The 2 biggest insect threats to corn and soybeans

Have you heard that soybean aphids shouldn’t be a problem this summer because it’s an even-numbered year? Everyone has his or her own theory on what influences a soybean aphid flush in any particular season. The reality is that perhaps there is no steadfast answer for geopgraphies. The best way to manage aphids and prevent damage to your crop is to prepare, regardless of annual predictions.

The 2 biggest insect threats to corn and soybeans

Scouting for Aphids

You or your agronomist may have started scouting for soybean aphids just a week or two ago. If not, following are recommendations on how to scout:

  1. Late-planted soybeans seem to be more susceptible to soybean aphids. Scout these fields first.
  2. Scout when your soybeans are in the R1 through R5 growth stages. These are the stages when aphid activity is the most detrimental to soybean yield. Treating aphids at R6 or later doesn’t seem to provide yield benefits.
  3. Identify 20-30 plants/field. This sample should represent 80% of your field.
    1. Early in the season, focus on the undersides of new soybean leaves.
    2. As the season advances, remember that soybean aphids move further into the soybean canopy.
  4. Count the number of aphids you find on each plant.
  5. If your findings average at least 250 aphids/plant, you are over the economic threshold and should spray.
  6. If you happen to find aphids concentrated on the soybean plant stem, it’s safe to assume the population is more than 400 aphids/plant without even counting. That’s because the stem is an inferior food source. The only reason aphids would look there for food is if the rest of the plant is overpopulated.

Economic Threshold Explained

The number of aphids per plant isn’t the only consideration when you are deciding whether to treat aphids. This is especially true if your count is just under the economic threshold.

Your state university Extension has localized resources to help you make decisions based on aphid thresholds. University of Wisconsin Extension, for example, calculates potential profit scenarios based on aphid levels and various soybean prices. The site also explains why 250 aphids/plant is the economic threshold. Once aphids reach 250/plant, it will take about seven days to make a treatment decision and coordinate application. While aphids will reproduce during those seven days, yields usually aren’t affected unless you treat after that window.

Choosing Soybean Aphid Treatment

If you find soybean aphids surpass the economic threshold, you can still protect your yield from these invasive pests. A product such as Leverage 360® insecticide protects against aphids that suck on soybean leaves. Unlike many other insecticides, Leverage 360 moves in a way that gets to the underside of the leaves, where soybean aphids often feed.  It also safeguards plants from environmental stresses and can be applied in a tankmix with a fungicide.

Corn Rootworm (CRW) Resistance Footprint Increases

As Farm Journal reported earlier this year, four states (Nebraska, Iowa, Illinois and Wisconsin) now have confirmed corn rootworm resistance to the Bt toxin Cry3Bb1. While researchers will continue to study this development, your concern right now is this year’s corn crop. Will resistance happen in your county? Is your own crop at risk?

Scouting for CRW

Diligence is the No. 1 way to protect your crop.  Whether or not you plant Bt hybrids, you should scout all of your corn fields for adult corn rootworm. You should start this now and continue to do so weekly from now until late September. Even if ears fill out, the adults can still damage the standability of your corn. This would cause some of the grain from those filled ears to be lost during harvest.

Unfortunately, CRW scouting is a little more complicated than soybean aphid scouting. Fortunately, the University of Nebraska-Lincoln provides a very easily understood explanation of two ways to scout for adult CRW.

As that document explains, the economic threshold isn’t as clear-cut for CRW infestations either; it depends on your plant population and whether the infested field is continuous corn. The thresholds vary from 0.42 beetles/plant up to 1.28 beetles/plant.

Treating CRW

Long-term CRW management should include a number of cultural and chemical practices — from crop rotation to rotating insecticide modes of action. In the short term, if you find a field this year with CRW over the economic threshold, an insecticide application can be considered.

At this stage, you will probably want to choose a foliar application of an insecticide such as Baythroid® XL insecticide. This is because during this time of year, the larvae have developed into adult beetles and you will want to control them before they have an opportunity to lay eggs.

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