Thistle caterpillars are being reported in soybeans in southern and western Iowa. Extension crop specialists Mark Carlton (Albia) and Clark McGrath (Lewis) both report finding them in their areas. The thistle caterpillar is a sporadic pest of Iowa soybeans.
In 1992, the thistle caterpillar was common in Iowa and caused significant defoliation in very young soybean.
In 1995, they appeared again and heavily defoliated fields in eastern Iowa during the late summer. The last episode that merited attention was in 2001.
The thistle caterpillar is the larval stage of the painted lady butterfly. The adult insect migrates into Iowa each year from Texas and Mexico. Eggs are laid on both thistle and soybeans. The caterpillar is easily identified by the spines that resemble Christmas trees that have lost all their needles.
Thistle caterpillars construct webs in upper soybean leaves, tying leaves together with silk and feeding inside their protective nest. There they consume approximately 40 square inches of soybean leaves, causing 97 percent of the leaf removal during the last two larval stages (when they are about 3/4 to 11/4 inch long). Only if a soybean field has very small plants and the caterpillar population averages about one insect for every other plant across the entire field, or if defoliation reaches 20-25 percent, should these insects be of concern. If high densities are found in small areas of a field, then these could easily be spot sprayed. There will be at least one more generation of thistle caterpillars in Iowa soybean, but they may not necessarily occur in the same field.
This article originally appeared on page 121 of the IC-494(15) -- June 20, 2005 issue.