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	Issue100 Prairie Grains


	Prairie Grains is the official publication of the Minnesota Association of Wheat Growers, North Dakota Grain Growers Association, Montana Grain Growers Association and South Dakota Wheat, Inc.




	Section 4 - Insect Management

	Scouting for Armyworms in Wheat Armyworm infestations occasionally develop followingmigrations of moths from southern states in late spring and early summer. Moths typically lay their tiny white eggs at the bases of plants in moist, shady areas, and prefer sites with lodged and/or hail-damaged grassy weed or crop plants. Eggs hatch and larvae quickly begin feeding on nearby plants. When armyworm larvae are young they will be pale green turning a dark green as they mature. Fully developed larva will grow to a length of 1 to 2 inches. A series of stripes down the back and side of the body will be found. Moth flights can occur over a few weeks time, so it is not unusual to find a wide size range of larvae present. Early detection of armyworm infestations is difficult because they feed at night and hide under vegetation or in loose soil during the day. Damage is sometimes obscured by wind injury or the dense small grain plant canopy itself. Armyworms are a favorite food of birds, so significant bird activity in a grain field can be an indicator of an armyworm infestation. Scouting for armyworms in small grain fields involves parting the plant canopy back and searching for feeding injury or fecal pellets. If either is evident, look for larvae under plant trash, soil clods or in soil cracks. Fields can also be checked during evening hours (when larvae are most active) by shaking plants and using a flashlight to count dislodged larvae on the ground. Whether sampling during the day or in the evening, it is important to sample as many locations in a field as practical (at least 5 sites per field). To avoid economic loss from armyworms, treat with a registered foliar insecticide when an infestation reaches 4 to 5 larvae per square foot. As the name implies, armyworms will assume the “army” habit and move in large numbers to neighboring fields when the food supply becomes depleted in the original feeding site. Applying a foliar insecticide as a barrier treatment to a strip of plant material ahead of the infestation in the direction of movement can provide good protection of neighboring fields if timed correctly. Options for armyworm control in small grains can be found in the 2009 N.D. Field Crop Insect Management Recommendations guide online at: www.ag.ndsu.edu/pubs/plantsci/pests/e1143w1.htm. Online NDSU bulletin: www.ag.ndsu edu/pubs/plantsci/pests/e830w.htm.Armyworms will also feed on other small grain crops as well as corn, alfalfa, clover, flax, and millet.

Scouting for Cutworms

Watch fields for any cut plants and dig up any cutworm larvae located under cut plants. Wet soil conditions will cause them to feed closer to the soil surface, and may make them easier to kill with insecticides. Cool conditions can slow their feeding activities and larval development. Since there are early and later season cutworms, cutworm activity can continue into late June.

Early detection is critical for effective cutworm control, especially in corn, dry beans, sunflowers, soybeans, sugar beets, and alfalfa. Cutworm damage is often localized in certain areas of the field, and in some situations insecticide sprays can be targeted at those infested areas. Economic thresholds include:

Corn: Begin scouting for cutworms at stand establishment and continue until mid-June. Treat when 3 to 6% of the plants are cut and small larvae (<3/4 inch) are present. Application rate of 15 to 20 gallons of water per acre by ground application is suggested.

Dry Bean: Treatment is warranted when one cutworm or more is found per 3 feet of row and the larvae are small (<3/4 inch long).

Soybeans: Economic thresholds for cutworm treatment decisions are not well established. Treatment guidelines used over the years include when one cutworm or more is found per 3 feet of row and the larvae are small (<3/4 inch long). Another guideline is when 20% of plants are cut or when gaps of 1 foot or more exist in the plant row. When making a final decision, consider that surviving soybeans are able to compensate for early stand reductions because of the plants long growth period.

Sunflower: Treatment is warranted when one cutworm or more is found per square foot or there is a 25 to 30% stand reduction observed. Keep in mind that while the seed treatment product Cruiser (a.i. thiamethoxam) has demonstrated success in controlling early-season insects such as wireworms, the product shouldn’t be expected to control insects beyond those listed on the product label. Cutworms are not listed on the Cruiser label. Some have observed that Cruiser may help protect plants from cutworms early in the season. But the product shouldn’t be expected to offer complete control, especially as the growing season progresses, since it is not labeled for cutworms.

A simple method of scouting cutworms to consider: Mix the labeled rate of an insecticide for cutworms in an ATV-mounted sprayer. Three or four days after planting, make a diagonal spray pass across a sunflower field. Come back about an hour before sunset and again before sunrise to check dead cutworms in the treatment path. Scout early after the sample spray to get a good count.

Consult the 2009 ND Field Crop Insect Management Guide for insecticide registered for cutworm control in specific crops. www.ag.ndsu.edu/pubs/plantsci/pests/e1143w1.htm.

Scouting Soybean Cyst Nematodes

Once Soybean cyst nematode (SCN), a microscopic parasitic worm, becomes established in a field, it is nearly impossible to eradicate. Therefore, keeping the SCN population levels to a minimum using crop rotation and resistant cultivars is the best way to manage this pathogen.

SCN survive as small cysts in the soil. The cyst is the actual body of a female nematode with hundreds of eggs inside her. When protected inside this cyst, the eggs are able to survive in the soil for multiple years. Swollen female nematodes on soybean roots are visible to the naked eye as a small lemon-shaped dot and can be white, yellow, or brown in color; the nematodes are much smaller than the root nodules.

