Skip Navigation

7/25/2011 - 7/31/2011

Insecticide and Insecticide-Fungicide Tank Mix Applications in Soybean

Rebekah Ritson, Matt O’Neal, Department of Entomology; Alison Robertson, Daren Mueller and Nate Bestor, Department of Plant Pathology and Microbiology

Trials were conducted at three Iowa locations (Sutherland, Ames and Nashua) over three years (2008 to 2010) to determine the effects of applications of insecticides (Asana ®, Leverage 2.7SE™) and fungicides (Stratego® YLD) applied alone or combined (i.e. a tank mix) at soybean growth stages R1 (beginning flowering) and R3 (beginning pod set) on soybean aphid populations and soybean yield in Iowa. Because these pesticides were applied based on plant growth stage, regardless of the level of fungal disease or insect pressure, these treatments are referred to as prophylactic treatments. We compared the prophylactic approaches to an integrated pest management (IPM) approach, in which an insecticide (Asana) was applied when soybean aphids reached an economic threshold of 250 aphids per plant.

What the research shows

  • All insecticide applications reduced aphid populations (Table 1).
  • Using the IPM guidelines we applied insecticide only 57 percent of the time. During 2010 we did not apply insecticide as aphid populations did not reach 250 aphids per plant.
  • During this study, applying insecticides at R3 or according to IPM guidelines reduced aphid populations more effectively than application at R1.
  • Fungicides did not effect soybean aphid populations.
  • Use of an insecticide-fungicide tank mix resulted in higher yields than use of an insecticide alone, but these differences were not statistically significant.

Table 1. Comparison of peak aphid populations and yield for soybeans grown with fungicides and insecticides, applied alone or in combination, for nine field trials in Iowa from 2008 to 2010

Economic considerations

We conducted a simplified economic analysis based on a break-even yield gain analysis to determine the economic viability of each of the management plans used in the field trials. Costs of pesticides, costs of application and scouting services, expected crop price, and expected yield were used to calculate a gain threshold (GT). A gain threshold is the increase in number of bushels of yield required to cover the costs of the applications. We used prices that were typical for 2010 to estimate the cost of scouting and application service cost.

Application of an insecticide based on IPM guidelines resulted in approximately 80 percent probability (on average) of a yield increase great enough to surpass the gain threshold. In other words, 80 percent of the time, the use of an insecticide resulted yields being high enough to pay for the cost of the insecticide and its application. Due to differences in overall costs (i.e. increase cost for scouting) and yields, prophylactic application of an insecticide or an insecticide-fungicide tank mix at soybean growth stage R3 provided slightly higher average probability (between 86-89 percent) of recouping treatment costs. However, due to risks associated with prophylactic pesticide application, such as yield loss due to ground application (Hanna et al. 2007) and development of resistance to pesticides (Bradley 2010), we recommend that scouting still be used to ensure a pest problem is present before applying either insecticides or fungicides.

Table 2. Probability ranges and mean probabilities of breaking even with ground applied fungicides, insecticides and tank mixes for soybeans ($12 per bushel)


Bradley, C.A. 2010. Frogeye leaf spot pathogen with reduced sensitivity to fungicides found in Tennessee soybean field. University of Illinois Extension Bulletin. 24:172.

Hanna, S., S. Conley, and J. Santini. 2007. Managing fungicide applications in soybean. Purdue Extension: Soybean Productions Systems. SPS-103-W.


Rebekah Ritson is a graduate research assistant in the Department of Entomology. She can be reached at or 515-294-1999. Alison Robertson is an associate professor in the Department of Plant Pathology and Microbiology with extension and research responsibilities; contact at or phone 515-294-6708. Daren Mueller is an extension specialist with responsibilities in the Corn and Soybean Initiative and ISU's IPM program. Mueller can be reached at 515- 460-8000 or by email at Matt O'Neal is an associate  professor in the Department of Entomology with teaching and research responsibilities. He can be reached at or at 515-294-8622. Nathan Bestor is a graduate assistant in the Department of Plant Pathology and Microbiology. He can be reached at 515-294-1741 or

Don’t Stop Now!

By Alison Robertson, Department of Plant Pathology and Microbiology

Now is not the time to stop scouting. It may not be pleasant out there (think pollen and hot), but the weather the past couple of weeks has been favorable for gray leaf spot development. Gray leaf spot development is favored by mean daily temperatures between 72 F and 85 F, and high humidity (higher than 90%). I have had several reports this past week of fields in which the gray leaf spot has developed up to the ear leaf. Northern leaf blight development has also been progressing in several fields in central Iowa. 

