By Ken Ostlie and Bruce Potter
Corn rootworm (CRW) management is not getting easier. In addition to ongoing Bt resistance issues with populations of western corn rootworm (WCR), resistance to Bt has now been documented in some northern corn rootworm (NCR). Corn rootworm populations were low in many, but not all, 2019 corn fields. Additionally, prevent plant acres may have dramatically changed rootworm populations. Finally, anecdotal reports indicate northern corn rootworm populations may be increasing. What does this mean for 2020 and beyond?
Rootworm survival
Rootworm eggs, particularly those of the WCR, can be killed if exposed to cold temperatures while they overwinter in the soil. This past winter, both species likely survived reasonably well in the southern part of the state. Areas with minimal snow cover when brief, but intense, cold snaps occurred may have seen some mortality of WCR eggs.
To this point, 2020 spring rainfall has been much more moderate than 2019. Soil moisture has little effect on the diapausing eggs but small, newly hatched larvae can “drown” in soils saturated from heavy rains. From hatch until they tunnel into roots, young larvae (< 1/8”) are most susceptible to drowning.
Later planting dates typically mean smaller root systems at the time of rootworm hatch with fewer larvae able to successfully colonize root systems. With the exception of the northwestern part of the state, most corn planting was very timely this spring. This year’s warmer weather should provide corn with well-developed root systems for larvae to colonize.
Later-emerging, later silking fields are attractive to immigration of beetles from fields were pollination has ceased and silks have dried. In most areas, the timing of corn emergence has been relatively uniform among 2020 fields. As a result, there could be less beetle movement between fields.
The large number of 2019 prevent plant acres in many areas of the state are expected to generally reduce corn rootworm populations, extended diapause NCR from rotated corn acres and both species from continuous corn acres. Depending on weed management timing and success during 2019, beetles responded to pollinating weeds and volunteer corn in individual fields.
What’s your risk of rootworm damage?
The best way to determine risk for a particular field is to scout for beetles this summer. On the other hand, an ability to understand near- and long-term changes in overall WCR and NCR populations, including resistance to Bt traits and rotation, can provide an early indication that risks may be changing. This requires information from a large number of fields over the geography and cropping systems of Minnesota.
Can you help?
We would like your help in collecting CRW data! The University of Minnesota will be coordinating a network for corn rootworm yellow sticky trap data this year with the help of funding from the Minnesota Corn Research and Promotion Council.
We can supply a relatively pain-free sampling protocol, data forms/software and a limited number of yellow sticky traps. Field-specific information will be gathered for statistical purposes but will not be made public. The fields sampled would be up to individual cooperators, but ideally, we would like data from a minimum of 3 continuous and 2 rotated corn fields from Minnesota counties with significant corn acres.
Source : umn.edu