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Thistle Caterpillars in Soybeans (07/06/17)

Thistle caterpillars are being observed at low or non-economic populations in soybeans.

Thistle Caterpillars in Soybeans

Thistle caterpillars are being observed at low or non-economic populations in soybeans. This insect is the larva of the butterfly known as the Painted Lady. This butterfly does not overwinter in North Dakota, but migrates from southern states each spring. These caterpillars are brown to black with yellow stripes along each side of the body. They are covered with spiny-hairs that give the caterpillar a prickly appearance. Full grown larvae are about 1½ inches long. The caterpillars feed on the leaves, webbing them together at the feeding site.

To determine the level of feeding injury caused by defoliating caterpillars, estimate the percent defoliation from 10 randomly selected plants per site and 5 sampling sites throughout the field. The growth stage of the soybean plant is important. Under most conditions, moderate defoliation early in the season has little effect on final soybean yield. As plants reach the flowering and pod filling stages, then defoliation poses a greater threat to yield. Research indicates that the soybean plant can sustain a 35% leaf loss prior to the pre-bloom period. From pod-set to maturity, the plant can tolerate only a 20% defoliation level.

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Janet J. Knodel

Extension Entomologist



 

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