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ISSUE 11  July 16, 1998

 

WHEAT MIDGE CONCERNS MOVE NORTH

    Wheat midge concerns are now confined to the northern most central counties including Bottineau, Rolette, Towner, and Cavalier.

    Reports from Tuesday (July 14) of this week should help define where problems may still be observed:

    Bottineau - South of Westhope >1 midge per head, susceptible fields were still at economic levels (Source N. Ag Management). North of Newburg - fields were at economic levels for wheat on wheat or durum and in susceptible growth stages; wheat on other crops had lower numbers. Larvae have been found in heads of early plantings and eggs found on later plantings (Source: Ag Zone). Souris - Some fields that were heading were still at economic levels (Source N. Ag Management); wheat on sunflower ground had low numbers during night observations. Overly - fields of wheat on wheat that were heading were above economic thresholds with as many as 1midge per head!

    Pierce - Wolford area had many fields with economic levels of midge and these fields were being sprayed. FEW midge were found in other areas of the county.

    Towner - Sarles area had high counts of 1midge per head to 1 per 5 heads. Bisbee area was still high. Areas SW of Cando still reported high of 1per head and 1per 2 heads

    Cavalier - Midge were found across entire county. Still had 75% of wheat fields in susceptible growth stage with many having economic levels of midge.

    Benson - York area is still reporting economic threshold levels of midge. This area is well above 1600 DD. Activity, if present should be very isolated.

    Ramsey - Areas east of Churchs Ferry reported economic levels of midge at 1 midge per 3-5 heads.

    We appreciate all the reports that have been submitted during the past three weeks pertaining to wheat midge activity. Population levels appear to have been greater this year compared to last based on these reports. A fall soil survey is being planned for this year to document the extent of the overwintering population.

 

CORN BORER SCOUTING CRITICAL

    Moth captures peaked and appear to be declining in black light traps across the southeast quarter of the state. Decisions to treat for corn borer in most areas of North Dakota are likely to be made during the next ten days.

    Egg masses can be found with little difficulty in many fields. Shotholing of upper leaves is becoming more apparent. Inspections for larvae reveal that the older corn borer are in the 5 to 6 day age, though these larger larvae are still a small part of the overall potential population. Corn borer larvae will begin to bore in after about ten days of age.

    Scout corn to determine the percent of plants with egg masses, shotholing, and levels of live larvae. Use these numbers to aid in determining if treatment is warranted OR if it is time to apply insecticides to control larvae prior to tunneling. Remember, spraying too early may leave egg masses that will hatch after the application. The foliar insecticides used should provide control of many of the newly hatched larvae for about 5 to 7 days after treatment. However, this will be influenced by many factors such as leaf area coverage, temperature, rainfall, insecticide exposure to sunlight. Beyond these number of days, any further control of hatching larvae will be suspect.

    When scouting tassel stage or older corn, treatment decisions using the following worksheet are most often based on field scouting egg masses. Examine the underside of the middle 7 leaves (3 leaves above and 3 below the ear leaf) on 20 plants from 5 locations in the field. Multiply the number of egg masses by 1.1 (correction factor for eggs on other leaves). By completing the following worksheet, a recommendation on the profitability of treatment will be calculated.

Worksheet for Tassel stage or older corn . . . You fill in the blanks

1.__ egg masses per plant*

2.__ borers per plant

3.__ percent yield loss

4.__ bushel loss per acre

5.__ loss per acre

6.__ preventable loss/acre

x 4.5 borers per egg mass

x__ percent yield loss per borer*

x__ expected yield (bu. per acre)

x__ price per bushel

x 80 percent control**

___ cost of control per acre

=  __ borers per plant

=  __ percent yield loss

=  __ bushels per acre loss

=$__ loss per acre

=$__ preventable loss/acre

=$__ profit (loss)/acre

*Cumulative counts taken five to seven days later can be added here

**Use 4% for pollen_shedding corn, 3% if kernels are initiated

    Correction: The spreadsheet printed in issue number 9, for use in making corn borer treatment decisions in whorl stage corn, had the expected level of control for foliar insecticides footnoted as 50%, that should read 75 or 80%.

Phillip Glogoza
Extension Entomologist


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