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ISSUE 4   June 4, 2009

OPTIMIZING CUTWORM CONTROL

More field observations of cutworm damage were reported on pulse crops in the northwest region, and on sunflower and canola in the southwest this past week. One producer recently found only one dead cutworm after spraying his pea field with a pyrethroid insecticide (source: K. Brown). When cutworm populations are high, it is easy to find many dead cutworms on the soil after spraying. So what is affecting the cutworm control in this situation? It is probably the dry soil conditions. Cutworms typically feed below the moisture line where the dry and moist soil meet. Under very dry soil conditions, cutworms restrict their movement to underground tunneling and feeding only where the soil moisture is greater. When cutworms feed below the soil surface, it is difficult to kill cutworms and plants are cut at varying depths below the soil surface, usually at or below the growing point. In normal ‘moist’ soil conditions, cutworms feed above the ground on the plant foliage and crawl around on the ground , thus exposing them to insecticides through ingestion and contact. Regrowth of cut seedlings is possible in some crops like corn when cutting occurs above the growing point. Neighboring plants can sometimes compensate somewhat for lost or damaged plants.

To optimize control of cutworms in dry soil conditions, insecticide applications should be followed by shallow cultivation to mix the soil and insecticide to the below-ground cutworms if possible. This is not always possible with no-till and minimum till fields, where we are trying to conserve moisture. Cutworm control is most effective when soil conditions are moist. Always apply rescue foliar insecticides at night when cutworms are actively feeding. It is important to remember that seed applied insecticides do not control cutworms.

Some of the cutworms that overwinter as partially grown larvae (caterpillars), such as dingy cutworm, should be completing their development within the next week and are close to 1˝ inches long when mature. These cutworms are primarily done feeding and will enter the pupal stage. The adult moths eventually emerge from the pupae in July. However, the late-season cutworms, like red-backed cutworm (Fig. 1) are just getting started. These cutworms will be small (<˝ inch long) and actively feeding. So, continue to monitor the fields for cutworms until the end of June.


Figure 1.
Red-backed cutworm in corn.
(Photo by J. Knodel)

 

FOLIAR RESCUE SPRAYING FOR CRUCIFER FLEA BEETLE IN CANOLA

The toxicity of seed-applied insecticide treatments for control of flea beetles may be declining if your canola fields were planted in early May. In cool springs, seeds are slow to germinate and grow. For these systemic seed-applied insecticide treatments to work, plants must be actively growing to take up the insecticide. Since flea beetles are becoming more active with the recent warmer temperatures, it is good insurance to scout your canola fields for flea beetle feeding injury (pitting) and the presence of dead flea beetles on soil (if the wind doesn’t blow them away). Remember, flea beetles still need to feed on plants to some extent to ingest the chemical even when the seed-applied insecticide is effective. The economic threshold for applying rescue foliar insecticide sprays is 25% defoliation on cotyledons and first true leaves. After canola reaches the 4-6 leaf stage, plants can usually outgrow flea beetle feeding injury without any yield loss.

For a list of insecticides registered in canola for flea beetle control, consult the 2009 Field Crop Insect Management Guide.

http://www.ag.ndsu.edu/pubs/plantsci/pests/e1143w1.htm

 

CORN ROOTWORM SURVIVAL IN FLOODED FIELDS

Although corn rootworms are not typically a problem on our corn grown in North Dakota, there were some questions this past week about how flooded and saturated soil conditions affect corn rootworm survival. The survival of corn rootworms depends on when the flooding occurs and soil temperatures. Research indicates that when saturated soil conditions exist early in the spring, egg survival is not affected by flooding, especially if the flooded water/soil was cool(<50 F). After larvae (Fig. 2) hatch from eggs, typically in mid- to late June in North Dakota, larvae are susceptible to saturated soils, especially in warm conditions (>77 F). Once the larvae dig into the root tissue, they become more tolerant and can survive short durations of saturated soils. Since most of our flooding occurred early in the spring, corn rootworm was in the egg stage. As a result, we can expect little or no mortality due to the flooded soils. If you have a corn rootworm problem use the recommended soil insecticides or Bt corn rootworm hybrids.


