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Millipedes May Be Responsible for Reduced Soybean Stands in 2020

When we think of millipedes, it is usually as a nuisance pest indoors. However, there have been reports in South Dakota and neighboring states of millipedes being more than just a nuisance in 2020. In fact, there are cases of millipedes causing significant injury to soybean and other crops in no-till fields. Although millipedes are saprophagous, which means they feed on decaying organic matter, they will occasionally feed on living plants.

So why are millipedes an issue in 2020? To answer that, we must think back to the field conditions in much of eastern South Dakota this year. We received large amounts of rainfall, which left many fields under water or very soggy. If we add the extra residue present in no-till fields to the equation, we now have optimal conditions for millipedes. Millipedes require cool, moist environments to prevent their bodies from drying out. No-till fields provide a stable food source of decaying organic matter and the residue slows down the soil drying out because there isn’t direct sunlight on it. With optimal environmental conditions present, it is likely that millipede populations increased in some fields and they began feeding on emerging soybean.

In some cases, millipedes may not be the direct cause of stand reduction. Millipedes will often feed on decaying seeds or on plants that were compromised by disease or soil conditions. In South Dakota, soybean that were planted early in 2020 may have had some of these issues due to slow emergence.

Identification

An adult, brown-to-black millipede feeding on a green leaf.
Figure 1. Adult millipede. Courtesy: Joseph Berger, Bugwood.org.

Millipedes are arthropods but not insects. This means they have some characteristics that are like insects but also have some that are very different.

Millipedes have an exoskeleton and a segmented body (Figure 1). They differ from insects in that they have two pairs of legs per body segment, and the total number of legs present varies from species to species.

Scouting and Management

Monitor fields with high residue early in the season for signs of millipede feeding. Feeding will normally occur at or below the surface of the residue on the soil. Millipedes will begin feeding on the softest parts of the plant first, which may include the stem and in some cases the cotyledons.

Because millipedes are not normally a pest of crops, there hasn’t been a lot of research done to evaluate the efficacy of insecticides on them. However, some research has shown that insecticide seed treatments do not reduce millipede feeding or their populations. Foliar insecticides may reduce millipede populations but must come into contact with the millipedes to be effective.

In no-till fields, the best management option is to plant in soil temperatures that will promote rapid seedling growth. This is especially true when planting into high residue fields. In some cases, millipede pressure may be increased near groves or shelterbelts. These areas of the field may be slower to warm up in the spring, which will provide a longer window of potential feeding for the millipedes.