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Thistle
Caterpillars are easily recognized by the multibranched spines, which
resemble Christmas trees that have lost their needles. The larvae are mostly
black and yellow with no distinctive pattern. The adult butterfly, called the
painted lady, is common along roadsides this summer. Feeding injury is
noticeable as three or four soybean leaflets held together with silk, usually in
the upper canopy of the plant. The caterpillar hides inside the webbing, feeds
on the leaflets, and deposits numerous black fecal droppings. Inside the webbing
the caterpillars consume approximately 40 square inches of soybean leaves,
causing 97 percent of the leaf removal during the last two larval stages (when
larvae are 3/4 to 11/4 inches in length). An economic threshold in
reproductive-stage soybean is 20 percent defoliation.
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