at Iowa State University. This is an excellent rating system and more information is available at http://www.ent.iastate.edu/pest/rootworm/nodeinjury/nodeinjury.html . Essentially, the injury scale uses a decimal system. The number to the left of the decimal indicates the number of complete nodes (or equivalent number of nodes) of roots pruned back to within 1 ½ of the stalk. The number to the right of the decimal indicates the % of the next node of roots pruned. A root rating of 1.2 indicates the equivalent on one complete nodes of roots is pruned and 20% of the next.
There are several UW Extension resources available to guide you through the root rating procedure and rating system.
Rating corn roots for rootworm feeding publication.
Scouting for corn rootworm beetles is every bit as important as it was decades ago. Perhaps more so because of the need to diversify management practices that reduce the potential for resistance. The Pest Survey Bulletin has reported low beetle counts in the 2015 and 2016 growing seasons. Several locations did not report finding a single beetle.
Counting adults during the egg laying period (mid-August to early-September) will give a good indication of expected larval damage in next year’s corn. The process is simple and doesn’t take a great amount of time when compared to the overall value. By establishing the level of adult infestation during the current year, you can determine whether preventative treatments will be necessary in the following year’s corn crop. The grower will need to manage corn rootworm larval populations if you find an average of 0.75 beetles per plant during any one of the three field samplings. This data will help you make appropriate management decisions based on observed field populations. Several UW Extension resources are available to help with beetle scouting.
Corn Rootworm Beetle Scouting publication.