Last week, a team of us met up at the Iowa State University Southeast Research Farm (SERF) to evaluate root injury in a small trial for corn rootworm management. The trial had four treatments replicated eight times. The treatments included no management (glyphosate-tolerant only; no Bt traits or insecticide), granular soil-applied insecticide (Aztec), SmartStax®, and SmartStax® Pro (RNAi).
We began by digging up four randomly-selected plants from each plot (4 plants x 8 reps = 32 roots per treatment). Each root was tagged with the treatment and rep number so we could keep track of them during the washing and rating process. Washing the roots was certainly the most time-consuming step. We had several buckets filled with water to soak the roots in an attempt to dislodge most of the soil prior to washing them. Ideally, roots would soak overnight for the best results. Then we used a power washer to clean the rest of the soil off. Having clean roots is critical so you can see the full extent of the injury during the rating process.
We used the ISU 0-3 Node-Injury Scale (NIS) to assess root injury. A rating of 1 indicates that an entire node of roots (or equivalent) was pruned within 1 ½ inch of the soil line or stalk. This rating would translate to approximately 15% yield loss, and each subsequent node is an additional 15% yield loss. Economic injury occurs when just 2 or 3 individual roots have been pruned.
Figure 1 shows the results of our root injury assessment. In the small trial at SERF, we found that the three corn rootworm management treatments (Aztec, SmartStax®, and SmartStax® Pro) had similar average NIS ratings, and those ratings were all significantly lower than the treatment without corn rootworm management. Additionally, the average NIS for those three treatments was below the economic threshold (0.25 to 0.5 NIS, depending on environmental conditions, corn price, and management costs). This supports other research that suggests soil-applied insecticides alone provide similar protection from corn rootworm larval feeding as Bt traits and are a good alternative to Bt, especially when resistance may be suspected in a field.
This research plot at SERF has been a trap crop for corn rootworm for many years, so we expected to see significant pressure in this field. Based on past management strategies and low injury scores, the corn rootworm population appears susceptible to the Bt pyramid and RNAi.
The team consisted of three extension field agronomists (Rebecca Vittetoe, Virgil Schmitt, and Clarabell Knapp), two extension entomologists (Erin Hodgson and Ashley Dean), a Dekalb technical agronomist (Lance Goettsch), and SERF staff (Cody Schneider, Myron Rees, and Chad Hesseltine).