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Corn Rootworm Update Northern Corn Belt​

May 26, 2016

Corn Rootworm Species Shift in the Northern Corn Belt

Root damage is greatest when the majority of larvae have completed the 3rd instar (larval stage). Often this is around tasseling and falls in July or August. It can occur as early as June with above average temperatures in spring and early summer.

The 2016 growing season is appearing to be much different in regard to the populations of both species. The population of WCRW has greatly decreased because of two fairly harsh back-to-back winters (2013/2014 and 2014/2015); conditions that WCRW do poorly in but favor NCRW. However, in southeast MN, the populations of WCRW appear to be healthy. This is likely attributed to the hilly terrain that holds more snow compared to the more level terrain of western MN. Populations of WCRW appear to be high also in Wisconsin.

Corn Rootworm Differences

The general biology of NCRW and WCRW are similar; however, NCRW has developed the ability to produce eggs that lay dormant for two winters and hatch the second year after being laid - instead of the normal first year. This phenomenon, known as extended diapause, has made crop rotation a less than desirable Northern corn rootworm (NCRW) management method. During a non-host rotational crop year, WCRW populations die off because of an inadequate food source for the hatching larvae.

Compared to WCRW eggs, the ability of NCRW eggs to tolerate lower soil temperatures gives it a distinct advantage in growing seasons that immediately follow harsh winters (Figure 2). Repopulation of WCRW in a corn field is dependent on the adults migrating into the field from other fields; however, NCRW variant larvae are already within the field and can quickly become established. Knowing which species is present in a field is important for future management.

Bacillus thuringiensis (B.t.) Insect Protection

The primary tactic for managing CRW species in the Northern Corn Belt is the use of corn products that contain Bacillus thuringiensis (B.t.) insect protection. Four primary B.t. proteins (mCry3a, Cry3Bb1, eCry3.1Ab, and Cry34/35Ab1) are currently commercially available in insect protected corn products.

Isolated cases of suspected resistance have been documented for each of the proteins mentioned above.1 Not all CRW populations are alike and field populations may exhibit varying levels of tolerance to one or more of these proteins based on the protein’s history of use in a particular field. Cross resistance between mCry3A and Cry3Bb1 has been observed; therefore, a CRW population resistant to one of the proteins is likely to demonstrate some level of resistance to the other protein. Recent information reported at the Annual Entomological Society of America meetings indicates that cross resistance may exist between mCry3a and eCry3.1Ab as well as with Cry3Bb1.1 As a result, corn products containing a single protection protein have not shown the effectiveness to prevent root feeding as they have in the past. The use of a single CRW B.t. protected product, when CRW pressure is known to be present, is not recommended. Cross resistance to mCry3a, eCry3.1Ab, and Cry3Bb1 has not been shown to exist with Cry34/35Ab1.

Research and Results

Test plots were planted in 2015 in MN and WI as well as other areas of the Corn Belt to collect data on the performance of single and double protein CRW protected products with and without soil applied insecticides (SAI). In MN and WI, significant larval feeding occurred on most of the single B.t. protein protected products compared to products with two B.t. proteins, especially SmartStax® technology products that have the combination of (Cry3Bb1 + Cry34/35Ab1) proteins for CRW protection (Tables 1,2, and 3).

The data from the 3 individual non-replicated trials supports the position that the use of an SAI on SmartStax® technology products provides a nominal level of additional protection which does not consistently translate to a significant increase in yield because of the very small differences in the NIS root scores of products C, G, and I in Tables 1,2 and 3, respectively. However, the reported yield differences of products C, G, and I in Tables 1,2, and 3 with a SAI were +6, +16, and –18 bu/ acre, respectively. A recent analysis of 79 locations that had medium to high pressure CRW resulted in an improvement of NIS root scores from .20 to .11 when a SAI was used (Figure 3). In the same studies, average yields of SmartStax® technology products were not statistically different with and without the use of an SAI (Figure 4). The cost of an SAI ranges from $15 - $25/acre; therefore, about 4- 8 bu/acre in increased yield is needed to cover the cost. If using a single trait product in a known CRW pressure situation, a SAI can significantly improve NIS root scores and in many cases, the insecticide cost can be absorbed through increased yield. However, from a resistance management perspective, the use of an SAI with CRW traits may mask the development of resistance, and therefore is no longer a recommended practice for CRW management.

Discussion

Determining how much root feeding can occur before there is yield loss is difficult because research has shown that correlating yield loss to root feeding is highly variable. Considerable root feeding occurred in 2015; however, there was very little yield loss. Mid-summer rainfall is often a mitigating factor in potential yield loss due to CRW feeding because plants with ample moisture can regenerate roots, which leads to better water and nutrient uptake. It is generally accepted among CRW researchers that for each root feeding score (0-3 scale), there is on average a 15% loss in crop yield.2 Therefore, a score of 0.5 would equate to a 7.5% yield loss, a score of 2 would equate to 30%. With a typical crop yield of 220 bu/ acre, 1 node of injury would equate to 30 bu/ acre loss. Severe CRW root feeding could easily lead to losses of $100 - $200/acre. During dry summers, yield losses can be higher because of the inability to absorb water and nutrients. Research has shown that even a NIS of 0.25 can result in yield reduction.2

Lodging from CRW generally doesn’t occur until there is fairly severe root pruning. It is likely that the year prior to lodging the field suffered economic yield loss as a result of management decisions.

Scouting

A well-planned scouting program is important as it allows for the collection of useful information regarding species present and infestation level. This information is important for making management decisions. A threshold of 1 adult beetle/plant may result in larval populations that are high enough the following year to cause yield reductions. During the scouting process, corn roots should be dug for evaluation (Figure 5).

Fields that are to be rotated out of corn should be scouted as the presence of NCRW populations have the potential to cause injury if the field is planted back to corn the second year. Unless rotated fields in Western MN and in ND and SD were scouted in 2014, there is a potential risk for NCRW feeding in 2016.

Summary

The CRW situation has changed dramatically over the past few years, especially in Western MN, SD, and ND. The shift from WCRW to NCRW forces one to utilize a different management style in the presence of significant populations of NCRW. Crop rotation, though still effective against WCRW in the most northern states can not be counted on when NCRW is present. The use of a CRW dual mode of action protected product, such as a SmartStax® technology product is a good management strategy, especially in a field going to corn that was in continuous corn for 3-4 years or more and then rotated for one season.

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