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ISSUE 13  July 26, 2001

 

CORN BORER NUMBERS ARE UP

Corn borer larval numbers are higher this year than in the previous two seasons. Consultants in the northern counties are even reporting the presence of shotholing with larvae in the whorl. Consultants in the southern counties have reported spray activities since late last week (July 20). The oldest, but still the fewest, larvae being found are in the third instar and are preparing to tunnel. Scouting and making treatment decisions for corn borer is strongly recommended at this time.

There are always questions about performance of insecticides. Bill Hutchison, Entomologist, U of Minnesota, and others have written a summary reviewing the probability and risk efficiency for percent borer control with Bt traits and the four primary pyrethroid insecticides labeled in sweet corn. The insecticides are Baythroid, Capture, Pounce, and Warrior. You can find his newsletter article at:

http://www.vegedge.umn.edu/mnvegnew/vol3/vol3n9.htm

The following excerpt has been paraphrased to highlight some of the key points made regarding borer control:

Key points: For ECB, nearly all products and rates provide >90 or 95% control of ECB. Performance of the Bt sweet corn is very high for both MN and WI, and slightly less for IL (due to higher Corn Earworm and/or fall armyworm pressure). Probability of Control for ECB for any given level of % Control has considerable overlap among all 4 pyrethroids at both "low and high" rates for each.

Hutchison et al., Minnesota Vegetable IPM Newsletter, Vol. 3 No. 9, July 20, 2001

 

CATERPILLARS CONTINUE

The same caterpillars mentioned last week continue to dominate the concerns of the regions farmers. The only new caterpillars to throw into the mix include Cabbage looper and Yellow woollybear.

The cabbage looper is light to dark green with light stripes, one along each side and two along the back. The body is thick but narrows towards the head. These caterpillars have only two paid of fleshy prolegs on the abdominal segments. When they move, they hump up in the middle, resulting in the characteristic looping motion. They have been found in legumes (soybeans, dry beans, chickpeas, alfalfa).

The yellow woollybears have been found but generally at low, non-threatening levels. These caterpillars are covered with dense, soft hairs. The color of the hairs can be variable, ranging from the yellow being seen now to almost black. They are defoliators but rarely reach treatable levels where soybeans are grown.

 

LYGUS IN CONFECTION SUNFLOWER:

Lorsban 4E 2(ee) label for North Dakota and Minnesota

A state label for the use of Lorsban 4E to control lygus in confection sunflower has been issued by Dow Agrosciences for ND and Minnesota. Lorsban is currently labeled for use in sunflower, this label adds lygus to the list of insects controlled.

Directions for use are the application rate of 1 to 2 pints per acre. The label suggests the use of the higher rate where lygus populations are heavy. Application is recommended at the onset of pollen shed or 10% bloom. For best protection , a second application is recommended seven days later. For further details, refer to the complete product label.

 

SUNFLOWER INSECTS TO WATCH . . .

Banded Sunflower Moth Being Reported

As flowers begin to bloom, field scouting for red seed weevils and banded sunflower moth needs to begin. There are already multiple reports and observations of Banded moth beginning to stage in grassy borders around fields in the eastern, central, and northern areas of ND. The issue of insects feeding in confection sunflowers was addressed in NDSU Crop and Pest Report, Issue 11, July 12. The following is a summary of management information for oil seed sunflowers with focus on seed weevil and banded moth.

Calculating Treatment 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:

  1. the cost of insecticide treatment per acre
  2. the market price of sunflower in $ per pound
  3. the plant population per acre

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

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. 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 late 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 late July, a second spray may be necessary as more weevils continue to emerge.

 

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.

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The newly hatched larvae are off-white and 1/16 inch long. The head capsule is dark-brown. As the larva grows, there is a gradual color change to light pink or yellow, then to reddish or purplish and finally to green at maturity. The full-grown larva are about 7/16 inch long.

Banded sunflower moths begin to emerge from the soil about mid-July and are present in the field until mid-August. Adult populations (east central North Dakota) are at their highest levels between July 22 and 25 in normal years. 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. The average adult life span is 7 to 10 days.

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 Asana XL, Baythroid, Lorsban 4E, ethyl and methyl parathions, Scout X-TRA, and Warrior.

Phillip Glogoza
Extension Entomologist
pglogoza@ndsuext.nodak.edu


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