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ISSUE 12    July 26, 2007

LATE SEASON INSECT PEST UPDATE IN CANOLA

Good news for North Dakota canola producers! So far, larvae of Bertha armyworm and the second generation of diamondback moth have been low (below threshold). Pheromone trap catches are also at ‘low’ risk for Bertha armyworm throughout North Dakota, and diamondback moth are decreasing from peak trap catches last week. The flight of the second generation diamondback moth will be too late for larvae to cause significant injury to flowers or pods in most canola fields. However, it’s always good "common sense" to scout any canola fields for pod-feeding injury from either insect pest, especially in late planted fields. There is localized spraying for Bertha armyworm in canola in Manitoba, Canada this year, which shows that populations can vary greatly within a region as well as between fields within a region. Canola swathing should be getting started in next couple of weeks with the hot weather pushing crop development.

 

RED SUNFLOWER SEED WEEVIL EMERGING

It’s time to get out and scout! Red sunflower seed weevil can be found in R4 and early blooming sunflower fields in major sunflower producing areas - north central, northeast, and southern tier. The map below illustrates the "hot spots" of the red sunflower seed weevils from the 2006 Sunflower Survey in North Dakota. Last year, populations were high in the southwest, south central, central, and southern part of the north central regions. So far, 2007 field reports indicate high populations in the north central and the northeastern (Grand Forks) regions.

Sunflower survey map

Adult red sunflower seed weevil

 

Adults are small about 1/8 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. 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. The easiest method for scouting for adult seed weevils can be done with a can of insecticide/mosquito spray with DEET, such as Deep Woods Off. Spray the sunflower heads and wait 15 or more seconds for the adult weevil to move to the front of the head. Then, count the number of adult weevils on numerous sunflower heads at several locations and calculate the average number of adult weevils per head. If you use mosquito repellent sampling technique, please use the conversion table below to calculate the absolute number of adult seed weevils.

Estimation of absolute red sunflower seed weevil adults when sampling using a commercial formulation of mosquito repellent.

No. counted in the field

Absolute number

No. counted in the field

Absolute number

1

1.4

10

17.8

2

2.9

11

19.5

3

4.4

12

21.3

4

5.8

13

23.1

5

7.3

14

24.9

6

10.7

15

26.6

7

12.4

16

29.3

8

14.2

17

31.1

9

16.0

18

32.9

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 0.20 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, current thresholds are only at 2-4 weevils per head this year!

Economic Threshold Values for Red Sunflower Seed Weevil in Oilseed Sunflower

Price for Oilseed Sunflowers = 20 cents per lb

Plant population plant/acre

Treatment Costs ($/acre)

$6.00

$7.00

$8.00

$9.00

$10.00

$11.00

17,000

2.5

2.8

3.4

3.8

4.2

4.6

18,000

2.4

2.7

3.2

3.6

4.0

4.4

19,000

2.3

2.6

3.1

3.5

3.9

4.3

20,000

2.3

2.5

3.0

3.4

3.8

4.1

21,000

2.2

2.5

2.9

3.3

3.6

4.0

22,000

2.1

2.4

2.8

3.2

3.5

3.9

23,000

2.0

2.3

2.7

3.1

3.4

3.7

24,000

2.0

2.2

2.6

3.0

3.3

3.6

25,000

1.9

2.2

2.5

2.9

3.2

3.5

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. If spraying is done too early, weevils can re-infest a field requiring a second treatment. Banded sunflower moth and red sunflower seed weevil can both be controlled with a well-timed spray, usually near 10% of the head shedding pollen or R5.1 (see photo). Fields should always be scouted for both insect pests.

Sunflower R.5

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

RSSW injury

 

BANDED SUNFLOWER MOTH UPDATE

Adult moth trap captures have continued to increase in Prosper, Fargo, and Mapleton, probably due to the recent hotter weather pushing emergence. Adults live for 7 to 10 days, and will be present for a total of eight weeks due to their extended emergence period. The map below summarizes the peak pheromone trap catches during July. Field spraying has started in many regions including the north central and northeastern regions.

Banded sunflower moth trap catch map

Be sure to continue scouting for eggs of Banded Sunflower moth in sunflower fields. In going around to NDSU Field Days at Carrington, Minot and Langdon this past week, it was easy to find large numbers of egg per heads (over 100 eggs per head in some cases!) indicating high populations of adult banded sunflower moths. It takes about 5- 8 days for the eggs to hatch.  Eggs turn a yellowish color when they are close to hatching. Spraying should target the early instar larvae (2nd instar) about 4 days after egg hatch.  The first instar larvae feed inside the bracts and will be difficult to kill with insecticide, where as the second instar larvae move to the more exposed areas of the developing sunflower head. The current action thresholds are summarized in the table below for number of eggs per 6 bracts using a market value of 20 cents per pound.

