Western corn rootworm larvae feed on corn roots. This feeding injures corn roots, which can result in lost yield. Research has shown that for every one node of root pruned, there is an average of 15 percent yield loss.
Research over the years has led to the development of multiple strategies to minimize rootworm feeding injury, from broadcast insecticides to crop rotation, from insecticides applied to seed or in-furrow to the development and deployment of hybrids that produce one or more insecticidal protein (rootworm Bt traits).
Western corn rootworms in particular have evolved resistance to several of the practices and products that have been developed to manage them. Some populations have evolved resistance to select insecticides, others to crop rotation with soybean, while still others have developed resistance to specific Bt proteins.
Understanding that rootworm management must remain a priority in those areas of the Corn Belt that have damaging rootworm populations many farmers are again considering in-furrow, planting-time insecticides. These products may be being considered by those people that: 1) do not plan on planting a hybrid with a rootworm Bt trait(s), 2) have a rootworm population that is either known or suspected to have resistance to one or more Bt traits but still plan on planting a hybrid expressing the trait, 3) are interested in throwing everything at the rootworms and plan to both plant a hybrid with rootworm Bt traits AND use a planting-time in-furrow soil insecticide.
Research has shown that scenario #3 may not be the best idea for several reasons. In 2013 researchers from both Iowa State University and the University of Illinois published the results of a 2-year field study:
Petzold-Maxwell, J.L., Meinke, L.J., Gray, M.E., Estes, R.E. and Gassmann, A.J. 2013. Effect of Bt Maize and Soil Insecticides on Yield, Injury, and Rootworm Survival: Implications for Resistance Management. Journal of Economic Entomology 106(5): 1941-1951.
As part of this study the researchers looked at the effects of planting a Bt hybrid (expressing Cry3Bb1) and the near-isoline to the Bt hybrid (genetically similar to the Bt hybrid minus the Bt trait) with or without either a seed treatment insecticide or an in-furrow insecticide. For each of the treatment combinations they measured several factors: rootworm emergence and emergence timing, root injury, and grain yield.
The data showed that combining an insecticide with the Bt hybrid did not reduce root injury or increase yield.
Patterns in rootworm emergence also suggested that this combination may speed the evolution of resistance.
Planting refuge hybrids (structured or in-the-bag) is required from a resistance management standpoint to allow emerging rootworms that had been exposed to Bt proteins (those populations that may havea higher frequency of resistance alleles) to mate with those individuals that have not been exposed to the Bt protein (those populations that may have a lower frequency of resistance alleles).
The research showed that while both male and female rootworms emerge later in the season on Bt corn, adding an insecticide further delayed emergence. This could increase the chance that females rootworms emerging from non-Bt corn could have already mated with a male that had also emerged from non-Bt corn by the time males emerge from the Bt corn – like ships passing in the night – thereby speeding the evolution of Bt resistance in the rootworm population in that field.
See University of Illinois post-doctoral researcher Dr. Nick Tinsley discuss the potential value of inputs geared towards reducing yield losses due to rootworm feeding injury in THIS WEBCAST.
Hear Dr. Mike Gray, University of Illinois emeritus professor discuss corn rootworm management in THIS ONLINE COURSE.