Managing Insects in Soybean Fields

July 22, 2020

In mid-July, there’s generally an increase in the insect population and it’s time to deal with the pests that ravage your plants’ leaves. With the dry conditions we’ve been experiencing since the beginning of the season, soybean plants that have been suffering through the heat are more susceptible to disease and insects. Plus, insect infestation levels vary each year; they’re unpredictable. 

The most common insects

The bean leaf beetle, flea beetle, Japanese beetle, painted lady, soybean aphid and two-spotted spider mite cause the most damage to soybean crops in North America. 

Estimating defoliation

Do you think you might have defoliating insects in your soybean field? Because it can be hard to see the insects, you can determine if you need to intervene based on your crop’s percentage of defoliation and stage of growth. Start by taking a sample of 10 separate areas of the field. Take a sample of trifoliate leaves that have completely unfolded from the middle of 5 different plants in each area. Discard the most and least damaged leaflets of all trifoliate leaves collected. Use this visual to determine the percentage of defoliation. 

Source: Malin Rice, University of Iowa

In general, here are the established thresholds:

  • Vegetative stage: 30%
  • Flowering and pod fill: 15%
  • Pod fill at maturity: 25%

The procedure is different for the soybean aphid. To screen for this pest, take a random sample of at least 20 soybean plants from all over your field. Then count all the aphids on each part of the plant and calculate the average. Note the soybean’s phenological stage and amount of natural enemies like predators, parasitoids and entomopathogens. We also recommend screening twice in the same field when the insect population reaches an alert threshold, which is more than 250 aphids per plant. 

What can I do?

Integrated, or organic, pest management is an excellent and completely natural way to fight insect infestations. Predators like ladybugs, Neuroptera larvae, maggots and pirate bugs subsist largely on aphids and are considered a producer’s allies. Parasitoids and fungal entomopathogens also help curtail infestations. As a last resort, you can use a chemical insecticide when the alert threshold has been reached and beneficial insects are not controlling pests that could hurt production. 

Vigilance is key at this point in the season. Have questions? Contact your Prograin representative!