NDSU Crop and Pest Report

Entomology


ISSUE 5   June 3 2004

ALFALFA WEEVIL MANAGEMENT:

Slow start with cooler weather

Southwest North Dakota has had recent problems with infestations of alfalfa weevil. A key points that needs to be emphasized for this insect pest is early scouting . . . well before leaf damage becomes visible from the road.

There are temperature, or degree day, models that have been developed for alfalfa weevil predictions, though they have not been evaluated in our state. The alfalfa degree day guidelines used by Iowa State University are based on a 48EF base temperature. Remember, degree days are used to predict when important biological events are likely to occur. With these events predicted, we should be able to better time field scouting to determine insect populations and have confidence in knowing where we are in the development of the population.

Life Stage

Activity

Degree Days

Egg hatch

 

300

1st - 2nd instar

Light leaf feeding

301 - 438

3rd - 4th instar

Major leaf feeding

439 - 595

pupa - adult

Feeding stops

596 - 810

Begin scouting for alfalfa weevil larvae when 250 DD have accumulated from January 1 (we use March 1 as a practical starting date for our insect models). Currently, we are approaching this number in the southwest portions of the state where weevils have been a problem for the past two years. Monitor tip injury to assess infestation levels and damage. This method is relatively simple and provides adequate estimates for the pre-harvest damage potential from alfalfa weevil when planning management decisions.

Select 50-100 alfalfa stems, (10 to 20 randomly selected stems from each of 5 locations) and examine for signs of feeding damage in the leafbuds and growing tip leaves. Divide the number of stems with recent tip injury by the total stems collected, convert to a percent, and compare with the threshold.

Many of the problems in most years are with weevils surviving the first cutting. It is important, when cutting alfalfa that has weevil larvae feeding, to assess the need for post-harvest weevil management. Monitor regrowth for potential stubble infestations, particularly beneath windrows. After the hay has been picked up, sample the stubble and early regrowth in 20 one square foot samples, 4 chosen randomly from 5 locations. When regrowth after harvest is sufficiently tall, go back to monitoring tip injury.

Insecticides labeled for alfalfa weevil include Baythroid, carbaryl (Sevin), Furadan, Imidan, Lorsban, malathion, methyl parathion (including Penncap M), Mustang, permethrin (Ambush and Pounce), and Warrior.

Alfalfa Weevil thresholds

Before 1st Cutting

35% (weak stand) plants with feeding damage

40% (vigorous stand) plants with feeding damage and/or 2 live larvae/stem

After 1st Cutting in stubble

8 or more larvae/ft2, (6/ft2 on sandy soil); or larvae are suppressing regrowth

 

HESSIAN FLY ISSUES


Hessian fly adult female on wheat stem

Hessian fly problems occurred in some limited areas of North Dakota during the 2003 production year. It will be interesting to observe whether any of the problems repeat themselves in 2004.

Two areas of the state reported plant lodging in September during wheat harvest. They were located in the northern counties. In addition, some later planted research plots near Fargo had significant infestations as well.

Why Hessian fly in these areas when this insect has not been a serious problem for a number of years? Well, like the wheat midge, if conditions are favorable they will take advantage of it. One suspected reason is the rainfall that occurred in August and September of 2002 (for rainfall accumulations see the following two maps). There were two areas where rainfall accumulations were significant that resulted in poor harvest conditions and lots of volunteer plants . . . right when the final generation of Hessian fly adults would have been emerging, laying eggs, and hatching larvae were feeding.


"Flaxseed" or pre-pupa of the
Hessian Fly on wheat straw

Why Hessian fly in these areas when this insect has not been a serious problem for a number of years? Well, like the wheat midge, if conditions are favorable they will take advantage of it. One suspected reason is the rainfall that occurred in August and September of 2002 (for rainfall accumulations see the following two maps). There were two areas where rainfall accumulations were significant that resulted in poor harvest conditions and lots of volunteer plants . . . right when the final generation of Hessian fly adults would have been emerging, laying eggs, and hatching larvae were feeding.


Will Hessian fly be an issue this year? It is not known at this time. As part of the North Dakota Extension Service IPM crop and pest survey, we plan to sample plants from selected areas. This should help determine if Hessian fly are still with us or gone due to drier conditions which prevailed last August -September.

Unfortunately, there is no control for Hessian fly with a post-emerge insecticide program. In some areas of the country, they have looked at the seed treatments, such as Gaucho, as a control tactic, though it would be hard to justify a preventive program here with the problem still being rare. The BEST management strategy is still the avoidance of having suitable plants available for the flies to lay eggs on in the fall through either delayed planting or destruction of volunteers.

Phillip Glogoza
Extension Entomologist
pglogoza@ndsuext.nodak.edu

 

SUGARBEET ROOT MAGGOT: POPULATIONS DEVELOPING SLOWLY

Soil temperatures began warming up slightly ahead of normal this spring; however, the cool temperatures and rainy weather that have predominated throughout the Red River Valley during the past couple of weeks have likely slowed development of the overwintered sugarbeet root maggot populations. For several years, NDSU has been collecting root maggot emergence and adult fly activity data and correlating it back to soil and air temperature accumulation to develop a predictive model.

Peak emergence of flies from soil in previous-year beet fields typically occurs after an accumulation of at least 450 soil degree-day (DD) units. Peak fly activity in current-year sugarbeet fields on the first warm (80 degrees Fahrenheit or above) day following the accumulation of 600 air DD. Current DD accumulations are presented in the following table:

Degree-day (DD) accumulations for sugarbeet root maggot development in selected Red River Valley sites as of May 31, 20041

Site

Soil DD

Air DD

Soil Temp.

Cavalier

214.27

206.72

57.08

Fargo

280.90

338.06

58.04

Grand Forks

269.32

267.29

58.21

Hillsboro

191.84

293.23

56.50

Perley

230.31

313.56

57.35

Sabin

240.76

345.96

57.35

St. Thomas

261.59

231.36

56.77

1Data compilation by Dr. Robert B. Carlson,  Emeritus Professor , NDSU Entomology Dept.

These accumulations, combined with the anticipated daily accumulation rates from the extended weather forecast, suggest that peak activity is not likely to occur before mid-June, irrespective of location within the Valley. Temperatures during the next couple of weeks will dictate when the exact peak occurs. Bottom line: growers in high-risk areas need not rush to apply postemergence insecticides. Waiting until at least the second week in June will likely be a prudent decision.

Mark Boetel
Research & Extension Entomologist
mboetel@ndsuext.nodak.edu


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