NDSU Crop and Pest Report

Entomology


ISSUE 10  July 3, 2002

 

SOYBEAN APHID BEING FOUND IN SE NORTH DAKOTA AND NEIGHBORING AREAS

The first reports of soybean aphid in North Dakota have come in from the Wahpeton and Fairmount areas of Richland County. In addition, Ian MacRae, entomologist, U of M - Crookston, has reported small colonies of aphids in soybean fields at the Crookston Research farm. Also, the Minnesota Pest Report has reported soybean aphids present in Grant and Stevens Counties (south and east of Wilkin and Traverse Counties in MN).

None of these sightings has reported aphid numbers at the treatment threshold. Treating too early could result in the aphid population recovering more quickly in the absence of predators or parasites.

Scouting of fields will be important in the next two weeks. Flowering in soybean fields, or the important R1 to R2 stages, is approaching quickly. Scouting and management was summarized in the June 20 (Issue 8) Crop and Pest Report. Management results from 2001 (Michigan, Wisconsin, and Minnesota) indicate management at the R1 to R2 stage provided the best return . . . contingent on having soybean aphid present in fields at abundant levels.

A new publication is available for Soybean Aphid Management in North Dakota (E-1232). The publication is available through the internet at:

Web Version (html):

http://www.ext.nodak.edu/extpubs/plantsci/pests/e1232w.htm

Portable Document File (PDF):

http://www.ext.nodak.edu/extpubs/plantsci/pests/e1232.pdf

For a printed copy of the circular contact:

Distribution Center, NDSU Extension Service
Morrill Hall, P.O. Box 5655
North Dakota State University
Fargo, ND 58105-5655
Voice: (701) 231-7882
FAX: (701) 231-7044

 

WHEAT MIDGE EMERGENCE UNDERWAY

Degree day (DD) accumulations have reached the point where wheat midge females are emerging in southeast North Dakota.  Counties farther north and west will see emergence of the midge female begin over the next week.  The anticipated dates of emergence are summarized in the following map.

02_midge_emerdate.jpg (55024 bytes)

Fortunately, wheat midge populations are low across the region, based on the fall soil survey (see Crop and Pest Report, Issue 1, May 2, 2002). Populations were at economic levels in the northwest areas of the state last year, however, the dry August and September appears to have reduced the larval numbers present in the soil.

Another good situation is the rapid development of the crop in many areas. In the southeastern counties, where the DD model indicates emergence is underway, the wheat crop is well past the susceptible stage (heading to flowering) in most fields. In other areas, if midge emergence is underway and the crop is heading, scouting is recommended to determine if treatable numbers of midge are present. The high risk planting windows, when planting date results in wheat head emergence and midge emergence occurring together, is defined as wheat planted from 200 to 600 DD. These risk windows occurred later this year than in the past four years.

02_midge_plantdate.jpg (61765 bytes)

Wheat midge treatment threshold for yield protection in HRSW is when an average of 1 adult midge per 4 to 5 wheat heads is present. Recent research from Canada has resulted in an additional threshold recommendation for quality protection, such as with durum wheat, is desirable. The revised treatment threshold in this case is 1 midge per 8 to 10 wheat heads.

 

GRASSHOPPER ACTIVITY PICKING UP

Grasshopper activity is increasing, particularly in the south central counties impacted by the dry conditions. In Emmons, Logan, and McIntosh counties, grasshopper nymphs are looking for green plants. The concentration of these grasshoppers on the limited areas available will intensify the problem. When adults are present later, they will further concentrate at feeding sites.

There are other reports of localized hot spots around the state. John Kringler, NDSU Extension Agent-Cass County, reported grasshopper spraying around Harwood and Kindred; spray reports have also come from Page. Jan Knodel, NDSU Plant Protection Specialist-Minot REC, reports some grasshopper treating in McLean County.

Phillip Glogoza
Extension Entomologist
pglogoza@ndsuext.nodak.edu

 

BERTHA ARMYWORM POTENTIAL FOR CANOLA-MUSTARD IN NORTH CENTRAL COUNTIES

Sex pheromone traps are used to detect bertha armyworms in a general area. Bertha armyworms have already been trapped in most counties in the north central region. High trap catches generally provide insight into the larval populations that follow. The pheromone trap catches from Bottineau and Renville Counties are unusually high for this early in the season, about 100 moths per trap night near Westhope and 35 per trap night near Mohall. Trap catches in other areas, like Minot and southwest ND, are much lower at about 4-6 per trap night. Growers in this "HOT" spot should keep a close watch on their blooming canola and mustard fields for localized infestations. Other preferred hosts are rapeseed, alfalfa, and lamb’s quarter. Secondary hosts include flax, peas, and potato.

BAW_Agr_AgFood_Can.jpg (19191 bytes)
Bertha armyworm adult

The adult moth is about 1˝ 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 late 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. Their hiding makes them difficult to find! Mature larvae are about 1˝ inch long and vary in color from brown to 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 (>˝ 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 larvae’s life. Mature larvae will even continue to feed on pods in the swath. Fortunately, populations are kept low most years due to natural environmental factors like harsh winters and a number of biological control agents (diseases and parasites).

Monitoring and Economic Thresholds - Knowing When To Control Bertha armyworm

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. The economic threshold is 18-22 larvae per square yard when larvae are feeding on the leaves. If larvae are feeding on the seed pods, thresholds should be lowered to 2-3 larvae per square foot. Two larvae per square foot is estimated to reduce yields by 1.16 bushel per acre.

Capture 2 EC, ethyl-methyl parathion 6-3, and methyl parathion 8 EC are registered in North Dakota for controlling bertha armyworm in canola. The higher rate of Capture 2.1-2.6 fl. oz./acre should be used.

The key to controlling bertha armyworm is:

Jan Knodel
Area Extension Specialist Crop Protection
North Central Research Extension Center
jknodel@ndsuext.nodak.edu


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