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ISSUE 11   July 22, 2010

GET READY TO SCOUT FOR RED SUNFLOWER SEED WEEVILS

Adult red sunflower seed weevils have been observed in the southeast region of North Dakota. These fields are R3 to R4 and will be blooming by the end of the week. Red sunflower seed weevil adults (Fig. 1) are reddish brown and ⅛ inch long. Adult weevils begin to emerge in mid-July and continues through mid-August. Peak emergence occurs in late July. Start counting adult seed weevils when the yellow ray petals are just beginning to show. Counts should continue until the economic threshold level has been reached or most plants have reached 70% pollen shed. A plant that has reached 70% pollen shed has few seeds still suitable for red seed weevil egg laying. Fields where most plants are at the 70% pollen shed stage should no longer be susceptible to further significant damage.

 
Figure 1. Red sunflower seed weevil adult
(Ext. Entomology, NDSU)

When sampling, use the X pattern and begin counting at least 70 to 100 feet into the field to avoid field margin effects. Count the number of weevils on five plants at each site for a total of 25 plants. The ideal plant stage for treatment is when most plants are at 40% pollen shed. However, we recommend that treatment be considered when three out of 10 plants are at 10% pollen shed (or R5.1) (Fig. 2).


Figure 2.
R5.1 or 10% pollen shed on flowering
sunflower head
(L. Charlet, USDA-ARS)

Economic Threshold for Oilseed Sunflowers:

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) is based on: 1) the cost of insecticide treatment per acre, 2) the market price of sunflower in dollars per pound and 3) the plant population per acre. The ET can be calculated using the following formula:

 Oilseed ET (Weevils per head) = 

Cost of Insecticide Treatment

(Market Price x 21.5) (0.000022 x Plant Population + 0.18)

 Tables 1 and 2 show the number of weevils per head (ET) when the cost of the insecticide is at $8 and $10 per acre, respectively.  It looks like the ET is between 4-6 weevils per head this year.

Table 1. Economic Threshold for Oilseed Sunflowers - Number of adult red sunflower seed weevil per head when the cost of control equals $8 per acre.

Market Price

Sunflower Plants per Acre (x 1,000)

$ per lb

14

15

16

17

18

19

20

21

22

23

24

25

0.07

12.4

11.8

11.3

10.8

10.3

9.9

9.5

9.1

8.8

8.5

8.2

7.9

0.08

11.3

10.7

10.2

9.7

9.3

8.9

8.5

8.2

7.9

7.6

7.4

7.1

0.09

10.3

9.8

9.3

8.9

8.5

8.1

7.8

7.4

7.2

6.9

6.7

6.4

0.10

9.5

9.0

8.5

8.1

7.8

7.4

7.1

6.8

6.6

6.3

6.1

5.9

0.11

8.8

8.3

7.9

7.5

7.2

6.8

6.6

6.3

6.0

5.8

5.6

5.4

0.12

8.2

7.8

7.4

7.0

6.7

6.4

6.1

5.8

5.6

5.4

5.2

5.0

0.13

7.7

7.3

6.9

6.5

6.2

5.9

5.7

5.4

5.2

5.0

4.8

4.7

0.14

7.2

6.8

6.5

6.1

5.8

5.6

5.3

5.1

4.9

4.7

4.5

4.4

0.15

6.8

6.4

6.1

5.8

5.5

5.2

5.0

4.8

4.6

4.4

4.2

4.1

0.16

6.5

6.1

5.8

5.5

5.2

4.9

4.7

4.5

4.3

4.2

4.0

3.9

0.17

6.1

5.8

5.5

5.2

4.9

4.7

4.5

4.3

4.1

3.9

3.8

3.7

0.18

5.8

5.5

5.2

4.9

4.7

4.5

4.2

4.1

3.9

3.7

3.6

3.5

0.19

5.6

5.2

4.9

4.7

4.5

4.2

4.0

3.9

3.7

3.6

3.4

3.3

0.20

5.3

5.0

4.7

4.5

4.2

4.0

3.9

3.7

3.5

3.4

3.3

3.1

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 2. Economic Threshold for Oilseed Sunflowers - Number of adult red sunflower seed weevil per head when the cost of control equals $10 per acre.

Market Price

Sunflower Plants per Acre (x 1,000)

