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ISSUE 13  July 27, 2000

 

BEGIN SCOUTING FOR SUNFLOWER HEAD FEEDING INSECTS

    As the earliest flowers begin blooming, there are reports of red seed weevil moving to fields. Growers in South Dakota have reported some populations in the range of 9 to 13 per flower head. Maybe of greater concern are the reports of banded sunflower moths congregating in field borders. Concerns about BSM have been expressed by Scott Knoke, Benson County extension agent, who is reporting large numbers staging in border grasses around fields in the bud stage.

    The following is a review of management decisions and strategies for these two pests.

Red Seed Weevil
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 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:

a. the cost of insecticide treatment per acre;

b. the market price of sunflower in $ per pound;

c. the plant population per acre.

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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. 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.

 

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 1/2 inch. It has a brown triangular area in the median portion of the forewing.

    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.

Life Cycle

    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 oviposit. Moths flutter from plant to plant but do not feed. The average adult life span is 7 to 10 days.

    Within a week after emergence, the moths begin to 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. Eggs will be present through early August, hatching in five to eight days after being deposited. Eggs are small, cream-colored (newly laid) to a pale-orange color (near hatching), difficult to find, and are not normally sampled.

Damage

    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.

    Seed damage done by larvae of the banded sunflower moth resembles damage caused by the red sunflower seed weevil. However, the banded sunflower moth normally consumes the entire kernel, whereas the seed weevil larva 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.

    In most cases banded sunflower moth larvae will have exited the seeds by the time the heads are harvested. Damage from banded sunflower moth is less likely to be detected by examining harvested seeds. Often the seed kernel is entirely consumed by the larva and the seeds will normally pass through the combine.

Sampling and Treatment Decisions

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

    A sampling strategy based on numbers of moths observed in fields during the day has been suggested since 1995. Sampling sites should be at least 75 to 100 feet from the field margins. In monitoring a field, use the X pattern , counting moths on 20 plants per sampling site for a total of 100 to 200 plants. The economic threshold recommendations (Table 1) are based on sampling conducted during the late morning to early afternoon hours and the number of moths observed at these times. During the day the moths remain quiet, resting on upper or lower surfaces of the leaves of sunflower plants. When disturbed, they flutter from plant to plant.

 

Table 1. Number of banded sunflower moth larvae constituting an economic threshold when the cost of control equals $8.00 per acre.

Market Price ($) per lb

Sunflower Plants per Acre (x 1000)

14

15

16

17

18

19

20

21

22

23

0.07

11

11

10

9

9

8

8

8

7

7

0.08

10

9

9

8

8

7

7

7

6

6

0.09

9

8

8

7

7

7

6

6

6

5

0.10

8

7

7

7

6

6

6

5

5

5

0.11

7

7

6

6

6

5

5

5

5

4

0.12

7

6

6

5

5

5

5

4

4

4

 

    Table 2 summarizes the average adult density based on adult counts made during daylight hours in a field during mid-July (late bud stage) that will result in different densities of larvae per head. The information in the table is based on a multiple year study comparing average adult densities in sunflower fields in July with the final number of mature larvae found in the sunflower heads. The field must be monitored to determine if the adult densities corresponding to the economic threshold (larvae per head) have been reached. The scouting should be conducted prior to the R5.1 plant growth stage. If treatment is warranted, it should be applied at the R5.1 sunflower plant growth stage.

 

Table 2. Adult BSM field populations and resulting larval densities in sunflower heads.

Adults / 100 plants

Adults/no. of plants

Larvae / head

0.6

1 / 167

3

1.0

1 / 100

4

1.4

1 / 72

5

1.8

1 / 56

6

2.2

1 / 46

7

2.6

1 / 38

8

3.1

1 / 32

9

3.5

1 / 29

10

3.9

1 / 26

11

    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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