Vanessa cardui (soybean)
Authors: Buyung Hadi, Jeffrey Bradshaw, Jan Knodel, Ken Ostlie, Robert J. Whitworth, J. P. Michaud and Phillip E. Sloderbeck
The thistle caterpillar, Vanessa cardui, is a globally distributed butterfly from family nymphalidae. The adult butterfly of the caterpillar is commonly known as painted lady butterfly.
Full grown thistle caterpillar is about 1.25 in (32 mm) long. The caterpillar body is black to brown with yellow stripes on each sides. The thistle caterpillar is conspicuous due to branched spikes covering the upper side of the body.
The chrysalis or pupa of thistle caterpillar ranges from light to dark brown in color with bumps on the surface. The adult butterfly is colorful with red orange color dominating the top sides of the wings. The top sides of the wings also bear black and white markings. The wings' bottom sides are pinkish brown with black, white and blue spots.
Life Cycle and Seasonal History
The painted lady butterflies survive the winter in Florida along the gulf coast, extreme southwest Caifornia and Mexico. Annual flight northward between April to July is well documented (Williams 1970). They can reach states as far north as North Dakota as early as May. Upon arrival, mated female butterflies lay barrel-shaped eggs singly on the top of host plant leaves. The eggs and larvae fail to develop at temperatures under 65˚F (18.3˚C). The resulting larvae (caterpillars) feed on host plant leaves before they pupate. The thistle caterpillars produce silken web attaching the leaves they feed on together. After 5-6 days, adult painted butterfly emerge from the pupae. The development thistle caterpillar is temperature dependent. It takes about 73 days for development from egg to adult emergence at 65˚F (18.3˚C). At 90˚F (32.2˚C), it takes about 22 days for egg to develop into adult. Depending on the timing of butterfly arrival and the weather afterward, there may be one or two generations of thistle caterpillar found in Northern Plains. There has been few sightings of southward flight from northern states such as Minnesota, New Jersey and New York beginning in August to later part of autumn (Williams 1970).
Plant Injury and Damage
The thistle caterpillar feeds on over a hundred host plants, mainly in plant families Malvaceae, Compositae and Leguminoseae. It can be found on Canada thistle, maize, sunflower and soybean. The thistle caterpillar inflict injury by defoliating soybean plants. Caterpillars of later instars consume higher amount of soybean foliage. The webbing of soybean leaves has little effect on yield. In years when large painted lady butterfly migration occur, the subsequent thistle caterpillar population may reach sufficiently high population to attain pest status.
Scouting and Threshold
Scouting for thistle caterpillar in early stage soybean to help get a grasp of the caterpillar's situation for the growing season. One reference suggests an economic injury level of 3 thistle caterpillars per row-foot for V3-V4 soybean (Pedigo 1994) while others suggest using a defoliation assessment to determine the need for caterpillar control. In general, treatment is recommended when there is 25-30% defoliation pre-bloom or 20% defoliation after bloom or during pod set.
Other Online Resources
For information regarding labels of chemical control options, please visit Agrian.com