Bryan Jensen, Department of Entomology and IPM Program
Perhaps not all that surprising was a phone call I recently received regarding poor performance of the Herculex RW (CRY34/35ab1) trait. Although resistance has been documented in other states it was the first time that I had a very reliable source indicate performance issues for that trait in Wisconsin. It is not a widespread problem, as far as I know, but certainly is an issue we need to identify, respect and manage for years to come.
As a result of this report all 4 CRW Bt traits have had performance issues somewhere in the state. The first step is documentation that the problem exists (or doesn’t). Both are important to know. For background information on digging/rating roots please go to a past WI Crop Manager Article on monitoring corn roots for guidance.
It is not too late to dig roots although you will see more root regeneration now than in July which can mask rootworm damage. You cannot and should not assume fields which do not show symptoms of lodging will not have significant rootworm feeding. Furthermore, do not assume that all lodging is a result of rootworm feeding. You will have to do the legwork. Use of an unmanned aerial vehicle could be of assistance to identify field areas which are lodged. Once those areas are identified good boots, time, a shovel and a power washer are necessary for verification.
Expect to see some root feeding even on pyramid hybrids. They are not immune to feeding. However, if you have average root ratings greater than 1.0 on single traits and 0.5 on pyramids it would typically be considered higher than damage which you typically expect. If so, future mitigation practices should be implemented and include practices such rotation (preferred), use of alternate CRW traits (if reliable), use of soil applied insecticides on conventional corn or the use of the rootworm rate of seed treatments if population are verified to be low through beetle scouting. Strongly consider contacting your seed sales representative to initiate reporting of this unexpected damage or at least to verify the Bt trait was present in that hybrid.
Although this information is not good to hear. It is important that we understand the potential, appreciate its significance and recognize that changes in control practices are going to be important for durability of all management practices.