Crop & Pest Report


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Migratory Insects (06/11/20)

With the recent thunderstorms, it is not surprising that migratory insects will be blown into North Dakota with the southerly winds.

With the recent thunderstorms, it is not surprising that migratory insects will be blown into North Dakota with the southerly winds. Some of these insects are pests that are looking to feed on our emerging field crops. Two insect pests that have been observed at non-economic levels are:

ARMYWORM does not overwinter in ND. Adult moths, also called miller moths, are a light brownish gray moth with a conspicuous white spot about the size of a pinhead on each front wing. When expanded, the wings are about 1½ inches across. However, larvae (caterpillars) do all of the feeding injury to crops. Full-grown larvae are green-brown with a brown head and longitudinal stripes on the sides. They grow to a length of 1½ to 2 inches.

Field scouting for armyworms should be done in field margins, low areas with vigorous plant growth, and areas where plants have lodged. Indications of armyworm feeding include leaf defoliation, worm frass (droppings) around the base of plants, and defoliated leaves in lodged areas of wheat fields. Larvae complete feeding in 3 to 4 weeks, staying in the area where they hatched until they run out of food. If all food is consumed, larvae often move in hordes or “armies,” eating and destroying vegetation as they move.

For proper pest management, it is important to scout and control armyworms while they are small larvae and before extensive feeding damage has resulted. Look for larvae beneath plant debris around the base of plants and on heads of wheat or barley.

Since armyworms feed at night, spray in early evening and use adequate water volume to get the insecticide into the crop canopy (3-5 gallons per acre by air). The economic threshold is 4-5 armyworms per square foot in wheat or barley.

For more information, see the NDSU Extension publication The Armyworm and the Army Cutworm E830 (revised).



PAINTED LADY BUTTERFLIES are beautiful butterflies of our pollinator gardens, but the larval stage (caterpillar) can become a pest when large numbers of painted lady butterflies migrate into North Dakota from the southern states. Thistle caterpillars have become a more regular insect pest of sunflower, soybean, dry bean and vegetable plants. Of course, it prefers to feed on Canada thistle as its name implies. However, thistle caterpillar does not control the noxious thistle weed due to its extensive root system. Painted lady butterflies are often attracted to crop fields that are weedy with Canada thistles for egg-laying.

Thistle caterpillars are dark brown or black with yellow stripes on the side of the body, spiny hairs, and are about 1¼ inch long when mature. It feeds on leaves for two to four weeks causing severe defoliation when densities are high. Loose webbing creates a nest in leaves when the caterpillars feed. Black fecal pellets can also be found in the webs. Mature caterpillars pupate, and adult butterflies will emerge in 7 to 10 days. Two generations are typical in North Dakota.

Minnesota Extension Specialists have also reported CEREAL APHIDS and ASTER LEAFHOPPERS in small grains, but there are no reports of either pest for North Dakota yet.


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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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