What’s your risk from corn rootworms? Assess it by scouting and join the MN Rootworm Survey Project

Ken Ostlie, Extension entomologist and Bruce Potter, IPM specialist

Adult corn rootworm emergence will begin soon.
What will it reveal about your risk for 2020?
Photo: Dave Hansen

Corn rootworm management is not getting easier. In addition to the ongoing issue of Bt resistance issues with populations western corn rootworm, resistance to Bt has now been documented in the northern corn rootworm. With corn rootworm populations already at low levels in 2018, this year’s weather and planting delays will further shift threat levels and raise management questions for 2020.

Winter survival

The brutal cold weather this winter may have caused egg mortality, especially with western corn rootworms. However, much of the state had decent snow cover (>8”) by the time brutal cold arrived. But, if you were in an area with minimal snow cover, egg mortality could be significant and further drive the surviving populations towards northern corn rootworms.

Spring rainfall

Small, newly hatched larvae can “drown” in soils saturated from heavy rains but soil moisture has little effect on diapausing eggs, which have little metabolic activity and require little oxygen. From hatch to their tunneling into roots, young larvae (< 1/8”) are most susceptible to drowning. While 2019 spring rainfall challenged records throughout much of MN, most of the saturated soil conditions occurred before hatch. We could still see extensive mortality in low-lying and poorly drained areas. Cooler, wetter conditions this spring also had another significant prospective impact: delaying egg hatch and larval development.

Delayed planting

Later planting dates typically mean smaller root systems at rootworm hatch with fewer larvae successfully colonizing the root system. The rainfall-induced delays in planting in 2019 will reduce larval success and effectively shift emergence of survivors later (because earlier hatchers have more mortality than late hatchers). Planting delays can also accentuate the attractiveness of later-planted, later silking fields, or areas within fields, to immigration of beetles from earlier planted fields (earlier silking fields). Watch out for late season re-distribution off beetles as they respond to the evolving silk situation.

Prevented plant acres

Prevented plant acres generally reduce corn rootworm populations throughout the area: extended diapause northern corn rootworms from rotated corn acres and both species from continuous corn acres. Depending on weed management timing and success, beetles may respond to pollinating weeds and silking volunteer corn to alter risk locally.

Risk of rootworm damage

What does that mean for Minnesota with respect to corn rootworm damage in 2019 and risk in 2020? The best way to determine risk for a particular field is to scout for beetles this summer. On the other hand, to understand near and long-term changes in western and northern corn rootworm populations, resistance to Bt traits and rotation, and the risk of economic damage, requires information from a large number of fields over the geography and cropping systems of Minnesota. It also requires a knowledge of cropping history for the fields from which these data are collected.

With the help of funding from the Minnesota Corn Research and Promotion Council, the University of Minnesota will be coordinating a network for corn rootworm yellow sticky trap data this year.

We can supply a relatively pain-free sampling protocol, data forms/software and yellow sticky traps. Field specific information will be used for statistical purposes but will not be made public.

If your farm or company would be interested in participating and sharing your data, complete the attached interest form and send an scan or photo to rootworm@umn.edu. If you have questions, please contact either of us:

Bruce Potter at bpotter@umn.edu or (507) 276-1184
Ken Ostlie at ostli001@umn.edu or (612) 750-0993.