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ISSUE 5   June 12, 2008

GET READY TO SCOUT FOR CEREAL APHIDS

Cereal aphids have been reported in South Dakota on winter wheat. These aphids will be migrating into North Dakota soon. The greenbug, English grain aphid and bird cherry oat aphid are the principal species that cause problems in North Dakota small grains. These aphids transmit the disease barley yellow dwarf virus (yellowing from the tip of the flag leaf - see photograph). When aphid populations are high, the disease can spread quickly through small grain fields. At greatest risk are later-planted fields, which attract migrating aphids that are moving from more mature fields.

Barley yellow dwarf virus
Barley yellow dwarf virus

Descriptions:

Greenbug - pale green with dark stripe down back.
Bird Cherry Oat Aphid - olive green, brownish patch at bases of cornicles.
English Grain Aphid - bright green with long black cornicles.

Cereal aphids

When to Scout: For wheat, field scouting should begin at stem elongation and continue up to the heading stage.

Thresholds for Small Grain Aphids: English Grain, Bird Cherry Oat, Greenbug
To protect small grains from yield loss due to aphid feeding, three different treatment thresholds are available:

- 85% of stems with at least one aphid present prior to complete heading.
- 12-15 aphids per stem prior to complete heading
- 100 aphid days prior to complete heading (=0.6 bu/acre yield loss)

If you averaged 10 aphids per stem and had 12 days to reach heading, you would have 120 aphid days (10 aphids x 12 days). This is above the 100 aphid day threshold and an insecticide should be applied to prevent yield loss. On the other hand, if you averaged only one aphid per stem and had 12 days to reach heading, you would only have 12 aphids days (1 aphid x 12 days). Thus, no control action would be necessary.

Susceptible Crop Growth Stage:

Crop growth stage also affects its susceptibility to aphids. Vegetative to boot stages are the most susceptible stages to aphid feeding and subsequent yield loss. For example, in the 4-6 leaf stage, injury caused by aphids is stunting, decreased number of kernels per head, and decreased kernel weight. In the boot stage, only kernel size and kernel weight are affected by aphid feeding (not number of kernels per head). By heading, only kernel weight is affected. After flowering, small grains are less susceptible and producers are discouraged from spraying.

Natural Controls:

Lady beetles, aphid lions, syrphid fly, and parasitic wasps play a major role in reducing aphid populations. When natural enemies are present in large numbers, and the crop is well developed, farmers are discouraged from spraying fields.

Janet Knodel
Extension Entomologist
janet.knodel@ndsu.edu

 

SUGARBEET ROOT MAGGOT: LOW FLY ACTIVITY NOW, PEAK POSSIBLE IN 10 DAYS

Degree-day (DD) accumulations for development of sugarbeet root maggot (SBRM) populations this year are running about 7-10 days behind normal. This, combined with frequent events of windy, rainy weather during the past several days, has resulted in very little SBRM fly activity thus far. NDSU Entomology, in cooperation with American Crystal Sugar Cooperative, MinnDak Sugar Cooperative, and Pembina County Extension, is monitoring fly activity using sticky stakes in 40 grower fields throughout the Red River Valley (RRV) this summer. As of June 9, only 8 flies have been captured in the entire program.

Fly activity is not likely to increase significantly during the next 3 to 4 days because rainy and breezy weather is expected to persist. However, a rapid increase in fly activity could occur if warm, dry, and low-wind conditions develop in the next week or so.

Peak fly activity usually occurs shortly after the accumulation of 600 DD. It is important to note that warm weather (around 80 F), and calm to low-wind conditions are most conducive to fly activity. Flies will remain relatively inactive in cool, rainy, or windy conditions.

An extended forecast of anticipated DD accumulations and associated peak fly activity dates is presented in Table 1. Depending on location, peak fly activity in current-year beet fields should occur shortly after the third week of June, depending on location. The forecast is based on site-specific extended weather forecasts through June 17. Actual peak dates will vary somewhat from these estimates.

Table 1. Degree-day (DD) accumulation forecast and predicted peak activity dates for sugarbeet root maggot flies in Red River Valley

Site

Degree Day forecast
(for June 17)

Peak
activity forecast*

Baker, MN

537

June 21 + 80 day

Ada, MN

582

June 19 + 80 day

Grand Forks, ND

586

June 18 + 80 day

St. Thomas, ND

517

June 22 + 80 day

*Peak fly activity in current-year beets is most likely on the first calm or light-wind day to reach 80 F after the required 600 air DD are accumulated.

Fly emergence and movement into beet fields can be accelerated quickly following a major increase in air temperatures. Growers in high-risk areas for SBRM infestation should consider applying a postemergence insecticide, especially if a low or moderate rate of an at-plant soil insecticide was applied. Growers preferring granular products should apply them immediately or as soon as soil moisture conditions allow. It is better to err on the early side of peak activity when applying granules. Postemergence liquid insecticides should be applied within 3 days of the expected peak (before peak is best). This will provide control of both adults and larvae.

For more guidance on postemergence control strategies, consult the "Insect Control" section of the 2008 Sugarbeet Production Guide or the "Sugarbeet Insects" section of 2008 Field Crop Insect Management Recommendations. Online versions of these publications are located at:

www.sbreb.org/Production/production.htm

and

www.ext.nodak.edu/extpubs/plantsci/pests/e1143w1.htm

Mark Boetel
Research & Extension Entomologist
mark.boetel@ndsu.edu


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