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ISSUE 12  July 18, 2002

 

SOYBEAN APHID: EASIER TO FIND IN EASTERN COUNTIES, NUMBERS STILL LOW

Soybean aphid are becoming easier to find in fields, especially in the southern valley. Field scouting outside the valley has not found any soybean aphids, yet. Where aphids have been found, the percent plants infested remains low. Colonies are still relatively small, averaging around the 10 to 15 aphid range. Occasionally, a lone plant with 200+ aphids may be found. There have not been any reports that indicate the need to treat in North Dakota.

Though we are not past the risk window, the longer the populations stay small, the better we certainly are. Soybean aphid populations have remained low in soybean production areas south and east of the region. Numbers are still increasing and treating may still occur. Continue to scout fields for the next two weeks.

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SUNFLOWER INSECT FORECAST ?

As sunflowers approach late bud stages and some flowering, field inspections for two key head feeding insects should begin. The Banded sunflower moth and the red seed weevil were both present last season in significant numbers. We would anticipate problems again this year.

Last season, these two insects had the largest populations and the greatest impact on sunflower in the southeast quarter of North Dakota. The National Sunflower Association coordinated survey in assessed insect damage associated with these two pests.

The following maps provide an overview of seed damage observed in surveyed fields. This summary is based on a 100 seed sample from each field location (Source: Dr. L. Charlet, Entomologist, USDA, ARS-Fargo).

Banded moth damage was found throughout the state, but the greatest damage was found in the southeast and areas near Devils Lake. This map summarizes damaged seed found on the field edge (within 50 feet of the border). Damage levels decreased the further into the field samples were taken. This helps illustrate the nature of banded moth infestations and the edge effect that often occurs as moths migrate to new fields.

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The red seed weevil was also present in the greatest numbers in the southeastern areas. However, their impact was significant across all the southern counties. Field scouting for seed weevils should be high on the list when flowering gets underway.

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Banded Sunflower Moth

The banded sunflower moth, Cochylis hospes, is a small, straw-colored moth about 1/4 inch long with a wing span of about inch. It has a brown triangular area in the median portion of the forewing.

Banded sunflower moths begin to emerge from the soil about mid-July and are present in the field until mid-August. Although some moths are in the sunflower field during the day, many rest in vegetation along field margins. At twilight, females move into the field to deposit eggs. Moths flutter from plant to plant but do not feed.

Moths lay eggs on the bracts of the sunflower heads in the late bud stage. Females oviposit more eggs on pre-bloom to bloom stage sunflower heads (R4-5) than on early bud (R2-3) or post-bloom (R6) sunflower heads. The majority of eggs are deposited on the outer whorl of bracts, and some eggs are laid on the underside of the sunflower head.

Newly emerged larvae are usually found on the bracts later moving to the disk flowers where they feed on pollen. Third and later instars tunnel through the disk flowers and feed on young developing seeds. As the seeds mature and harden, larvae chew into the seeds to feed. Each larva penetrates and consumes the contents of several seeds. The maximum density of larvae in the sunflower head occurs in mid-August. After feeding to maturity, larvae drop to the ground and spin cocoons in the soil where they pass the winter.

Banded sunflower moth larvae normally consume the entire kernel, whereas seed weevil larvae consumes only about one-third of the kernel. Also, the exit hole in the seed created by the banded sunflower moth is slightly larger than the one made by the seed weevil larva and is usually located on the top rather than on the side of the seed.

A treatment guideline of 1 moth per 2 plants when scouting at dusk or later has been used for a number of years.

Insecticides registered for managing these head feeding sunflower insects include Asana XL, Baythroid, Lorsban 4E, ethyl and methyl parathions, Scout X-TRA, and Warrior.

Red Seed Weevil Management in Oilseeds

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

ET =

                        Cost of Insecticide Treatment                   

Market Price x 21.5 ((0.000022 x Plant Pop/n) + 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

0.07

7

8

9

10

12

13

0.08

6

7

8

9

10

11

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

Price for Oilseed Sunflowers = $0.09

Plant Population

Treatment Cost ($)

6.00

7.00

8.00

9.00

10.00

11.00

17,000

6

7

7

8

9

10

18,000

5

6

7

8

9

10

19,000

5

6

7

8

9

10

20,000

6

7

8

8

8

9

21,000

5

6

6

7

8

9

22,000

5

5

6

7

8

9

23,000

5

5

6

7

8

9

24,000

4

5

6

7

7

8

25,000

4

5

6

6

7

8

Sunflower plant stage is used to time insecticide treatments. For oilseeds, 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. 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.

Insecticides registered for seed weevil control inlcude Asana, Baythroid, Lorsban, Methyl parathion, 6-3 parathion-methyl parathion, Scout X-TRA, Warrior.

Confection or Hulling Market

The banded moth, seed weevil, and the Lygus bug have all impacted quality of these sunflowers the past three to four seasons. 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 be planning now for treatment schedules. When flowers begin blooming across the region, competition for access to aerial applicators increases.

Reducing Insecticide Rates or Volumes ?

Insecticide labels have recommended rates and volumes that users should follow. The rates are based on trials and have been set to provide consistent results over a range of conditions. For many of our sunflower head feeding insects, the best field results have often been obtained with mid to upper rate ranges.

Volume is also important. When reducing the total volume of spray per acre there is the risk of reducing the effective coverage of our treatment target. If the droplet coverage on the plant surface is reduced, there is less chance of contacting an insect during the application and with droplets leaving residual for insects to contact later. Reduced volume further risks evaporation of the spray under hot dry conditions, limiting the amount of product that ever reaches the target.

 

UNIVERSITY OF MINNESOTA APHID ALERT: APHID SITUATION, JULY 12

(the following information is from Dr Ted Radcliffe, Entomologist, U of MN)

No green peach aphid have yet been captured in 2002 in traps at any location in the Aphid Alert Network. However, potatoes are abundantly colonized by green peach aphid at Rosemount (25 mi south of St. Paul campus of the University of Minnesota) as of June 30. Spring 2002 has now seen six major wind events (low level jets) likely to have brought green peach aphid to the Northern Great Plains. We encourage potato growers, especially seed potato producers, to closely monitor their fields and apply effective aphicides at first detection of green peach aphid colonization. Some buckthorn aphid and potato aphid have been captured at Manitoba locations. Low numbers of bird cherry-oat aphid have been caught at some locations in Minnesota and North Dakota. Previous observations suggest that bird cherry-oat aphid is an important PVY vector in the Northern Great Plains. Turnip aphid colonies are developing on canola and wild mustard. Turnip aphid has been implicated as PVY vector.

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Phillip Glogoza, Extension Entomologist
pglogoza@ndsuext.nodak.edu


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