Crop & Pest Report

Accessibility


| Share

Thistle Caterpillars Again (07/18/19)

Mature thistle caterpillars about 1¼ inch long are still being reported in soybeans and sunflowers throughout the state.

Mature thistle caterpillars about 1¼ inch long are still being reported in soybeans and sunflowers throughout the state. Caterpillars feed on the foliage causing defoliation. These caterpillars are at the end of their larval feeding and development of 2-4 weeks.  So, no treatment is advised now. In fact, some of the caterpillars are already dead and have been infected with a disease. Caterpillars will be forming a chrysalis (or cocoon) soon, which is the non-feeding stage. During the chrysalis stage, the insect transforms into the adult butterfly, painted lady butterfly, in about 7-10 days. Crop defoliations from the 2nd generation of thistle caterpillars should be scouted for in about 2-3 weeks.

ent.1

Economic Thresholds:

SOYBEAN- vegetative to bloom - 40% defoliation; bloom to pod fill - 20% defoliation; and pod fill to harvest - 35% defoliation. An average infestation of 4 to 8 larvae per row foot typically causes 20-30% defoliation. As plants reach flowering and pod filling, defoliation poses a greater risk for yield loss.

SUNFLOWER – 25% defoliation and when larvae are still small (<1¼ inch long).

ent.2

 

Janet J. Knodel

Extension Entomologist

This site is supported in part by the Crop Protection and Pest Management Program [grant no. 2017-70006-27144/accession 1013592] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed are those of the website author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

USDA logo

Creative Commons License
Feel free to use and share this content, but please do so under the conditions of our Creative Commons license and our Rules for Use. Thanks.