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ISSUE 11  July 13, 2000

 

ARMYWORM ALERT - Many Fields with Treatable Infestations in Red River Valley Areas

    More fields are being found with armyworms at the treatable level of 4 to 5 larvae per square foot. If head clipping is observed, back the threshold down to 2 larvae per square foot. Indications are that many larvae are less than 1 inch in length. Most are feeding on leaves, but some stem clipping is beginning.

 

WHEAT MIDGE AT TREATABLE LEVELS

    Wheat midge emergence is underway statewide. There are reports of treatable populations in northwest North Dakota (Divide, Burke, and northwest Ward counties). Scouting reports indicate that fields with treatable levels have 1 midge per 4 to 8 wheat heads. There was also a report from areas west-northwest of Devils Lake of fields with populations in the 1 midge per 6 to 8 wheat heads. The recommended treatment threshold is 1 midge per 4 to 5 wheat heads.

    Research from Canada indicates that the treatment threshold for wheat midge in Durum wheat should be revised slightly. In order to prevent reductions in quality, a treatment threshold of 1 midge per 8 to 10 heads is being recommended. These reports can be found at:

http://res2.agr.ca/saskatoon/rslt/rslt0006fs.html

http://res2.agr.ca/saskatoon/rslt/rslt0005.html

 

SUGARBEET ROOT MAGGOT FLIES STILL ACTIVE

    NDSU sticky stake counts taken on July 7 indicate that sugarbeet root maggot fly activity in the northern end of the Red River Valley is remaining quite high for this point in the season. In addition, producers from the Minto, Nash, and St. Thomas areas were also reporting observations of high fly numbers in their fields. However, preliminary assessments of maggot feeding injury in commercial fields indicate that insecticides are performing well and that they have protected the beets from any significant maggot injury that would have otherwise occurred when the plants were smaller and more vulnerable to attack. Producers need not be alarmed by the apparent flux in fly activity since most beet fields are at a stage when they are able to withstand some scarring injury without resulting in measurable yield impacts. Currently, tap roots of the beets are quite deep and the moist sub-surface soil conditions that exist in most areas are resulting in maggots remaining higher in the soil profile. Larvae are more likely to exposed to insecticides in these upper zones. Also, feeding injury from surviving larvae should be mostly limited to surface scarring on the upper region of beets and will not have any major economic consequences. Only very late-planted beets would be at risk from any significant injury by the larvae that will come from the currently active mating flies that are being observed in the area.

Mark Boetel
Sugarbeet Entomologist
mboetel@ndsuext.nodak.edu

 

POTATO: APHID ALERT WEB SITE

(coordinator’s note: The following information is from Dr. Ted Radcliffe, Dept of Entomology, U of Minnesota. His research group is providing updates on aphid activity in potato again this year. The web site can be found at:

http://ipmworld.umn.edu/alert.htm

PAG, coordinator NDSU C&PR)

 

    We collected our first green peach aphid July 3 at Climax, MN. There is still not a lot of aphid flight activity at any of the locations. While that means that there is not presently a lot of aphid dispersal occurring, it does not mean that aphids aren't there. They probably are! We have observed unusually high populations at our Rosemount, MN station.

    It is important to scout your fields now. Aphids are hard to sample with a sweep-net or by shaking over a tray. The best means of sampling for potato colonizing species such as green peach aphid is to examine leaves from the lower third of the potato plant. The action threshold for treatment of seed fields is 10 green peach aphids per 100 leaves (leaves not leaflets). This is scarcely above the level of first detection. But, it is essential for seed growers not to let aphids become abundant - when that occurs, it is very difficult to re-establish control (even effective insecticides will never contact every
aphid in the field). Also, if aphid numbers reach high levels it is probable that much virus spread has already occurred.

    There is no effective means of sampling for non-colonizing aphids other than to use pan traps. You should place these in the headland (where they won't interfere with farm operations). Unfortunately, there is not much you can do to prevent virus spread by non-colonizing aphids. Insecticides do not prevent competent vectors from transmitting non-persistent viruses, e.g., PVY.

Edward (Ted) B. Radcliffe
Department of Entomology
University of Minnesota
WWW: http://ipmworld.umn.edu

 

APHID MANAGEMENT IN POTATO

    Insecticides effective against aphids in potato have been limited to the products Admire (at-planting treatment) and Provado (foliar) containing imidacloprid, and Monitor containing methamidophos. There is a new product avialable this year called Fulfill (pymetrizone).

    In general, all recommended insecticides (Monitor, Provado and Admire) when used at labeled rates give adequate control of green peach aphid. In studies conducted by Radcliffe, Ragsdale and others from the U of Minnesota, rates of Monitor and Admire below the recommended labeled rate failed to give satisfactory control. Cutting the rate to save on insecticide costs would compromise control.

