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Weeds, Late Season Aphids, Wheel Traffic

University of Minnesota Extension, Stearns County News
August 19, 2015         
Source:  Dan Martens, Extension Educator
University of Minnesota Extension
Stearns, Benton & Morrison Counties

Weeds, Late Season Aphids, Wheel Traffic
By Dan Martens, University of Minnesota Extension

FOLEY, Minn. (08/19/2015) —Take note of weeds showing up in fields now. This picture shows a tall waterhemp plant, now reaching a foot or so above the crop canopy. I’m holding the white board high enough to capture the weed above the soybeans, so the 30 inch mark doesn’t mean much. These beans were about waist high. One plant this year will produce enough seed to be a significant patch next year that will be spread wider with tillage work. Patches soon become wide spread areas of fields – if not dealt with.

Some people are pulling isolated weeds that have potential to become bigger problems in fields. If seed is forming now, they might even carry the weeds off the field, maybe burning them on a brush pile - if that can be done safely. We all have a limit to the amount of time we can or care to spend pulling weeds. We can do some work to actually identify what the weeds are and where they are. This is real information we discuss at our farm stores or with agronomy advisors in making plans for 2016. 

Waterhemp is part of the amaranth or pigweed family. It usually has a leaf that appears somewhat longer and narrower than red-root pigweed. The seed flower is different too. We have about 5 variations of pigweed in Minnesota. They can cross pollinate to create a wider variety of combinations. Waterhemp develops resistance to glyphosate more quickly than some weeds because it has male and female flowers on different plants. This offers greater genetic capacity for multiplying resistance.

Other weeds that might be on a top 10 list to watch include giant ragweed, common ragweed, lambsquarter, velvetleaf, yellow nutsedge, nightshade, and kochia. Kochia is more common to the west and northwest in Minnesota. Some of these have more potential to germinate, emerge and produce seed later in the growing season.

LATE SEASON SOYBEAN APHIDS. Extension entomologist Ian McRae offers the following: Our recommendation is to treat if the field is at the treatment threshold of an average of 250 aphids/plant with >80% of the plants having aphids until the field is at R6 – pods fully packed with green seed.  It may necessary to treat fields to avoid yield loss in the late R5 and early R6 stages. The thresholds for R6 are not known. Indications are that much higher populations are required to cause economic loss.  There will be a lot of white cast skins on the plants which you do not count. Do count small light colored live aphids (“white dwarfs") - that can produce more typical larger, green aphids in late R5. 

Considering Cumulative Aphid Days might help in making late season treatment decisions.  Before R6, it takes over 5000 Aphid Days for economic yield loss.  If there’s an average of 500 aphids on a plant over the last 10 days of R5, for example, that field will hit the economic loss level. Late season control with marginal thresholds, might be useful where the crop is stressed for other reasons, such as dry weather stress. 

If the field is below threshold, continue to keep an eye on it. Hold off treating until the threshold is reached. You never know - heavy rains with wind, fungal disease, predation and parasitism can all knock aphid populations back.   We may also begin to see aphids moving back to Buckthorn in the next couple of weeks as beans mature and if weather patterns change. We've also seen late season population increases, sometimes quite significantly in late planted or fields affected by iron chlorosis.

WHEEL TRAFFIC DAMAGE. A Purdue article says soybeans do not compensate for wheel damage made from the R3 early pod development stage and later. At this stage, soybeans always had yield loss in 15 inch rows or less… and about half the time in 30 inch rows.

Obviously wheel traffic loss was less with wider sprayers. Additional passes in the same tracks did not seem to increase loss. Yield losses with a 30 foot spray boom ranged from 3.2 to 6.7%; at 60 feet from 1.6 to 3.4%; at 90 feet from 1.2 to 2.6%; and at 120 feet from 0.8 to 1.7%. Of course, a number of variables affect the results. Logically as the beans develop and the canopy fills in, wheel traffic differences are quite similar between 7, 15 and 30 inch rows. One conclusion is to base row width decisions on other factors; not based on potential wheel traffic loss. Farmers might prefer aerial applications if pest control is needed later in the soybean season.

Bee Aware of bee colonies and other neighborhood concerns; and make safety a priority.


IMAGE:  Waterhemp plant in soybean field


Daniel Martens
Extension Educator, Ag Production Systems
(320) 968-5077
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