Bertha Armyworm (Mamestra configurata)
The adult moth is about 1 1/2 inch and mainly gray-black with a silvery/whitish kidney shaped spot and silvery/whitish fringe on each forewing. Moths emerge from the overwintering pupae in early-mid June and emergence continues through early August. These night fliers are particularly attracted to blooming canola fields for their nectar and egg laying. Eggs are laid on the lower side of leaves in cluster of 50-500 eggs in a typical honeycomb pattern, and hatch in about one week. The emerging larvae (1/10th of a inch) are usually green in color and hide underneath leaf litter and clumps of soil during the day. Making them difficult to find! Mature larvae are about 1 1/2 inch long and vary in color from brown or velvety black. Larvae develop for six weeks and then drop to the ground in mid-late August to pupate. If the autumn is unusually warm, some adult moths may emerge from the puparium only to perish when winter arrives.
As the canola plant drops its leaves, the mature larvae (>1/2 inch) begin to feed directly on the pods which causes the economically important yield losses and premature shattering. Feeding by the mature larvae also account for 80-90% of the plant material consumed during a larvaes life. Mature larvae will even continue to feed on pods in the swath. Fortunately, populations are kept low due most years due to natural environmental factors like harsh winters and a number of biological control agents (diseases and parasites).
Pest Management: Monitoring and Economic Thresholds - Knowing When To Control?
Sex pheromone traps can be used to detect bertha armyworms in a general area. Bertha armyworms have already been trapped in Bottineau, Renville, and Burke counties. The recommended trap design is the green unitrap or bucket trap suspended above the crop canopy near the fields edge. High trap catches generally indicate the level of larval populations to follow. Fields should be monitored about two weeks after peak trap catch and scouted regularly to minimize crop losses. Check several locations per field, and continue scouting until economic threshold is reached or the crop is swathed. Assuming a crop price of $6.00 per bushel and a spraying cost of $10.00 per acre ($6.50 for an insecticide and $3.50 for the application cost), the economic threshold would be three bertha armyworm larvae per square foot. Two larvae per square foot is estimated to reduce yields by 1.16 bushel per acre or $6.96 per acre. The key to controlling bertha armyworm is:
early detection of adult moths for their
presence or absence and relative abundance in an area
Prepared by: Janet J. Knodel, Crop Protection Specialist, North Central Research Extension Center, NDSU
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For more information see the Insect Control Guide at http://www.ext.nodak.edu/extpubs/plantsci/pests/e1143w1.htm