    Fulfill gave good control by 6 days after application. Fulfill has a unique mode of action that needs some explanation. This product paralyzes the muscles associated with the aphids sucking mouthparts. Death is essentially by starvation. Soon after applying Fulfill, feeding ceases, but death takes several days.

    Some insecticides such as Asana and Penncap_M cause aphid populations to increase dramatically. Most of the older compounds often used to control potato leafhopper and Colorado potato beetle will flare green peach aphids. This is especially evident if the products are used more than once. For more details on these trials, refer to the Aphid Control Section found at:

http://ipmworld.umn.edu/aphidalert/alert4.htm

    Some final comments on Fulfill. The use rate is 2.75 fl oz per acre for an application. Only 5.50 fl oz may be applied per acre in a season. Allow 7 days between application. The post harvest interval is 14 days. Apply in sufficient water to ensure good coverage; use a minimum of 5 gals/A by air and 10 gals/A by ground. The addition of a penetrating type spray adjuvant is recommended.

 

SUNFLOWER MIDGE EMERGENCE UNDERWAY

    SF midge emergence began during the July 4th week at Mapleton, ND according to Dr. Gary Brewer and the sunflower insect research project group. Though there still is no treatment recommendation for this insect, field scouting may reveal adult midge, eggs and soon larvae at the base of the bracts. Last year, the greatest infestation levels by SF midge occurred in fields around the Starkweather-Webster area in northern Ramsey county. Sunflower fields surrounding this area are likely to have some activity this year.

 

TIME TO SCOUT FOR EUROPEAN CORN BORER

    Blacklight traps in North Dakota have been capturing moths since mid-June. Since about June 29, captures have increased indicating we are in the univoltine flight of corn borer moths. Emergence in cages from last year’s stalks has been very steady recently. Based on a degree day accumulation study, we should be at peak moth emergence.

    Borer populations declined across the region last year. We expect this to lead to lower populations this year. With the frequent, heavy rains that are persisting during this flight, we should see populations stay low. The lateness of many of these storms will hinder nightly activities of moths and moisture on plants should contribute to mortality of larvae.

    Scouting under these conditions becomes important because fewer fields are likely to reach treatable levels of borers. Even with populations down last year, scattered fields had infestations great enough to justify control.

Field scouting for corn borers:

    Whorl stage corn . . . . Pull the whorls from 10 plants at 5 locations across the field. Select whorls at random, avoiding damaged plants. Unwrap the whorl leaves; count and record the number of live larvae found.

    Use the corn borer worksheet to help make decisions about the profitability of treating an individual field.

Phillip Glogoza
Extension Entomologist
pglogoza@ndsuext.nodak.edu

Worksheet for Corn borer in whorl stage corn . . . You fill in the blanks

1. ____ % of plants infested      x ____  Avg no. borers/plant           = ____ Borers per plant

2. ____ borers per plant              x  ____ % yield loss per borer*      = ____ percent yield loss

3. ____ percent yield loss           x ____ expected yield (bu/acre)       = ____ bushels/a loss

4. ____ bushel loss per acre       x____ price per bushel                      = $ ____ loss per acre

5. ____ loss per acre                    x ____ percent control**                  = $____ preventable loss/a

6. ____ preventable loss/acre     - ____ cost of control per acre       = $____ profit (loss)/acre

*5% for corn in the early whorl stage; 4% for late whorl; 6% for pretassel
**80% for granules; 70% for sprays.

 

Economic Threshold (Corn borer/plant) when factoring Crop Value and Control Costs

Control Costs2 ($/acre)

Value of Corn Crop1 ($/acre)

200

250

300

350

400

450

500

550

600

6

0.75

0.60

0.50

0.43

0.38

0.34

0.30

0.27

0.25

7

0.88

0.70

0.58

0.50

0.44

0.39

0.35

0.32

0.29

8

1.00

0.80

0.67

0.57

0.50

0.45

0.40

0.37

0.34

9

1.12

0.90

0.75

0.64

0.56

0.50

0.45

0.41

0.38

10

1.25

1.00

0.83

0.71

0.63

0.56

0.50

0.46

0.42

11

1.38

1.10

0.92

0.79

0.69

0.61

0.55

0.50

0.46

12

1.50

1.20

1.00

0.86

0.75

0.67

0.60

0.55

0.50

13

1.63

1.30

1.08

0.93

0.81

0.72

0.65

0.59

0.54

14

1.75

1.40

1.17

1.00

0.88

0.78

0.70

0.64

0.59

15

1.88

1.50

1.25

1.07

0.94

0.84

0.75

0.68

0.63

16

2.00

1.60

1.33

1.14

1.00

0.89

0.80

0.73

0.68

                                                    1 Crop value = expected yield (bu/acre) X projected price ($/bu)
                                                    2 Control costs = insecticide price ($/acre) + application costs ($/acre)

 


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