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Thistle Caterpillar in Crops and Gardens (06/27/19)

We have received several reports of the colorful painted lady butterfly fluttering around fields and pollinator gardens.

We have received several reports of the colorful painted lady butterfly fluttering around fields and pollinator gardens. In one situation, severe defoliation by thistle caterpillars was reported in a sunflower field near Linton in Emmons County. Thistle caterpillars defoliated young sunflower plants down to just the stem with little leaf tissue left.

Thistle caterpillars are only an occasional pest problem in North Dakota since the butterflies get blown in annually. The last major outbreak of thistle caterpillars in North Dakota was 2012. Larvae (caterpillars) injure plants by defoliation. Host plants include Canada thistle, sunflower, soybean and vegetable plants. Larvae feed for two to four weeks. They are dark brown or black with yellow stripe of side of body, spiny hairs, and 1½ inch long when mature. Larvae create loose webbing when feeding on the leaves. Black fecal pellets also can be found in the webs. Mature larvae pupate, and adult butterflies will emerge in 7 to 10 days. Two generations are typical in North Dakota. Although the thistle caterpillar feeds on Canada thistle, it does not control this noxious weed due to the thistle’s extensive root system. Painted lady butterflies are often attracted to fields that are weedy with Canada thistles for egg-laying.

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Economic thresholds are dependent on the crop, crop stage and the presence of other foliage-feeding caterpillars, such as green cloverworm. All are lumped into the threshold for total leaf area lost. When doing defoliation estimates, sample foliage in the top, middle and lower canopies and average the percent defoliation. If the majority of larvae are >1¼ inch long, most of the feeding damage has occurred and treatment is not advised.

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 cause 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).

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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.

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