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ISSUE 12  July 23, 1998

 

SUNFLOWER MIDGE LARVAE FEEDING

    Reports and field inspections from throughout the eastern part of the state indicate sunflower midge larvae are present in buds. The flat, oval, creamy-white larvae can be found at the base of the bracts. The scarring caused by their feeding can be found with the larvae.

    Some of the biggest concerns about the infestation are from an area from Hope to Cooperstown and south to Sanborn. This area is just to the west and north of the worst infestations found during the 1997 season.  There are reports of SF midge the whole length of the valley, including southern Manitoba, but many of the reports indicate tolerable levels of larvae with infested buds concentrated on field margins.

    It is still too early to assess the level of damage these infestations may inflict on the plants. Variables that influence the level of injury include the number of larvae present, bud size at the time of infestation, and sunflower hybrid being grown. Last year, many fields that displayed damage symptoms still yielded respectably.

    Other impacts the midge had on plants last year were delayed flowering and destruction of ray petals. In fields where injury was concentrated on the ray petals, fields did not take on the normal yellow appearance we expect when flowering is underway. Closer inspection revealed that the florets in the face of the flower sustained less damage and seed were produced in the flower. The important message is not to judge too hastily the potential losses.

    It is time to inspect flowers for the presence of larvae, determine if infestations are confined to field margins or throughout the field, and begin to assess extent of damage (scarring in bract areas only, feeding throughout the bud with floret damage, severe cupping of heads).

 

RED SUNFLOWER SEED WEEVIL AROUND THE CORNER

    Early reports of red seed weevil activity in the state. Adults could be seen in many fields in the southeast corner of the state. These fields were from early to late bud stage. Reports from around the Grand Forks area indicate some treating in early blooming, confection fields.

Treatment Threshold - Confection sunflower

    The economic threshold for red sunflower seed weevil on confection sunflower is based on the need to keep seed damage below the 3 to 4 percent industry standard. Assuming confection sunflower contains 800 seeds per head, the number of damaged seeds per head would therefore need to be kept below 24 to 32, to remain below the industry standard of 3 to 4 percent seed damage. Research on oilseed sunflower indicates that for each weevil sampled in the early bloom stage, 27 damaged seeds resulted. This suggests about one weevil per head as an economic threshold for red sunflower seed weevil on confection sunflower.

Calculating Economic Thresholds for the Red Sunflower Seed Weevil-Oilseed sunflower

    To decide whether to use an insecticide treatment to control red sunflower seed weevils, it is necessary to determine the economic threshold for this year. The economic threshold (ET) for red sunflower seed weevil depends on the following variables:

a. the cost of insecticide treatment per acre;
b. the market price of sunflower in dollars per pound;
c. the plant population per acre.

Cost of Insecticide Treatment

ET =


Market Price x 21.5 ((0.000022 x Plant Pop.) + 0.18)

 

 

 

Red Seed Weevil Economic Thresholds
(Weevils per head)

Plant population = 18,000 per acre

Market
Price ($)

Treatment Cost ($/A)

6.00

7.00

8.00

9.00

10.00

11.00

0.09

5

6

7

8

9

10

0.10

5

6

6

7

8

9

0.11

4

5

6

7

7

8

0.12

4

5

5

6

7

7

0.13

4

4

5

6

6

7

0.14

3

4

5

5

6

6

0.15

3

4

4

5

5

6

Price for Oilseed Sunflowers = $0.12

Plant
Population

Treatment Cost ($)

6.00

7.00

8.00

9.00

10.00

11.00

17,000

4

5

6

6

7

8

18,000

4

5

5

6

7

7

19,000

4

5

5

6

6

7

20,000

4

4

5

6

6

7

21,000

4

4

5

5

6

7

22,000

4

4

5

5

6

6

23,000

3

4

5

5

6

6

24,000

3

4

4

5

5

6

25,000

3

4

4

5

5

6

Timing treatments

    Sunflower plant stage is used to time insecticide treatment. The ideal plant stage to treat is when most plants in the field are at 40 percent pollen shed. However, we recommend that treatment be considered when more than half of the plants in the field are just beginning to show yellow ray petals to 30 percent of the heads shedding pollen and the rest of the plants in the field are still in the bud stage. This difference between the ideal plant stage (40 percent pollen shed) to treat and the earlier plant stage (just beginning pollen shed) is based, in part, on the fact that aerial applicators - because of a busy schedule or adverse weather - will not always be available to spray at the ideal stage of sunflower development. The consideration of treatment at the early bloom stage should allow growers a sufficient cushion of time to have their fields treated. Growers must be aware, however, that if weevil populations are high and/or spraying is done too early, a reinfestation may occur and a second insecticide application may be necessary.

    Although insecticides applied to sunflower at the bud stage will kill weevils, treatments at that stage are not economical or effective because (1) seeds have not developed to a stage suitable for oviposition, (2) eggs within the weevil are not mature, and (3) adult weevil emergence is still continuing. Sunflower normally reaches the bud stage in late July at which time only about 30 percent of the weevils in the soil have pupated and emerged. Most weevils emerge from the soil by the first week of August. If growers were to spray bud stage sunflower in mid to late July, a second spray may be necessary as more weevils continue to emerge.

 

CORN BORER UPDATE

    Many treatment decisions are being made this week around the state. In the southeast, treatment is underway in field corn. Fields farther north, treatments are being considered. In the Missouri River area, treatments were begun last week. Reports indicate the oldest larvae are reaching the 3rd instar and preparing to tunnel into the stalk. Most larvae are in the 1st and 2nd instars. Egg masses can still be found, but several people have commented that their numbers appear to be declining. Not every field has treatable numbers of larvae, but decisions should be possible this week.

    The next major challenge will be to monitor the August flight of moths to determine if any significant infestations result.

Phillip Glogoza
Extension Entomologist


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