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6/22/2009 - 6/28/2009

Weeds are Winning the 2009 Battle, Plan Now for Next Year

By Mike Owen, Department of Agronomy

I spent an enjoyable Wednesday driving around central, southeast and east central Iowa looking at some on-farm demonstrations we have established.  It was wet and muddy to be sure and there is no question that herbicide applications have been delayed by the weather. 

The consistent observation I made was that the soybean crop is going to be less than it could have been if growers had practiced weed management earlier in the season.  We still may have a good “weed killing” opportunity, but untold bushels of soybeans have been lost due to weed competition.  

Most of this yield loss could have been avoided if an early preplant herbicide application had been used in early April. The pictures below illustrate the problem and the solution.

clean field

Note the weedy foreground of the picture and the clean field where a soil-applied residual herbicide had been applied. 


weedy field

This weedy field may be controlled eventually, but to what end? The 2009 soybean yield potential has already been dramatically reduced regardless of how dead those weed eventually become.


My advice is to look closely at the fields now, determine which weeds are most problematic in the fields and devise a management plan to be initiated early next growing season. The 2010 yield you protect will more than pay for the cost of the residual herbicide treatment.


Note: This article is a blog entry on the CropWatch Blog; this and the entries of Iowa State University Extension field agronomists discuss Iowa weed issues.



Micheal Owen is a professor of agronomy and weed science extension specialist with responsibilities in weed management and herbicide use. Owen can be reached by email at or by phone at (515) 294-5936.

Soybean Aphids Are Found in Young Soybean

Erin Hodgson, Department of Entomology

Throughout the North Central Region, 2009 reports indicate soybean aphids are present in young soybean (VE-V2). This includes Iowa, other states (Wisconsin, South Dakota, Michigan, Ohio, Minnesota) and southern Canada. Northeastern Iowa Extension Field Agronomist Brian Lang closely monitors early season establishment of soybean aphid. He reported 70 percent of plants in his research plots were infested (5.5 aphids per plant) on June 21.

Other people have recently reported finding aphids in central and northwestern Iowa. But what does it mean to have soybean aphids so early in the season? Right now, it is too early to make any predictions about widespread aphid outbreaks because there are several scenarios that could take place this summer.

Natural enemies
The first hypothetical option is that natural enemies can help regulate early season numbers and fields will not need to be treated (see circles in Figure 1). Beneficial insects, such as ladybeetles, lacewings, predatory bugs and flies, and parasitic wasps are excellent at finding aphids at low densities. Of course natural enemies are susceptible to broad spectrum insecticides and will be wiped out if the aphids are sprayed, so suppress the urge to make a "just in case" application at this time. Iowa also has naturally-occurring fungi that can quickly wipe out an aphid population. Although it is certainly humid enough for aphid fungi, the evenings are not cool enough to induce sporulation at this time.

figure 1

Figure 1. Soybean aphid populations can show up on young soybean but fade out before reaching the economic threshold.


Fly away
The second possibility is that fields get heavily infested early and young plants get crowded, forcing colonies to produce alates, or winged adults. These winged aphids are capable of short and long distance flight, and is historically how much of Iowa soybean is infested each year. When scouting early in the season, look for winged aphids that might be on the move. Fields that have high numbers of winged aphids may dissipate and never build up to damaging populations (see triangles in Figure 1).

Exponential growth and population increase
The third situation that might happen is that aphids start exponential growth and double populations every week. If this type of reproduction is seen, it is likely the economic threshold (250 aphids per plant) could be reached before mid-July (see Figure 2).

But daily temperatures and humidity have to be moderate for optimal reproduction, and the current weather forecast prediction for Iowa is too hot and humid for that trajectory. Fields that are sprayed before bloom may require a second insecticide application if aphids rebound or if the field is reinfested (see circles in Figure 2). It is important to keep scouting for aphids through seed set even if treated with an insecticide earlier that season.

figure 2

Figure 2. Soybean aphid populations can show up on young soybean and surpass the economic threshold, sometimes twice in one season. Red arrow indicates a properly-timed insecticide application.


Regularly sample every field
There are other possible directions soybean aphid might take this year as well. The most accurate way to predict soybean aphid growth is to SAMPLE EVERY FIELD on a regular basis. Counting aphids every 7-10 days will allow you to estimate aphid densities and gauge whether the population is moving up, going down or staying the same.

Especially early in the season, look on the undersides of leaves for first and second instars and winged adults. If you are trying to find aphids in soybean, stop at several locations per field and examine at least 20 to 50 plants per location. After aphids have established, you can reduce the number of sampled plants.

soybean sampling

Scouting early in the season may help prevent yield loss from soybean aphid.



Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at or phone (515) 294-2847.

June 22 Iowa Crop and Weather Report

By Doug Cooper, Extension Communications

Iowa State University Extension climatologist Elwynn Taylor returns to the weekly crop and weather report this week. Taylor is joined by integrated pest management specialist Rich Pope and corn agronomist Roger Elmore for this week's roundtable discussion on the 2009 growing season.

Taylor says hot humid temperatures will be around for a week or so, but July temperatures should moderate. Near ideal growing conditions are expected for the next couple of weeks.

Pope reports that weeds are still a topic of discussion at the weekly field specialists teleconference. Insects are also starting to show up in some Iowa corn and soybean fields.

Elmore talks about corn recovering from hail damage and how volunteer corn in corn fields is difficult to control with herbicides, saying cultivation is probably the best solution depending on the stage of the corn.

Degree Days - A Warm Start to Summer

By Rich Pope, Department of Plant Pathology

Iowa weather took a turn to the warmer in mid June. Although uneven corn stands are still easy to find, particularly in corn following corn, the warm days have allowed plants to green up and make up in development.

accumulated degree days in Iowa since May 1 2009

Last week I wrote that the cold-limited crops only put in a 4- to 5-day week; this week produced some overtime. The average degree-day accumulations for the week of June 14-21 netted nearly one extra normal June day of  performance.

A few early planted southwestern Iowa soybeans fields have just begun to flower, marking growth stage R1.  On that note, producers in Iowa should begin monitoring fields for soybean aphid.  The first Iowa sitings of aphids were about three weeks ago near Decorah; numerous reports of small populations have been noted in central Iowa.  None of these reports are near the treatment threshold  - 250 aphids per plant and the population increasing - but monitoring important nonetheless.



Rich Pope is a program specialist with responsibilities with Integrated Pest Management. Pope can be contacted by email at or by calling (515)294-5899.

This article was published originally on 6/29/2009 The information contained within the article may or may not be up to date depending on when you are accessing the information.

Links to this material are strongly encouraged. This article may be republished without further permission if it is published as written and includes credit to the author, Integrated Crop Management News and Iowa State University Extension. Prior permission from the author is required if this article is republished in any other manner.