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ISSUE  10   July 12, 2007

SCOUT FOR EGGS OF BANDED SUNFLOWER MOTH IN SUNFLOWER

Eggs of banded sunflower moth (BSM) have been observed in R1 stage(miniature floral head) sunflowers located in Prosper (Cass County) and Finley (Steele County). Female adults typically prefer more mature sunflowers in the R2-R3 crop stage (immature bud elongates 0.5 to 2.0 cm above the nearest leaf, yellow ray petals not visible) for egg-laying. However, females will lay eggs in more immature fields if R2-R3 fields are not available. Pheromone trap catches indicate peak trap catches of adult moths in the southeast region this past week. In the northern tier, flights of adult BSM have started.

Banded sunflower moth adult

Egg Sampling:

The potential for banded sunflower moth damage is determined by counting eggs (see photo) on the outer layer of floral bracts in the field. Because the eggs are very small a magnifier is needed to accurately count eggs.

Banded sunflower moth eggs

We recommend using a head-mounted 3.5X magnifier to leave both hands free for moving the bud around. Egg counts are usually made when most of the plants in the field are at plant stage R3 (distinct bud elongated ¾ inch above the nearest leaf, yellow ray petals not visible). However, this year scouting should start when adult moths are increasing. Egg sampling steps include: 1) Divide each side of the field into two sections, 2) Sample the center of each section at 20 feet into the filed from the field edge, 3) Randomly select five buds, 4) From each bud, randomly select six bracts from the outer whorl and count the eggs on each bract, and 5) Average the egg counts from the five buds. Next, calculate the economic injury level. The economic injury level (EIL) is the density or number of insects expected to cause damage that is equal to the cost of control. For BSM, the EIL is the number of eggs per 6 bracts and considers treatment cost ($/acre), market price ($/lb), and plant population per acre.

EIL =     

Treatment Cost ($)

Market Price ($) x Plant Population x 0.00078

For example, an insecticide spray at $8.00 per acre, plant population of 20,000 per acre, and market value of $0.16 per acre, the EIL would be 3.6 eggs per six bracts. A calculator on the NDSU Department of Entomology Web site is available for automatically calculating the egg EIL - http://www.ndsu.nodak.edu/entomology/ext.htm. For more information, please see Extension Bulletin E823 Banded Sunflower Moth.

http://www.ag.ndsu.edu/pubs/plantsci/pests/e823.pdf

 

UNIVOLTINE EUROPEAN CORN BORER EMERGING

It’s time to scout non-Bt corn fields for European corn borer eggs and larvae (see photos).

European corn borer eggs
European corn borer eggs

European corn borer larvae
European corn borer larvae

European corn borer adult
European corn borer adult

Look for eggs masses on the underside of leaves and early signs of larval feeding (irregular pinholes in corn leaves or whorl). The best time to apply insecticide is when larvae are in the early instars, only a 7-day window, before it enter the corn stalk. Once the larvae is inside the corn plant, no insecticides can kill it. Only 25 - 50% of the univoltine-type European corn borer moths have emerged in northern tier of North Dakota, while 75 - 90% have emerged in the southern tier of North Dakota (see map).

European corn borer map

The table below summarizes the degree day (Base 50 F) model for moth emergence of the univoltine-type European corn borer.

Accumulated Degree Days

Proportions of Emerged Moths

911

10 %

986

25 %

1078

50 %

1177

75 %

1274

90 %

To continue monitoring degree day accumulation in your part of the state, refer to NDAWN, select Applications, then select Corn Degree Days for the nearest town, and enter 2007-03-01 for planting date to accumulate degree days for corn borer.

http://ndawn.ndsu.nodak.edu/corndd-form.html

With high temperatures, heat units will accumulate faster resulting in faster insect development. Control should be considered in field corn when 45 - 50 percent of the plants in dryland corn or 25 - 35 percent of the plants in irrigated corn have shot-holing in the whorl, egg masses on the undersides of leaves or live borers visible in whorls. Specific economic thresholds (corn borer/plant) are available that factor the crop value and cost of insecticide. Please see the following weblink:

http://www.ag.ndsu.nodak.edu/aginfo/entomology/entupdates/ecb/mngborer.htm

 

BERTHA ARMYWORM TRAPPING IN CANOLA

Cumulative trap catches for Bertha armyworm are still below <300 moths for most canola producing areas (see map).