$ per lb

14

15

16

17

18

19

20

21

22

23

24

25

0.07

15.5

14.8

14.1

13.5

12.9

12.4

11.9

11.4

11.0

10.6

10.3

9.9

0.08

14.1

13.4

12.7

12.1

11.6

11.1

10.7

10.3

9.9

9.5

9.2

8.9

0.09

12.9

12.2

11.6

11.1

10.6

10.1

9.7

9.3

9.0

8.6

8.3

8.0

0.10

11.9

11.2

10.7

10.2

9.7

9.3

8.9

8.5

8.2

7.9

7.6

7.3

0.11

11.0

10.4

9.9

9.4

9.0

8.6

8.2

7.9

7.5

7.3

7.0

6.8

0.12

10.3

9.7

9.2

8.7

8.3

7.9

7.6

7.3

7.0

6.7

6.5

6.3

0.13

9.6

9.1

8.6

8.2

7.8

7.4

7.1

6.8

6.5

6.3

6.0

5.8

0.14

9.0

8.5

8.1

7.7

7.3

7.0

6.6

6.4

6.1

5.9

5.7

5.4

0.15

8.5

8.0

7.6

7.2

6.9

6.5

6.3

6.0

5.7

5.5

5.3

5.1

0.16

8.1

7.6

7.2

6.8

6.5

6.2

5.9

5.7

5.4

5.2

5.0

4.8

0.17

7.7

7.2

6.8

6.5

6.1

5.9

5.6

5.4

5.1

4.9

4.7

4.6

0.18

7.3

6.9

6.5

6.1

5.8

5.6

5.3

5.1

4.9

4.7

4.5

4.3

0.19

7.0

6.5

6.2

5.9

5.6

5.3

5.1

4.8

4.6

4.5

4.3

4.1

0.20

6.6

6.3

5.9

5.6

5.3

5.1

4.8

4.6

4.4

4.2

4.1

3.9

Economic threshold for Confection or Hulling Sunflower Market

Red sunflower seed weevil control on confection sunflower is based on a need to keep seed damage below 0.5% due to industry standards. Treatment is recommended when 1 to 2 weevils (ET) are found per plant. The banded sunflower moth, seed weevil and Lygus bug have all impacted quality of these sunflowers the past seasons. So, it is recommended at this time that sunflowers grown for these markets be treated a minimum of two times, once at early flowering and again 5 to 7 days later. With this type of program, a window of protection should be provided to minimize impact from all three of these seed damaging insect pests.

Growers should plan treatment schedules early. When flowers begin blooming across the region, competition for access to aerial applicators increases.

 Insecticides registered for control of red sunflower seed weevil are listed in the sunflower section of the NDSU E-1143 2010 North Dakota Field Crop Insect Management Guide.  http://www.ag.ndsu.edu/pubs/plantsci/pests/e1143w1.htm

 

SUNFLOWER MIDGE DAMAGE OBSERVED

It is a good time to get out and inspect sunflowers for the presence of small (about 0.9 inch) cream to yellow sunflower midge larvae (Fig. 3), and to determine if infestations are confined to field margins or throughout the field, and also the extent of the injury (bract only or floret damage).  Heavy infestations of sunflower midge have been reported near Thompson and Grand Forks in Grand Forks County, North Dakota. It may be too early to determine the amount of damage to sunflower heads (Fig. 4), which is affected by the midge densities present, bud size at the time of infestation, and sunflower hybrid being grown.


Figure 3.
Sunflower midge larva (P. Beauzay, NDSU)


Figure 4.
Deformed sunflower head from
sunflower midge damage (G. Endres, CREC)

Remember, insecticides will not control sunflower midge. The best management strategy has been rotation to crops other than sunflower in the vicinity of severe infestations. Staggering planting dates to promote different budding periods between fields aids in reducing risk of damage to all fields in the same geographic areas. Sunflower hybrids have recently been evaluated for their tolerance to sunflower midge. Selecting hybrids for their ability to tolerate infestations should be considered when choosing a hybrid for the upcoming season.

 

SCOUT FOR LYGUS BUGS IN FLOWERING CONFECTION SUNFLOWERS 

Lygus bugs (Fig. 5 & 6) are noted for being a pest of seed production to many crops. Their feeding preference is meristematic tissue or embryonic tissue. Lygus bugs  insert their mouthparts into the host, start a "pre-digestion pump" to inject saliva and start digestion, then suck the fluid into the stomach.


Figure 5.
Lygus bug adult (S. Bauer, USDA-ARS)


Figure 6.
Lygus bug nymph (S. Bauer, USDA-ARS)

This is where the seed injury originates, because the saliva is toxic to plant tissue. This injury causes a brown to black spot (Fig. 7) resulting from tissue death at that feeding site. The damage has been named “kernel brown spot.”


Figure 7.
Kernel brown spot caused by Lygus bug feeding (L. Charlet, USDA-ARS) 

To minimize the damage which results in a quality reduction, a general approach to protecting sunflower from Lygus and other seed-feeding insects is  recommended. Sunflower is susceptible to Lygus damage, from flowering through seed hardening. Lygus can be treated at the same time confection sunflower is treated for other insect pests, such as the red sunflower seed weevil and banded sunflower moth. The action threshold is one adult or nymph Lygus bug per 9 heads. This population level could result in economic loss to the producer through the reduction of seed quality. Two treatments are recommended to sufficiently protect confection sunflower heads from Lygus bug feeding: one application at the onset of pollen shed, or approximately 10% bloom, followed by a second treatment 7 days later. This program should adequately control any insect pests on confection sunflower throughout flowering, minimizing the potential feeding damage. Oilseed sunflower are not believed to be at risk to damage from Lygus feeding at this time.

Janet Knodel
Extension Entomologist
janet.knodel@ndsu.edu


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