SW MN IPM Stuff 2021 Issue 10



Just a quick update this time.

Crop weather

Rainfall, air and soil temperatures, degree-days, soil moistures, and other current and historical weather data for the University of Minnesota Southwest Research and Outreach Center (SWROC), a little spot about two miles west of Lamberton, MN, can be found at SWROC Weather


Corn rootworm beetles are abundant now.  I am hearing rumors of some unusual and perhaps irresponsible guidelines for treating rootworm beetles in corn.

The most useful reason to scout beetles is to determine risk from root damage in the subsequent corn crop.

Very heavy rootworm populations (and we have some this year) can clip silks and interfere with pollination. If corn is less than 50% pollinated and silks are being clipped to less than ½ inch, beetle control may pay. This amount of feeding often takes 10 beetles or more /plant. After pollination there is no benefit to the current year's crop. You can check the pollination of a corn ear by unwrapping the husk and gently shaking the ear. The silks will fall from the ovules that are already pollinated.

“Beetle bombing” to reduce egg-laying is sometimes used for fields that cannot be rotated for one reason or another. Insecticide should be applied when females gravid with eggs are observed. This is often later than foliar fungicides would typically be applied. Continued scouting is needed and one, or perhaps two, additional applications might be needed. I have not yet seen any gravid females and beetle emergence is not yet complete.

Do not base any decisions on the edge of the field. You need to scout off the headlands well into the field. Field edges concentrate beetles moving in or out of the field.

Beetles are most abundant on or near the ear and tassel. Early and late silking plants attract beetles and can give an impression of more beetles than are present on a per/plant basis. Look at average rootworm populations/plant not just the most heavily infested. 

Rootworms are highly adaptable. Heavy use of insecticides can encourage resistance development and further complicate management. Some of our most effective at-plant rootworm insecticides are organophosphates or pyrethroids.

A little restraint when managing rootworms is helpful. Overuse of Bt and refusal to rotate fields out of corn for a year has got us into big trouble with western corn rootworms. We are headed down the same path with northern corn rootworms. Let’s not add insecticide resistance to the mix. 

For those of you in dry areas - Pyrethroid insecticides can flare spider mites. Both pyrethroids and organophosphates can flare aphids and some two spotted spider mite populations.

Ken Ostlie and I would greatly appreciate hearing of any fields where predominately northern corn rootworm populations are damaging corn with Bt-RW traits.

For those who are trapping RW beetles for your company, or on your own farm, we would like to include your trap data if you are willing to share. Contact me for details.


I received my 1st report of two-spotted spider mites in southern MN.

There will be a webinar discussing the growth regulator - like herbicide injury symptoms prevalent this year on Friday, July 16 at 7:30 AM.  For details, see Field Notes: Dicamba vs. 2, 4 -D in 2021 - A tale of cupped leaves.


Potato leafhoppers are at threshold in some fields.

Meetings and other topics of interest

Happy trails,