ISSUE 10 July 7, 2005
THISTLE CATERPILLARS IN SUNFLOWER
Thistle caterpillars (or larvae) are abundant in sunflower fields now. The prickly caterpillar are found in webbed leaves where they feed causing defoliation. Larvae are about 1¼ inches (32 mm) in length when mature. The bodies are brown to black with yellow striping along each side of the body with stout, spiny hairs. The adult is known as the colorful Painted lady butterfly. The topside of the wings is red-and-orange with black-and-white spots. The bottom side of the front wings is rose-pink with olive, black and white markings. Bottom side of the rear wings is olive with white markings and blue spots.
Painted Lady Butterfly (courtesy Gerald Fauske)
Thistle caterpillars / Painted lady butterflies do not overwinter in North Dakota, but overwinter in tropical and subtropical areas. Butterflies migrate northward in the spring. With the strong southerly winds this May-June, numerous Painted lady butterflies have reached North Dakota. Upon arrival, females lay pale green, barrel-shaped eggs singly on the tops of host plant leaves. The caterpillars feed for 2 to 4 weeks, and then pupate. The butterfly emerges from the chrysalis in 7 to 10 days. Two generations are expected in North Dakota.
The caterpillar feeds on over 100 species of plants. The most common plants are Canada thistle, sunflower, soybean, canola, dry edible beans and vegetable plants in the garden. The Painted Lady butterfly prefers nectar from composites 3_6 feet high, especially thistles; also aster, cosmos, blazing star, ironweed, and joe-pye weed. Flowers from other families that are visited include red clover, buttonbush, privet, and milkweeds.
Economic thresholds for thistle caterpillars include:
Soybeans: In general, treatment of soybean is recommended when there is 25 to 30% defoliation prior to bloom; 20% after bloom or pod set. This normally requires about 4 to 8 caterpillars per row foot.
Sunflower: In sunflower, the threshold is 25% defoliation provided that most of the larvae are still under 1¼ inch long. If the majority of the larvae are 1¼ to 1 ½ inch long, most of the feeding damage will have already occurred and treatment is not advised.
A common question is whether the caterpillar's feeding will kill Canada thistle, a colony-forming perennial. The answer is no. The extensive horizontal root structure of Canada thistle makes this weed pest very hardy. Consuming the leaves may set back individual plants, but the root and its energy reserves will allow the plant to recover. Weed control directed at thistle will result in movement of caterpillars to the healthy crop plants. In these cases, spot spraying or the addition of an insecticide to the herbicide can be done to limit crop damage.
WHEAT MIDGE DEGREE DAY UPDATE
The southeast region of North Dakota is close to the end of adult wheat midge flight activity - 1800 DD. The rest of the state is in the major emergence period of the female wheat midge between 1300 to 1600 degree days (see map). Continue to scout wheat or durum fields in the susceptible crop stage (heading to early flowering) for economic threshold levels: one wheat midge per 4-5 head for wheat and one wheat midge per 7-8 heads for durum. Pheromone trapping for adult wheat midge indicates low populations of wheat midge in the Minot area so far.
Area Extension Specialist
North Central Research Extension Center