SW MN IPM Stuff 2019 Issue 4B

6/8/19

Soybean aphid watch 2019

When the berries on female buckthorn start to swell, we usually find the first soybean aphids (SBA) on soybean. Both of those events happened at the SWROC earlier this week.

Each spring, we spend some time checking buckthorn and early emerging soybean for SBA. Our efforts finding spring SBA populations on buckthorn consistently fail. On the other hand, as they leave buckthorn, we can usually find winged females and newly deposited nymphs on the largest soybeans, including volunteer soybeans plants.

On June 7th, a sharp-eyed intern found new SBA nymphs on a volunteer soybean plant. In spite of not being able to find any aphids on SWROC buckthorn last fall, it seems that at least a few aphids were there, reproduced and survived the winter.

In many fields, volunteer soybeans are numerous this spring. Depending on herbicide tolerance of the previous year’s soybean crop and corn herbicide selection, these volunteers can persist in corn. MN Soybean Checkoff funded research has shown that, when colonized, volunteer plants in corn can produce significant numbers of SBA to move to soybean fields later.

As you scout corn and soybeans for stand and weed management evaluations, you might want to look for SBA on a few plants. I would appreciate hearing when you start finding SBA. It will help begin the process of assessing SBA risk for 2019.

Weeds and soybeans

Limited time and other pest management issues means I don’t keep up on herbicides as well as I probably should. On the other hand, I do keep up with the seasonal development of crops and weeds. This spring, the late planting of soybeans in many Minnesota fields means that although soybean emergence will be rapid, it will coincide with the emergence of waterhemp... advantage waterhemp. Good crop management means starting at the start. Weed management does not need to be complicated.

If you are working with a Roundup Ready2 Xtend soybean field, there is no reason to wait for larger soybeans and emerged broadleaf weeds. I doubt there will be any changes to the June 20th application cutoff for Minnesota. Regardless of the June 20 application cutoff, consider applying a labeled dicamba product early, rather than later. This includes pre-emerge. There will be pre-emerge activity and labeled tank mix partners can extend the residual activity on waterhemp. Years ago, dicamba plus a residual herbicide used to be a commonly used and effective pre-emerge herbicide for corn.

There are other reasons to apply earlier rather than later. Volatility of dicamba is lower when applied to bare soils compared to vegetation. Volatility increases as temperature increases. Fewer soybeans will be emerged with earlier applications. Finally, flowering soybeans are most sensitive to dicamba. Late-planted soybeans will flower when small, perhaps as earlier as V3.

Dicamba applications continue to be under regulatory scrutiny. Read, understand and follow the label.

Weed management using other herbicide tolerant soybeans (e.g. glufosinate, 2.4-D tolerant) have their own nuances, but “earlier is better than later” is a general rule.

For more information on the weed management subtleties associated with late-planted soybeans, read the recent MN Crop News article on weed control in late-seeded soybeans by Jared Goplen and Liz Stahl.

Clean soybean fields will make it easier to find soybeans and aphids later this summer!

Happy trails,

Bruce Potter