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Damaging Soybean Insects Spotted in IA

July 24, 2019

Soybean fields in IA are experiencing a fair amount of insect pressure. A record number of thistle caterpillars (painted lady butterflies) have been spotted, and are feeding on the foliage of soybean plants. They are joined in the field by Japanese beetles and aphids, creating a multi-pest complex. Here are some updates and scouting tips on these insects.

Thistle Caterpillar/Painted Lady Butterfly

This agronomic image shows an adult thistle caterpillar, the painted lady butterfly

Generally, thistle caterpillar are not troublesome pests, but we continue to see more issues with this foliage feeder. While adult painted lady butterflies are pollinators, the immature caterpillars have been eating through soybean fields, threatening to significantly reduce yield. Below are some identifiers to keep in mind when scouting, and some background on this pest’s appearance in IA.

  • The lifecycle of the thistle caterpillar begins with adult butterflies migrating north in the spring.
  • Thistles are often a preferred host for egg lay, but with better weed management success making thistles harder to find, soybeans can become a target laying area.
  • The caterpillars emerge with brown/black bodies and yellow striping down each side, with spiny hairs covering the body.
  • Caterpillars feed for 2-4 weeks prior to pupating, and during this time create webbing that binds up the leaves of the plant, which can be confused for soybean webworm.
  • After they pupate, it takes another 7-10 days for the bright orange, black, and white painted lady butterfly to emerge.
  • In IA, it is believed that we are now entering the second generation of caterpillars

Japanese beetle

This illustrated image shows a Japanese beetle

Warm weather and lack of moisture are contributing stress factors to current soybean plants. Japanese beetles are a cause for concern, with clear defoliation in soybean fields. With some researchers worried that these Japanese beetles will start to head into corn fields, here are some tips on what to look for when scouting and how they ended up in your soybeans.

  • The Japanese beetle injures soybeans by feeding on foliage, identified by the skeletonization of the leaf with the veins remaining.
  • Adults begin to emerge from grassy areas in late June, and start to feed.
  • Adults are roughly ½” long, with a green metallic head, bronze back and 6 white tufts of hair on each side of the abdomen.
  • After mating, the female beetles return to grassy areas and lay eggs in the soil. Eggs typically hatch in about 2 weeks, and the small grubs feed on roots.
  • In early spring, the grubs pupate and hatch into adults.
  • Japanese beetles typically have 1 generation per year in IA.
  • Treatment for Japanese beetles is recommended if 20% soybean defoliation has occurred during the reproductive stages.

Soybean Aphids

This illustrated image shows a soybean aphid.

With soybean aphids currently making their way through IA soybean fields, they are contributing to foliar damage and potential yield loss. If you are wondering when to start insecticide applications, consider the information below.

  • Soybean aphids can reach economic threshold levels starting in late July.
  • If there are 250 aphids per plant with 80% of the plants infested, treatment is recommended through the R5 stage.
  • Cooler conditions favor reproduction, and populations can double in 2-3 days.

For fields where thistle caterpillars are the primary pest, we recommend Besiege® insecticide, which provides both rapid knockdown and long lasting residual control of lepidopteran, sucking and chewing insect pests.

When thistle caterpillars, Japanese beetles and soybean aphids are simultaneously spotted in your fields, determining threshold becomes much more difficult. For protection against all 3 insects, we recommend applying Endigo® ZC insecticide, which offers trans-stemic movement into leaves for better protection. It also serves as an excellent rotational product for a complete integrated pest management program and easily tank mixes with other products for added convenience.

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