Bertha armyworm trap map

This indicates a "low" risk of larval infestation. However, two sites in Cavalier County are approaching >300 indicating an "uncertain" risk level. In this case, infestations may not be widespread, but fields that were attractive to egg-laying females could be infested. Fields should be monitored about two weeks after peak trap catch and scouted regularly for larvae. Continue scouting until an economic threshold is reached or the crop is swathed. With the high price of canola at $16.4 per CWT and insecticide spray costs averaging around $6-10 per acre, the current economic threshold can be lowered to 10-16 larvae pe square yard. Fields above the economic threshold level should ideally be sprayed once the egg hatch is complete and when larvae about ½ inch long are young. Since larvae are active feeding at night, apply a well-timed insecticide in early morning or late evening. Use high volumes of water for good coverage of the dense canola canopy. Avoid spraying fields below the economic threshold level to avoid killing natural enemies of Bertha armyworm, which keeps it ‘in check’ most years.

 

PEA APHIDS IN PEAS

Pea aphids are increasing in fields in north central region of North Dakota; fortunately, most of the fields will be harvested within the next several weeks. No treatment is recommended when pods are close to pod fill and maturity. Pea aphid impact yield primarily in the flowering to early pod set causing reduced seed formation and smaller seed size. Protein and other quality factors are not impacted by pea aphids. The following information was extracted from ‘Manitoba Insect Updates’ from July 3, 2007 (J. Gavloski).

Scouting Tips. Tapping plant tips over a tray is an easy and quick means of determining approximately how many aphids per stem tip are on a plant. If you are tapping several stems at a time, divide the number of aphids found by the number of stems being tapped. Also remember when you do these counts that the thresholds are the average number of aphids per stem (so you also have to counts the stems that don't have any on them). You will almost always find some clusters of aphids if you look hard enough in a pea field, but it is the average that is important. If doing counts per plant tip, the economic threshold is 2-3 aphids per 20 cm (8-inch) plant tip on average.

Economic thresholds have also been developed so counts from sweep net sampling can be used as economic thresholds. If, at the beginning of flowering, there are on average 9 to 12 aphids per sweep (90-120 per set of 10 sweeps) an insecticide application when 50% of plants have produced some young pods would be cost-effective.

The following table relates the yield loss in peas for average aphid counts from 1 to 8 per 20-cm tip of a field pea stem when about 25 % of the crop has begun to flower.

Aphid per tip

% Yield Loss

1

3.4

2

4.9

3

6.1

4

7.1

5

8.0

6

8.8

7

9.6

8

10.3

 

SOYBEAN APHIDS STILL LOW

Soybean aphid can be found in soybean fields in the eastern tier of North Dakota now. Populations are still below economic threshold levels (250 aphids per plant). Avoid early spraying for sub-threshold populations of soybean aphids. Research from several states has demonstrated no yield benefits from spraying early with low populations (<250 aphids per plant). In fact, early spraying can kill the beneficial insects, which help control low aphid populations. Lady beetles, Aphid lions, and Syrphid fly play a major role in reducing aphid populations. When natural enemies are present in large numbers, farmers are discouraged from spraying fields. Continue to scout soybean fields.

 

WHEAT MIDGE EMERGENCE ENDING

The hot temperatures this past week has accelerated degree day accumulations and crop / in sect development. Most region of North Dakota are past 1600 DD when 90% of the female wheat midge have emerged (see map).

Wheat midge map

If moist and humid conditions continue, adult wheat midge may continue to survive for the next week.

Janet Knodel
Extension Entomologist
Janet.Knodel@ndsu.edu


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