By Bryan Jensen, UW Extension and IPM Program
You are probably aware that Bt CRW hybrid field performance issues are occurring. Several states in the Midwest, including Wisconsin, have experienced control problems for several of the Bt proteins. Although my impression is that Wisconsin’s situation lags behind that of some other corn producing states, it is nevertheless a valid concern. There are several monitoring techniques and management options that are available which can slow resistance. However, it is unlikely you will be able to reverse resistance to a Bt protein once it occurs. Therefore, 2015 will be the time to start implementing these practices.
Field monitoring has been, and always will be, an important IPM practice. Scouting beetles during the egg laying period (mid-August to early September) will give you an estimate of control needs in continuous corn. Additionally, that information can also be used to prioritize control practices that include insecticide seed treatments, soil applied insecticides as well as proper selection of Bt CRW proteins.
Evaluating corn roots for signs of feeding during mid-July to early August will also be useful. This information will verify the efficacy of your control practice. Don’t assume straight standing corn does not have significant rootworm feeding and do not assume that all lodged corn is a result of rootworm feeding. Dig and document. Evaluating root damage in Bt CRW hybrids will give you advanced warning regarding early levels of resistance. In first year corn it will give you information on presence (or absence) of rotation resistant rootworm.
Continue to plant refuges. Until we know different, refuges are a source of susceptible adults which could mate with resistant beetles. Although planting refuges is getting easier because of seed blends, block refuges may be difficult to understand especially when combined with lepidopteran Bt proteins. When in doubt, read the tag and/or consultant with your seed sales representative.
Using multiple modes of action and diversifying management practices will also be important components to a resistance management program. Topping that list of practices is crop rotation. Assuming you are not in an area where damage from rotation resistance corn rootworms is possible (your root assessments will tell you for sure!) crop rotation is an excellent method of rootworm management.
The high rates of insecticide seed treatments on conventional hybrids may be useful, but only if beetle counts are low. Beetle counts from the previous growing season will indicate which fields are possible candidates for this practice. Fields with moderate or high beetle numbers maybe a good choice for conventional hybrids planted with a soil applied insecticide. If Bt resistance is not an issue in your fields then traited corn remains an option, especially for those fields with higher beetle populations. However, make sure you rotate your Bt modes of action. If you have been using the same Bt protein for two years (or more!) switch proteins. Cross resistance is a potential issue in the family of Cry3 proteins (YieldGard, Agrisure, Duracade). Make sure you know what protein you have been planting and rotate to a viable option or switch to a soil applied insecticide. All proteins have had field performance issues so there is no silver bullet.
Hybrids with pyramid proteins MAY be a good alternative provided you do not have initial levels of resistance building up in your field. Pyramid hybrids have two modes of action for the same insect and can be a good resistance management tool provided both proteins are effective. If not, you essentially are using a single mode of action. That is, you will continue to select for resistance to the compromised protein while increasing selection pressure on the other. In that situation, you will be painting yourself into a corner without realizing. Just another reason to evaluate roots for damage.
Layering a soil applied insecticides with a Bt CRW hybrid has been a practice adopted by some growers. A soil insecticide with a Bt hybrid can be a valuable management tool ONLY IF beetle populations are high enough that control might be compromised when using a Bt hybrid by itself. Fortunately, Wisconsin had lower beetle populations in 2014 than 2013 in all districts except west central and southwest Wisconsin according to the Wisconsin Department of Agriculture, Trade and Consumer Protection’s (DATCP) Pest Bulletin. The southwest district reported a higher average than 2013 primarily because of one exceptionally high field. For more information on 2014 beetle counts go to the Pest Bulletin https://datcpservices.wisconsin.gov/pb/
Do not use a soil insecticide to mitigate performance issues with Bt hybrids. It won’t work. Soil insecticides are not designed to control beetle populations. Rather they protect a localized area of the root mass. Surviving beetles will continued to put selection pressure on the Bt protein.
In conclusion, diversifying corn rootworm management practices will help prolong all management options not just Bt hybrids. If you notice lodged corn, please don’t assume resistance to the Bt protein. Dig and evaluate roots for damage. Other factors including compaction, high winds and rain may also cause corn to lodge. Also, don’t assume that high beetle numbers in a field means Bt performance has been compromised. If you do see significant damage, in addition to calling the seed sales representative, PLEASE contact the local county extension agent. It will be important for us to know if, and how widespread these performance complains are.