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ISSUE 12   July 30, 2008

TIME TO SCOUT FOR RED SUNFLOWER SEED WEEVIL

Red sunflower seed weevil are emerging and can be found in R4 and early blooming sunflower fields in the major sunflower producing areas. The map below illustrates the "hot spots" of the red sunflower seed weevils from the 2007 Sunflower Survey in North Dakota (sponsored by the National Sunflower Association). Last year, populations were lower than 2006 and sunflower fields in the south central, central, east central and north central regions had high levels.

Sunflower survey map

Identification: Adults are small, about inch and reddish-brown in color. Newly emerged adults feed on the bracts, sunflower buds, and pollen. Peak emergence is usually in late July and early August. The female seed weevil must feed on pollen for fertile egg development.

Adult red sunflower seed weevil
Adult red sunflower seed weevil

Scouting: 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. Field scouting for adults should begin when plants are showing yellow ray petals (R5.0) to 30% of the head shedding pollen (R5.3), and should continue until most of the plants have reached 70% pollen shed (R5.7). A plant that has reached R5.7 has few seeds still suitable for red seed weevil egg laying and should no longer be susceptible to further significant damage.

Seed Weevil Damage: Research reveals that most seeds are only partially consumed or destroyed by the seed weevil larvae (see photograph) and that damaged seeds have lower oil content than undamaged seeds. The economic loss caused by larval feeding includes the loss of both seed weight and oil content.

Damaged sunflower seed
Damaged seed from larval feeding of red sunflower seed weevil.

Economic Threshold:

Oilseed Sunflower . . . The threshold can be calculated using the following formula:

Threshold (Weevils per head) =

Cost of Insecticide Treatment

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

The table below lists the economic threshold values based on 0.26 cent per pound for oilseed sunflowers. The current high prices for oilseed sunflowers has lowered thresholds for red sunflower seed weevil! Typically, we are at 6-8 weevils per head for the economic threshold; however, thresholds are only at 2-3 weevils per head this year!

Economic Threshold Values for Red Sunflower Seed Weevil in Oilseed Sunflower

Price for Oilseed Sunflowers = 26 cents per pound

Plant population plant/acre

Treatment Costs ($/acre)

$6.00

$7.00

$8.00

$9.00

$10.00

$11.00

17,000

1.9

2.2

2.6

2.9

3.2

3.6

18,000

1.9

2.1

2.5

2.8

3.1

3.4

19,000

1.8

2.0

2.4

2.7

3.0

3.3

20,000

1.7

2.0

2.3

2.6

2.9

3.2

21,000

1.7

1.9

2.2

2.5

2.8

3.1

22,000

1.6

1.8

2.2

2.4

2.7

3.0

23,000

1.6

1.8

2.1

2.3

2.6

2.9

24,000

1.5

1.7

2.0

2.3

2.5

2.8

25,000

1.5

7.0

2.0

2.2

2.5

2.7

CONFECTION sunflowers: The economic threshold is only ONE WEEVIL PER HEAD.

Insecticide Spray Timing: Insecticide spraying is targeted at the adult weevil to prevent egg laying. The best time to treat is when more than half of the plants in a field are beginning to show yellow ray petals (R5.0) to 30% of the head shedding pollen (R5.3) and the rest of the plants in the field are still in the late bud stage. 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. Considering treatment at the early bloom stage is the optimal insecticide timing for efficacy and should allow growers a sufficient 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 re-infestation may occur and a second insecticide application may be necessary. Banded sunflower moth and red sunflower seed weevil can both be controlled with one well-timed spray, usually near 10% of the head shedding pollen or R5.1 (see photograph). Fields should always be scouted for both insect pests.

Sunflower in R5.1 stage
Sunflower in R5.1 stage.

 

AVOID SPRAYING TOO EARLY FOR BANDED SUNFLOWER MOTH

Although sampling is conducted for eggs or adult moths, insecticide treatment is directed primarily at egg hatch (eggs turn yellow-orange prior to hatching) and the larval stage of the banded sunflower moth. Eggs (see photograph) are present through early August and hatch about five to eight days after being deposited. Females preferentially deposit more eggs on midsized buds (R3) than smaller or larger buds. Very few eggs are laid on plants at pollen shed (R5.1) and later. Most eggs are laid singly or in small clusters, and are deposited on the outer whorl of bracts. After hatching, larvae feed on florets preventing pollination and reducing the number of seeds the sunflower head produces. Once the larvae reach the third instar, they feed on seeds, usually consuming the entire kernel. The larva typically enters near the top of the seed and leaves by way of the same puncture after eating the kernel. Each larva typically consumes the kernels of six to seven seeds in addition to the early feeding that reduces seed number.

Sunflower moth eggs
Close-up of banded sunflower moth eggs.

Typically, the best sunflower plant stage to treat is the 5.1 growth stage (see photograph in previous article), or when pollen shed is just beginning. This is the time when most banded sunflower moth eggs have hatched and larvae are present, but before the head has seeds forming. At this time, the larvae are beginning to feed on the disk flowers, are exposed on the head and are susceptible to the insecticide treatment. Application at an earlier growth stage may be warranted if monitoring reveals earlier than normal egg-laying activity. On older plants where the seeds have started maturing, most larvae will be feeding in the seeds or underneath the florets, and will be protected from the insecticide. By then, much of the feeding damage already has occurred.

Remember to apply insecticides early in the morning or late in the day to minimize the adverse effect of the chemical on bees and other pollinators.

 

SECTION 24C FOR MUSTANG MAX AND GRASSHOPPER CONTROL IN FLAX

A Special Local Need (Section 24c) was registered for Mustang Max EC InsecticideŽ from FMC Corporation for control of grasshoppers and other insects in flax. Active dates are July 24, 2008 through September 1, 2009. The recommended use rate is 4 fl. oz. per acre. Do not apply within 7 days of flax harvest.

 

SOYBEAN APHIDS CONTINUE TO INCREASE

Soybean aphid numbers are increasing in the Red River Valley area and other eastern counties of North Dakota (see map). Most fields have higher counts than the previous week. Some fields are approaching or at the economic threshold level, and soybean fields in critical reproductive stages (R1-R3) will need to be treated. If weather conditions continue to be favorable (low to mid-80s F) for aphid reproduction, populations will continue to increase and scouting is crucial.

Soybean aphid map
Soybean aphid map from USDA Public PIPE website:
http://sbr.ipmpipe.org/cgi-bin/sbr/public.cgi

Janet Knodel
Extension Entomologist
janet.knodel@ndsu.edu


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