Agronomy Blog

Painted Lady Butterfly and Thistle Caterpillar

by Leonard Luebker | Aug 15, 2017

Many of you are now seeing a swarm of butterflies on the road.  See the photo’s​.
These are Painted Lady Butterflies.  The larval stage is known as Thistle Caterpillar.
Leonard-20170810_122935 2 citedLeonard-20170810_124334 1 citedThistle Caterpillar cted
This insect overwinters in the southern part of the United States and Mexico.  The butterflies fly into the central and northern corn belt in the spring to lay eggs.  Their targets include 100 different species of plants, but most notably the Canadian Thistle and Hollyhock.  Soybeans too can be a target of this insect.   When landing on the Canadian Thistle, they would be considered a beneficial insect consuming some of the upper leaf area of the plant.

Life cycle

  • Egg stage:  3-5 days to hatching.
  • Larva stage:  5-10 days.The eggs hatch, and the larva spins a web on the leaves they are hatched from.The larva feeds inside the web on the leaf, consuming the leaf area.The caterpillars will shed their skin 4 times before forming a Chrysalis.
  • Chrysalis stage:  7-10 days. The larva forms a chrysalis inside the webbing, changes to a complete liquid, then reforms itself into a butterfly.Within a couple of hours from emerging the chrysalis, the butterfly is ready to fly.
  • Butterfly stage:  The butterfly lives around 2 weeks to spread eggs, then restarts the life cycle over again.

With the onset of the large swarms of this butterfly observed this time of year, and their life cycle, it is apparent the butterfly will be depositing eggs in the soybean fields.  In the next few weeks, be watching for webbing in the top leaves of the soybeans.  The economic threshold suggested by many is around 20-25% leaf defoliation.  Some research suggests that approximately 4-8 caterpillars per foot of row is an economic threshold.  Since we are in the reproductive stages of the soybean plant, pod development and seed development may be affected.   In many years, spraying is not merited because the economic threshold is not being reached.  However, with the onset of such high numbers of the butterflies this year, one should watch their fields if severe infestations occur.

Typically, one may see more leaf webbing around the edge of a field, with less activity in the middle.  With this in mind, if spraying seems worthwhile economically, one may spray the edges of a field, and/or selected areas that are severely infested.  The insect does not overwinter in the corn belt, so treatment would be for preventing yield loss this year.

Soybean Defoliation Chart:
Soybean percent-defoliation-North Carolins Soybean Association cited














References and additional information:

  1. http://bulletin.ipm.illinois.edu/pastpest/articles/200321g.html
  2. https://www.extension.umn.edu/agriculture/soybean/pest/thistle-caterpillar/docs/thistle-caterpillar-in-minnesota.pdf
  3. http://entomology.k-state.edu/extension/insect-information/crop-pests/sunflowers/painted-lady-caterpillar.html
  4. http://iowabuglife.blogspot.com/2012/04/are-you-seeing-painted-lady-butterflies.html
  5. https://www.realagriculture.com/2017/07/soybean-school-painted-lady-caterpillar-dines-on-thistle-and-soybeans/
  6. http://www.ipm.iastate.edu/ipm/icm/2005/6-20/thistlecat.html

Note: The information in this issue is based upon field observations and third party information. Since variations in local conditions may affect the information and suggestions contained in this issue, LG Seeds disclaims legal responsibility therefore. Always read and follow label instructions.
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Download a copy of this technical bulletin: Tech_343 - Painted Lady Butterfly and Thistle Caterpillar