By Bryan Jensen
Entomology and IPM Program, UW Extension
I have been getting a few questions regarding the practice of controlling rootworms with a CRW traited hybrid plus a soil applied insecticide. First, I would like to suggest that this tactic would not be considered a resistance management tool. In general, using two effective modes of action can be a good way to manage resistance. However, in this situation, the soil applied insecticide is only an effective mode of action in a limited area at the base of the corn plant. It does not provide protection for roots that have grown outside the area influenced by the soil insecticide. Therefore, larvae feeding on those roots are exposed to a single mode of action and repeated use of the same, or similar protein, will continue to select for resistance.
Using CRW traited corn plus a soil insecticide may have economic value if you are planting into a field with verified high populations of rootworm adults in 2015. By “verified” I mean that detailed count(s) of adult beetles were completed during the 2015 egg-laying period and you had reason to believe excessively high beetle populations warranted a combination of control methods. While high beetle populations may be the case in a few fields around the state, it is definitely not the norm and certainly not consistent from field to field. This comment is supported by examining Wisconsin Department of Agriculture, Trade and Consumer Protection (WDATCP) whole plant count beetle surveys, Figure 1, and by reviewing the raw data graciously supplied by Krista Hamilton and her colleagues at WDATCP.
First, let me summarize the map. Overall, 229 fields were sampled. Only 58 of those fields had populations greater than the economic threshold of 0.75 beetles/plant, indicated by red dots on the map. Of those 58 fields, only 8 fields were higher than 3.0 beetles/plant. I am not implying that fields with greater than 3.0 beetles/plant are considered high, but just trying to indicate the distribution of field averages within that category are at the lower end of WDATCP’s scale. Furthermore, 12 fields had an average between 2.0-2.9 beetles/plant, 15 fields were between 1.0-1.9 beetles/plant and the remaining 23 fields were between 0.8-0.9. Of the remaining 171 fields with sub economic populations, 25 fields were in the 0.5-0.7 beetles/plant (yellow dots), 78 fields were in the 0.1-0.4 beetles/plant (green dots) and the remaining 68 fields (black dots) did not observe any corn rootworm beetles.
WDATCP’s data offers a unique snap shot of 2015 beetle counts across the state and is not intended to provide field level control recommendations. However, it does demonstrations the variability of rootworm counts and that a routine recommendation of traited hybrids plus a soil-applied insecticides is not always justified. If you are unsure of the validity of individual field recommendation this growing season, please consider leaving random check strips of either 1) CRW traited corn only or 2) CRW traited corn + soil applied insecticide depending on your decision. Follow up with assessment of root damage in July/early August. For help in assessing root damage, please view this YouTube video.
For 2016, the best option to accurately identify management options in continuous corn is to count beetles and base your recommendations on those counts. This field level data will help you choose appropriate recommendation for individual fields. For help scouting beetles please view this YouTube video.