Figure 2.
Larva of corn rootworm.
(Photo by F. Peairs, Colorado State Univ., Bugwood.org)

Janet Knodel
Extension Entomologist
janet.knodel@ndsu.edu

 

WIREWORMS DAMAGE AREA SUGARBEET FIELDS

Damaging wireworm infestations have been observed in central portions of the Red River Valley. In one field, planted using seed treated with Poncho Beta insecticidal seed treatment, plant stands have been reduced by 44%. Although NDSU performance trial data suggests that Poncho Beta provides some wireworm suppression, the infestation in that field is extremely high. Poncho Beta is most likely killing some wireworms, but each feeding larva is able to do some damage before succumbing to the insecticide. Thus, the combined effect of several larvae per plant is apparently resulting in sufficient damage to kill seedlings. Another contributing factor is that plants are unusually small in many fields this year. A common question asked in regard to wireworm damage is "Can an insecticide be applied to save the surviving plants?" In short, there is no proven effective treatment to save the field after planting. If stand is reduced substantially, replanting with an effective at-plant insecticide is the most effective option. In fields where the stand losses are patchy, replanting the affected areas might be sufficient.

 

SUGARBEET ROOT MAGGOT: EMERGENCE DELAYED BY COOL WEATHER

Overwintered sugarbeet root maggot (SBRM) populations in the Red River Valley are expected to emerge abnormally late this year as a result of this spring’s persistent unseasonably cool temperatures. Excessive soil moisture this spring also has contributed to delayed maggot development because wet soils take longer to warm up in the spring.

The NDSU root maggot development model uses soil and air temperatures to predict 2 important events in the SBRM life cycle. Peak fly emergence from soil in previous-year beet fields typically occurs after the accumulation of at least 450 soil degree-day (DD) units. Peak fly activity in current-year sugarbeet fields occurs on the first warm (80 degrees Fahrenheit or above), low-wind day following the accumulation of 600 air DD. Current DD accumulations for representative locations throughout the Valley are presented in the following table:

Degree-day (DD) accumulations for sugarbeet root maggot development as of June 3, 2009

Site

Soil temperature

Soil DD1

Air DD2

Fargo

61

182

350

Hillsboro

66

296

318

Grand Forks

64

283

293

Grafton

60

231

231

Cavalier

64

269

225

Raw data provided by the North Dakota Agricultural Weather Network (NDAWN).
1Target soil DD for peak emergence = 450
2Target air DD for peak fly activity = 600

These data and the extended weather forecast suggest that peak activity is not expected to occur before the third week of June, and it could be delayed as far as the end of the month. Temperatures during the next few weeks will have a major influence on peak activity timing. This means that growers in areas of high risk for damaging maggot infestations should not rush to apply postemergence insecticides. Postemergence granular insecticides should not be applied before the third week of June, and postemergence liquid materials should be timed to occur within 3 or 4 days before or 3 days after peak fly activity.

What about late-planted sugarbeet fields? Many fields in the Valley were planted exceptionally late this year. Late-planted beets are typically smaller and more vulnerable to attack when root maggot larvae begin feeding. Delays in root maggot development may reduce this effect. However, some of the more extremely late-planted fields may be more at risk. This underscores the need for effective postemergence insecticide protection in areas that experience moderate to high root maggot infestations.

Poncho Beta: how will it hold up against root maggots? Over 60% of the Valley’s sugarbeet fields were planted using seed treated with Poncho Beta insecticide. Poncho Beta has provided moderate sugarbeet root maggot protection in NDSU performance trials, but it should not be relied on for stand-alone protection in high-risk areas. Growers in areas of moderate to high risk of damaging maggot infestations should be vigilant about watching their fields for potential flare-ups in fly activity, and plan on applying an additive postemergence insecticide to ensure adequate protection of their crop.

Watch for updates on the root maggot fly forecast and for recommendations for postemergence insecticide application timing in future issues of the Crop & Pest Report. Detailed information can also be found in the "Insect Control" section of the 2009 Sugarbeet Production Guide or the "Sugarbeet Insects" section of 2009 Field Crop Insect Management Recommendations. Online versions of these publications are located at:

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

and

www.ext.nodak.edu/extpubs/plantsci/pests/e1143w1.htm

 

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


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