Economic Threshold Values for Banded Sunflower Moth

Price for Oilseed Sunflowers = 20 cents per lb

Plant population plant/acre

Treatment Costs ($/acre)

$6.00

$7.00

$8.00

$9.00

$10.00

$11.00

17,000

2.3

2.6

3.0

3.4

3.8

4.1

18,000

2.1

2.5

2.8

3.2

3.6

3.9

19,000

2.0

2.4

2.7

3.0

3.4

3.7

20,000

1.9

2.2

2.6

2.9

3.2

3.5

21,000

1.8

2.1

2.4

2.7

3.1

3.4

22,000

1.7

2.0

2.3

2.6

2.9

3.2

23,000

1.7

2.0

2.2

2.5

2.8

3.1

24,000

1.6

1.9

2.1

2.4

2.7

2.9

25,000

1.5

1.8

2.1

2.3

2.6

2.8

See NDSU Extension factsheet for more information about scouting for banded sunflower moth eggs and pest management.

http://www.ag.ndsu.edu/pubs/plantsci/pests/e823w.htm

 

NEW INSECTICIDE REGISTERED IN NORTH DAKOTA

Dow AgroSciences has released a new insecticide called "Cobalt Insecticide." It is labeled for soybeans, sunflowers, alfalfa, corn, wheat and more crops in North Dakota. It combines two different classes of insecticides: organophosphates (chlorpyrifos) and pyrethroids (gamma cyhalothrin). It provides quick knockdown (contact) and residual insect control. Crop coverage is enhanced through the fuming activity of Cobalt. Cobalt has tank-mix flexibility with other pesticides, nutrients or adjuvants. Please see website and product label for details.

http://www.dowagro.com/usag/prod/031.htm

Note: Mention of any product is not considered an endorsement by NDSU.

Janet Knodel
Extension Entomologist
Janet.Knodel@ndsu.edu

 

LYGUS BUGS IN AREA SUGARBEET

Lygus bugs have recently been found in many Red River Valley sugarbeet fields. Thus far, most infestations are relatively low; however, this week’s hot and dry weather will hasten the drying down of other area crops and small-seeded broadleaf weeds that often harbor Lygus bugs. Lygus bugs have caused significant injury to sugarbeet in late July through mid-August in past years. Therefore, fields should be monitored during the next few weeks to determine if an insecticide application will be needed.

Description. Lygus bug adults are about 1/4 inch long and 1/8 inch wide, and their color can range from dark greenish yellow to a dingy, mottled brown. Most have a pale yellow V-shaped mark near the middle of their back & two faint light yellow patches near their hind end (Fig. 1).

Lygus adult
Figure 1.  Lygus lineolaris adult.

Immature Lygus bugs are called nymphs. They pass through five nymphal molts before reaching adulthood. First-instar nymphs are very small (1/25 inch long), wingless, and look like a large, bright green aphid. Also, they have a faint black spot (actually a scent gland) on the center of their back. Older nymphs have four additional spots (Fig. 2).

Lygus nymphs
Figure 2.  Lygus lineolaris nymphs.

Damage. Lygus adults and nymphs use piercing and sucking mouthparts to feed on the plant. They pierce the plant and inject a salivary toxin which liquifies and kills plant tissue. The bugs then suck up the resulting liquid. Feeding injury in sugarbeet is usually concentrated in new leaves and petioles. Symptoms of Lygus feeding injury include leaf curling and wilting, seepage of a black oil-like exudate (Fig. 3.a.), swollen feeding scars on petioles (Fig. 3.b.), and often a gray to black sooty appearance on new growth near the plant crown. Leaf tip burn can also be a symptom of Lygus feeding injury.

Leaf curling caused by Lygus Lygus feeding injury
Figure 3.a. Leaf curling and black exudate
seeping from Lygus feeding sites.
Figure 3.b. Healed scar from Lygus feeding injury.

Lygus injury is believed to cause plants to respond by using carbohydrate reserves to produce new leaves and stems. This can lead to major sugar yield losses if it occurs later in the season when the reserves should be building up in the root.

Sampling. Sampling should be done with care because adults can fly away and nymphs usually hide or drop from the plant when the canopy is disturbed. Also, young nymphs blend in well with the beet canopy due to their green color, and can be difficult to detect. Threshold: an insecticide application is probably needed if harvest is at least 3 weeks away and if the infestation exceeds one Lygus bug per plant (adults and nymphs combined). An insecticide is not likely to be economically justified if harvest will occur within 3 weeks after the application. For more information on Lygus bug control, consult the Insects section of the 2007 Sugarbeet Production Guide. The online version is at:

http://www.sbreb.org/Production/production.htm

Mark Boetel
Research & Extension Entomologist
mark.boetel@ndsu.edu


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