I have been scouting my field plots the past couple of days and also have noticed that gray leaf spot and northern leaf blight have developed rapidly in the past two weeks. One thing I noted was disease severity was hybrid specific. I have several hybrids in my plots, but only one or two may be at threshold for a fungicide application (disease present on the third leaf below the ear leaf or higher). The affected hybrids always are rated more susceptible to disease.

Can a fungicide application be made after brown silk?

Yes. Most of the fungicides used on corn (Headline®, Headline AMP®, Stratego YLD® and Quilt Xcel®) have a pre-harvest interval of 7 days (Headline®) or 30 days (other products), which means in theory, a product could be applied up to R5 (dent). We have some data from 2007 through 2009 for foliar applications after R2 (blister; around brown silk), which is summarized in Table 1. Although the yield response with an R3/R4 application of fungicide was not as high as the other timings, there was low disease pressure in these trials. In general, yield responses to a fungicide application are greater when disease is present in the field.

Table 1.  Effect of application timing on the mean yield response of corn to a fungicide in Iowa


If disease occurs after brown silk, would a fungicide protect yield?

Good question. There are few data I am aware of. Consider that:

  • The reproductive growth period in corn (VT to R6) typically lasts approximately 64-65 days.
  • Dry matter accumulation starts at R2 and rapidly increases through approximately R5.75 (3/4 milk line).
  • And 55 percent of dry matter accumulating after R5 (Abendroth et al, 2011).

If the disease threshold is met at R3 or R4, could a fungicide application protect yield, bearing in mind dry matter accumulation is 20 to 30 percent complete?

Harkin and Arkridge (2009) evaluated one (R4), two (R4 and R3), three (R2, R3 and R4) and four (VT, R2, R3 and R4) applications of Headline® on two hybrids in double crop corn in Alabama. On one hybrid, northern leaf blight was predominant, while on the second hybrid southern rust was predominant. A single application of Headline at R4 significantly reduced rust severity and, although yield was higher (95.8 bu/acre), it was not significantly different at the 5 percent level from the unsprayed check (88.7 bu/acre). Two, three and four applications reduced rust severity and resulted in higher yields (104.9-119.1 bu/acre).  Similarly, with northern leaf blight, a single application of Headline at R4 reduced disease but multiple applications were more effective. Yields did not differ between treatments.

These data suggest that a fungicide application after R2 will slow disease development, although this may not always result in an increase in yield. However, reducing leaf disease could reduce stalk rot severity and therefore contribute to standability.

Abendroth, L.J., Elmore, R.W., Boyer, M. J. and Marlay, S.K.  2011.  Corn Growth and Development.  Iowa State University Extension, PMR 1009.

Hagan, A.K. and Arkridge, J.R.  2009. Headline programs compared for rust and northern corn leaf blight control on double crop corn, 2009.  Plant Disease Management Reports 5:FC009 



Alison Robertson is an associate professor in the Department of Plant Pathology and Microbiology with extension and research responsibilities; contact at or phone 515-294-6708.

Long Silks?

By Roger Elmore, Department of Agronomy

Very hot days and warm night temperatures the last couple of weeks concern agronomists and corn growers. Unfortunately, this period of hot weather occurred during tasseling and silking. However, the USDA-NASS July 25th Iowa Crops & Weather report indicates that 80 percent of the crop was still in good to excellent condition. Ninety percent of the crop had tasseled and 75 percent silked – both slightly behind last year but ahead of the five-year average. 

I’ve heard several accounts -- both in Iowa and other states -- of longer than normal silks this year. I hope these accounts are rare! Silks elongate an inch or more per day until they intercept pollen and the ovules are fertilized. Six inches of silk extending from ears -- like I’ve seen in photos from central Iowa -- could indicate four to six days of growth without pollination occurring. Silks remain viable for up to 10 days and turn brown and separate from ovules when ovules are fertilized.

Pollen shed and silking 

In older hybrids, pollen shed usually preceded silking. Since stress affects silking more than pollen shed, high temperatures, especially when coupled with moisture stress, resulted in barren ears. Pollen shed and silking usually happen simultaneously with modern hybrids and in many cases, silks may appear before pollen shed. This is one of the mechanisms that resulted in greater stress tolerance with modern hybrids. Silks develop first from near the butt of the ear and then proceed progressively to the tip.

Pollen shed occurs first from anthers that protrude from near the tip of the main tassel stem. Subsequently, shed moves progressively down the main tassel and from the tips of tassel branches toward the main tassel stem. The last anthers to shed pollen are those on the lowest tassel branches near the main tassel stem. Incidentally, scientists record the time difference between pollen shed and silking as a measure of stress among hybrids and/or experimental treatments. We call this the anthesis-silking interval (ASI).

What do long silks suggest? 

