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  • Writer's pictureRachel

Soybean Insecticide Investigation: Should I Spray?

Last week we covered foliar fungicide management in soybeans. We discussed when to apply, thresholds and yield response. However, in most studies, fungicide and insecticide were applied together and any yield benefit we saw was attributed to both fungicide and insecticide. So we thought this would be a good time to cover the other side of the yield response, and talk about using insecticides in soybeans.

Environmental Impact

The amount of insects we see in the field each year is very dependent on environmental conditions. We see more insect pressures in years with adequate or above average rainfall. We also see increase in pest populations after mild winters. Temperatures during the growing season affect populations to some extent, but not as much as early in the growing season. We are coming off a below average temperature winter, followed by above average summer temperatures. This temperature fluctuation can have an effect on overall pest pressures. Additionally, the below average precipitation we have experienced can also reduce pest pressures. Despite these facts, we still are seeing an increase in pest pressures over the last couple weeks.

How we make recommendations

As you are out in the field you will see a variety of insects. It is likely in our local area you will see some bean leaf beetles, Japanese beetles, stink bugs, grasshoppers, cloverworms, and all sorts of spiders. Because we see such a variety of insects, we cannot make recommendations based on numbers of insects present. Rather, we make decisions based on percent defoliation of leaves. Percent defoliation needed to trigger application depends on the size of the canopy, the stage of soybean and to a lesser extent, and the stage of insects present.

Canopy Size Matters

Soybean plants can sustain a surprising amount of defoliation before affecting yield. Some studies show that soybean plants can sustain up to 70% defoliation without affecting yield. For this statement to be true, the crop canopy must still intercept at least 90% of incident light. However, if the canopy is smaller, levels of defoliation as low as 20% can cause a yield reduction. Canopy cover can be assessed as area intercepting available light, and the ability of the crop to put on additional growth to compensate for lost leaf area. Larger canopies are better able to tolerate higher levels of insect damage. Smaller canopies or younger stands have a much lower threshold for percent defoliation.

Soybean Growth Stage

The growth stage of the soybean plant also matters when determining whether or not to trigger an application of insecticide. Plants in the vegetative state are much more able to sustain larger amounts of defoliation. Plants at this stage are still putting on new leaves and rapidly replace the leaf area lost to defoliation in new leaves. Most resources put the defoliation threshold around 30-40% during the vegetative stages. As the plant moves into reproductive stages, the tolerable level reduces to 20-25%. Plants are less able to replace leaf area and are also devoting much more energy to setting blooms and pods. This period is much more critical for treatment, particularly as pods begin to fill. Additionally, if plants are under stress and do not have adequate water, their ability to compensate for leaf loss is reduced.

Insect Stages

The stage of insect also matters when making application decisions. Knowing the life cycle of insects is very important for each pest in your field. Some pests, such as the Japanese beetle only have one life cycle per year. Consequently, as we approach the end of July we are getting close to the maximum amount of beetles that would be present in the field causing defoliation. Bean leaf beetles have two generations per growing season. Because of this, we may see some periods with less adults feeding and then after several weeks have a new generation of adults emerging and causing damage. Finally, when we consider pests like spider mites, we have to deal with 7+ generations per year, with each generation completing a life cycle in approximately 10-14 days. As you are looking at defoliation in your field, consider what part of the lifecycle the majority of the insects are at. If it is nearing the end of their lifecycle, it may be prudent to wait to spray, or not spray, as insect pressure may be decreasing to the point no future defoliation will occur.

Determining Defoliation

It can be difficult to determine defoliation objectively. The eye tends to be drawn towards the worst looking leaves in the canopy. While some of these leaves may look very damaged, as leaves higher in the canopy lose leaf area, leaves below will increase their photosynthetic rate with the increased light interception. Additionally, insects do not feed on all parts of the canopy evenly, with some insects feeding heavier in the lower canopy than upper and vise versa. One of the best ways to determine defoliation is by the method below. First pull three trifoliates off of one plant. One at the top of the plant, one in the middle, and one at the bottom. On each of those three trifoliates, remove the worst and best looking leaf. You should now have three remaining leaves for that single plant. For each of those leaves, determine percent defoliation and then average those values. Repeat this on 9 more plants nearby. Take the average of each of the 10 plants for the percent defoliation in that area. Then complete the process again at two different spots in the field. This gives us a more accurate assessment of defoliation.

Test your knowledge

So, it’s time for a little test. What percent defoliated do you think these plants are?

Plant 1:

Plant 2:

Do you have your answers?






No cheating! Get your guesses ready!






Let’s see how close you were!

The first leaf was 20% defoliated. The second leaf was 50% defoliated. I don’t know about you, but it is easy to assume a higher percent defoliation than is actually true for the leaf. Take along this handy infographic from the University of Nebraska for assessing percent defoliation as you are out in the fields.

Timing and Yield Return

As I mentioned in our last post, when applying fungicide and insecticide together, we can’t always match timing thresholds. Particularly in this year, it is more likely that we would need to apply an insecticide before a fungicide. Keep your eye on both but be aware that one will likely reach the treatment threshold before the other. On average, over the last three years, we saw an average of 5.9 bushels per acre increase by using a combo of fungicide and insecticide at R3. Similar DuPont Pioneer studies have shown a 5.3 bushel per acre increase by using a fungicide/insecticide combo.

Repercussions of using Insecticide

The last issue to consider is spider mites. Spider mites are often suppressed by insects. However, after applying an insecticide, the populations of predatory insects are killed off, leaving no check for the spider mite populations. Consequently, spider mite populations could surge after an insecticide application. Spider mites prefer hot and dry conditions. It is in that environment that they reproduce most rapidly. If you think you have the potential for spider mites, are seeing low levels currently in your field, or will have conditions suitable for their rapid reproduction, you may want to consider an insecticide that also includes a miticide or adding in a miticide to the spray mix. This should dampen populations long enough that the beneficial insects will rebound enough to keep spider mite populations low.

If you have questions on possible fungicide and insecticide combos, contact a member of the Pederson Seed Team for options.


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