Aboveground symptoms of SCN often include stunting, frequently expressed as a roller coaster effect. Infected plants may become yellow in July or August, and they have reduced vigor or mature earlier than those in surrounding areas of the field.

If SCN is suspected in a field, collect soil samples (6-8 in. deep) from the inside and margin of the affected areas, and send them to a lab (such as the NDSU Plant Diagnostic Lab or UM Nematology Lab in Waseca) to detect the presence of cysts or SCN eggs. A county extension educator or local agronomist can assist with identification and sampling. Soil samples can be collected from early spring to late fall, but fall after harvest is an excellent time to collect samples for fields that will be planted into soybeans the following year.

U of M SCN bulletin online www.extension.umn.edu/distribution/cropsystems/DC3935.html

SDSU SCN Factsheet: http://agbiopubs.sdstate.edu/articles/FS902A.pdf

Stem Maggot May Cause White Heads in Wheat

White heads caused by feeding of wheat stem maggot larvae can often be observed in wheat fields. Typically, only 1 to 5% of the heads are affected, and they appear to occur scattered

at random through the field. Normally the first indication

of its presence is the dying and whitening of wheat heads and upper internodes while the lower stem and leaves remain green. A single maggot will be found inside the straw just above the last node. The plant stem pulls out very easily and if larvae are not found, the stem is usually cut off. The presence of white heads alone is not always an accurate assessment of damage. The maggots can also infest young tillers prior to the boot stage, causing the affected tiller to abort. These aborted tillers can account for an unseen loss in yield. Little is known about this insect pest, and no chemical controls are recommended. Common root rot can also cause white heads, however with root rot, typically the whole plant is whitened and the whole plant easily pulls from the soil, with browned or blacked roots as well.For more information on the wheat stem maggot including photos, visit www.ag.ndsu.nodak.edu/aginfo/entomology/ndsucpr/Years/2008/june/5/ent.html

Orange Blossom Wheat Midge

Weather conditions during the spring and summer are very important in determining if economic injury will occur. If heading coincides with emergenceof the midge and weather conditions are favorable for the female to lay eggs, producers will need to monitor fields, even in areas where the survey says populations are low, to determine if treatment is necessary. High soil moisture, warm and calm conditions, and high humidity have favored midge egg laying in past years.

Wheat is susceptible to midge infestation from the time the head emerges from the boot until 80% of the primary heads have anthers visible. Treatments are warranted when 1 or more midge are observed for every 4 or 5 heads of spring wheat, and when 1 or more midge are observed for every 7 or 8 heads of durum. At these levels of infestation, wheat yields will be reduced by about 15% and lower the grade of harvested grains if the midge is not controlled. Treatments after 50% of the first heads have flowered are not recommended, due to reduced levels of efficacy and for the protection of a parasitic wasp that attacks the midge eggs.

Not every small fly in the crop will be a wheat midge. Wheat midge is a small, fragile, orange fly about 2 to 3 millimeters in size. Another small fly that is common in wheat fields is the lauxanid, which is larger (2.5 to 4 mm), more robust and yellowish brown. The wheat midge tends to flutter from plant to plant and rests with its head pointing skyward. The lauxanid flies and rests in a horizontal position with its head pointing toward the ground.

Scout fields after 9 p.m. if the temperature is above 59 degrees and the wind speed is less than 6 mph. Count the number of adult wheat midge present on four or five heads at several locations and then use an average per field.

A newer alternative to field scouting is using sex pheromone traps or yellow sticky traps to monitor for wheat midge in wheat fields. Traps are placed next to the wheat head at the beginning of heading and collected in three to five days. Three sex pheromone traps ($7.20 a trap) per 160-acre wheat field or 10 yellow sticky traps (60 cents per trap) per field are recommended. The number of wheat midge per trap is counted and the average per trap calculated.

Action thresholds are nine wheat midges per sex pheromone trap and five to 20 wheat midges per yellow sticky trap. Sex pheromone traps are available through PheroTech International (www.pherotech.com). Yellow sticky traps are available through Great Lakes IPM (www.greatlakesipm.com) or PheroTech International. Specific trapping guidelines also are available through distributors.

The wheat midge degree-day model developed in Canada has helped predict emergence and peak activity periods, as well as when field monitoring should take place. Using a base of 40 degrees Fahrenheit, 90% of female wheat midge has emerged by 1,600 degree days. Actual degree days are calculated and maps available for producers on the NDSU NDAWN Web site at http://ndawn.ndsu.nodak.edu/wheat-midgedd-form.html.

More information online: www.ag.ndsu.nodak.edu/aginfo/entomology/entupdates/Wheat_Midge/owbm.htm.

The 2009 forecast for populations of wheat midge continues to be favorable for North Dakota wheat producers. Overall, the wheat midge risk is lower than 2008. Wheat midge populations ranged from zero to 286 midge larvae per square meter, with most of the state having less than 200 midge larvae per square meter during 2008. Approximately 60 percent of the soil samples were positive for wheat midge larvae with an average of 70 larvae per square meter among the positive samples. In contrast, wheat midge populations ranged from zero to 678 midge larvae per square meter, and an average of 107 larvae per square meter among the positive samples during 2007.


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