Silks stop growing and turn brown when ovules they attach to are fertilized. If all anthers on all plants have shed pollen and silks are still yellow-green and growing, kernels on the ear remain unfertilized. Harvestable kernel numbers will be reduced unless there is another source of pollen nearby. Yield potential will be compromised.

Husk gently and shake ears

Kernel set should be “easy” to determine after completion of pollen shed. As mentioned earlier, browning of silks indicates successful ovule fertilization. If yellow-green silks are obvious, gently remove husks to expose silks and kernels. Hold the ear horizontal and shake or roll it carefully. Silks will detach from fertilized ovules. Silks remaining attached to ovules indicate that those ovules were not fertilized and thus will not produce kernels. Tip kernels often are not fertilized.

For more information on the silking process see:

figure 1 corn with long silk
Figure 1: Four-inch silks on ear prior to pollen shed, July 15, 2011. The reddish color likely is a hybrid trait. Photo by R.W. Elmore.

figure 2 husked corn ear with few silks
Figure 2: Husked ear showing a few silks still attached to unfertilized ovules at tip of ear, July 29, 2011. Photo by R.W. Elmore.


figure 3 husked corn ear with several silks
Figure 3. Husked ear showing several silks still attached to unfertilized ovules in upper third of ear, July 29, 2011. Photo by R.W. Elmore.

figure 4 husked corn ear showing many silks
Figure 4. Husked ear showing many silks still attached to unfertilized ovules along the entire length of ear, July 29, 2011. Photo by R.W. Elmore.


Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production. He can be contacted by email at or 515-294-6655.

Drought Stress Favors Grasshoppers and Spider Mites

By Erin Hodgson, Department of Entomology

Although some parts of Iowa have had adequate moisture, other areas could use some rain. Drought stress combined with high temperatures is good news for field crop pests like grasshoppers and spider mites. If your area is hot and dry, consider scouting fields now and throughout August.

The two-spotted spider mite thrives in hot, dry weather. Infestations typically start at field margins and progressively move to the center, if weather conditions remain favorable. Use a hand lens or shake plants on paper to see if mites are present.

figure 1 spider mites
Figure 1. Spider mites are small, so look for plants that are dirty or are covered in webbing. Photo by David Cappaert. 

Spider mite injury to soybean can resemble herbicide injury or a foliar disease; however, characteristic signs are tiny yellow spots, or stipples, on leaves. As the injury becomes more severe, leaves turn yellow, then brown, and finally die and drop off. Spider mite injury can reduce soybean yields by 40 to 60 percent, and cause pod shattering, wrinkled seed, and early maturity.

figure 2 spider mite damage
Figure 2. Heavy spider mite populations will cause a yellow speckling on leaves. Photo by Whitney Cranshaw. 

In corn, prolonged spider mite feeding will turn leaves yellow with stippling on the upper surface. Heavy infestations can cause premature drying, which results in the loss of leaf tissue, stalk breakage, and kernel shrinkage.

Under dry conditions, foliar treatments are recommended when plants have substantial stippling or leaf-yellowing and spider mites are active. Although spider mites are not insects, they are often treated with insecticides. Pyrethroid chemicals are not very effective at reducing outbreaks, so consider using organophosphates. Using pyrethroids to control other pests (e.g., soybean aphid) may actually flare spider mites in the field.

Warm and dry weather also favors grasshopper growth and development. When late summer grasshopper damage occurs, it usually is related to drought conditions and is frequently, but not always, restricted to field edges. There are two common grasshoppers in soybean in Iowa, the differential grasshopper and the redlegged grasshopper.

Young grasshopper nymphs eat irregular-shaped holes in tender leaf tissue and may consume the entire seedling. Older nymphs and adults can consume all of the leaf except the tougher veins. Grasshoppers chew through green soybean pods (which bean leaf beetles will not do) and destroy the seeds within. They also can feed on developing corn ears and destroy kernels.

figure 3 grasshoppers on soybean 
Figure 3. Grasshoppers can chew through the soybean pod and feed on the developing seeds. Photo by Marlin E. Rice.

Reducing grasses and other weeds within and around fields will discourage adults from feeding and mating in that area. The economic thresholds are based on leaf area consumed or percent defoliation. In soybean, a foliar treatment may be justified if defoliation exceeds 40 percent before R1 (full bloom) or 20 percent after R1. Consider an insecticide application in corn if grasshoppers are clipping silks or ear tips, or are removing foliage above the ear leaf. Border treatments are recommended if infestations are restricted to field edges.

figure 4 grasshoppers on corn
Figure 4. Grasshoppers can clip silks and damage corn ears during hot and dry conditions. Photo by Marlin E. Rice.

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.

This article was published originally on 8/1/2011 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.