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ISSUE 4    May 26, 2005

TIME TO SCOUT FOR ALFALFA WEEVIL

Degree-day (DD) accumulations in much of North Dakota indicate that alfalfa fields should be monitored during the next couple of weeks for the presence of alfalfa weevil larvae. Egg hatch for alfalfa weevil begins after 300 DD have accumulated in the spring. Fields should also be checked after the first cutting to determine if larval numbers in the regrowth are high enough to require insecticide treatment. To determine DD accumulations for your area, visit the NDAWN web page at:

http://ndawn.ndsu.nodak.edu/

Description & Damage: Alfalfa weevil larvae are legless, have a black head capsule, and range from yellow to light green in color (Fig. 1). When most damage occurs, larvae are typically about 3/8 inch long, plump-bodied, and have a distinct white stripe down the middle of their back. They feed for about 14 to 21 days to complete the larval stage. They emerge as adults after one to two weeks in the pupal stage.


Figure 1

Larval feeding injury first appears as small holes in leaves at or near the growing tip. Leaves take on a shredded, skeletonized appearance as feeding continues, and eventually entire growing tips can be destroyed. Damage becomes progressively more apparent in fields as weevil larvae continue to grow and mature. Severely damaged plants have a bleached, grayish, or frost-injured appearance. Larval damage is most severe to the first crop, although significant damage can occur in the second crop. Adult weevils (Fig. 2) usually affect the second crop more by feeding on developing buds and inhibiting regrowth.


Figure 2

Control: Generally, the first line of defense against alfalfa weevil larvae is promptly cutting the crop. Harvesting is simple, inexpensive, and usually the most efficient tactic for managing alfalfa weevil infestations, especially for the first crop of the season. Many larvae will die of starvation because fresh green buds and foliage are scarce for several days after cutting. Also, harvest exposes them to sunlight and high temperatures that can be especially detrimental to young larvae. Severe infestations may require chemical an insecticide application.

Economic Thresholds: 1) before first cutting ! 35 percent of plants with feeding damage (in a thin stand), 40 percent of plants with damage and/or two live larvae per stem (vigorous stand); and 2) after first cutting ! 8 or more larvae per square foot (6 per square foot on sandy soil) or if larvae suppress regrowth after first cutting.

Table 1 lists several insecticides registered for alfalfa weevil control in North Dakota. Important considerations for insecticide use in alfalfa are: 1) preharvest interval; and 2) hazards to pollinators, especially honeybees. Many insecticides are extremely toxic to bees. They should never be applied if alfalfa is in bloom or high numbers of flowering weeds are present. If an insecticide treatments is planned, notify beekeepers maintaining hives within a three-mile radius of the field so hives can be temporarily covered or moved out of the area. Also, insecticides should be applied when most bees are in hives and not in the alfalfa (8:00 p.m. to 8:00 a.m.). For more information on chemical control of this pest consult the North Dakota Field Crop Insect Management Guide. It is located online at:

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

Table 1. Insecticides labeled for alfalfa weevil control in North Dakota.

INSECTICIDE

DOSAGE
(# AI/acre)

PRODUCT  PER ACRE

PREHARVEST INTERVAL

permethrin          
Ambush 2E
Arctic 3.2E
Pounce 3.2EC

RUP


0.1 - 0.2


6.4-12.8 fl oz
4 - 8 fl oz
4 - 8 fl oz


14 days if above 0.1 lb ai/ac

Baythroid            

RUP

0.025 - 0.044

1.6 - 2.8 fl oz

7 days

carbaryl

1.5

rate varies by formulation

7 days

Furadan 4F        

RUP

0.25 - 0.5

0.5 - 1 pt

7 days at 0.25 lb, 14 days at 0.5 lb

Imidan 50 WP

0.75

2 lbs

7 days

Lannate LV
Lannate SP

RUP

0.9

3 pts 1lb

0 days (cutting), or 7 days (feed/graze)

Lorsban 4E

RUP

0.5 - 1

1 - 2 pts

14 days at 0.5 lb/ac. 21 days if over 0.5 lb

Malathion 57EC

0.9 - 1.25

1.5 - 2 pts

None

Methyl parathion 4EC

0.25 - 0.5

0.5 - 1 pt

15 days

Methoxychlor 2EC

1 - 1.5

2 - 3 qts

7 days

MustangMax

RUP

0.014 - 0.025

2.24 - 4 fl oz

3 days (cut/graze); 7 days (seed harvest)

Proaxis

RUP

0.01 - 0.015

2.56 - 3.84 fl oz

1 day for forage; 7 days for hay

Warrior

RUP

0.02 - 0.03

2.56 - 3.84 fl oz

1 day for forage; 7 days for hay

RUP = Restricted Use Pesticide

 

CUTWORMS DAMAGING SUGARBEETS

Reports of major cutworm activity in sugarbeet fields have been received from the central and southern Red River Valley and the western North Dakota/eastern Montana production area. Fields should be checked often during the next couple of weeks for wilting or dead plants. Inspection consists of lightly sifting through the upper 2 inches of soil around plants and within about 6 inches of the row. Several sites within a field should be inspected. Use rubber gloves when checking insecticide-treated soil to avoid exposure to the chemical. A range of larval sizes are currently present in fields, so smaller larvae may be overlooked during initial field inspections. Also, cutworm larvae may not be readily visible when a field is scouted during the daytime because most feeding activity occurs at night. Larvae can usually be found 1 to 2 inches below the soil surface near the base of wilting plants. They often follow the moisture line in the soil. Young sugarbeet plants are often cut off at or near ground level. During dry weather, larvae mostly feed just below the soil surface as they move along the row. They feed more on above-ground structures during wet weather.

Economic Thresholds: Cutworm control in young beet fields is suggested when 4 to 5% of seedlings are being cut by larvae. In late summer, a population of 3 to 5 larvae per square foot is needed before treatment will be justified.

Chemical control: Insecticides generally require moisture after application for optimal performance. Light rain showers or heavy dew is generally sufficient. Insecticide applications are most effective if applied during late afternoon or early evening. This maximizes the amount of insecticide material present during the first nighttime hours following application (when larvae are most active). Heavy infestations may require a second application. Liquid formulations generally provide better control of cutworms, especially during very dry periods. If a field has crusted over from recent rains, the crust should be broken up before or during insecticide application. Consult the "Insect Control" section of the 2005 Sugarbeet Production Guide for insecticide recommendations. The guide is online at:

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

 

SUGARBEET ROOT MAGGOT FLY EMERGENCE UNDERWAY

Sugarbeet root maggot fly emergence has begun in the Red River Valley. Numbers are very low at this time, and the expected stretch of cool weather during the next few days may slow development of overwintered larvae in the soil. Degree day (DD) accumulations in the area suggest that significant emergence is likely to occur in 10-15 days.

Peak activity of root maggot flies in current-year sugarbeet fields is not expected for a few weeks yet, but it is important to note that a stretch of several hot sunny days can warm the dark Valley soils up and accelerate maggot development. Watch for updates in the next few issues of Crop & Pest Report.

 

POST-EMERGENCE INSECTICIDES NEEDED IN SUGARBEET ROOT MAGGOT HOTSPOTS

Despite the early planting of many Red River Valley sugarbeet fields this year, below-normal temperatures prevailed in much of the production area after planting. As a result, seedling development in some fields is slightly behind normal, and those sugarbeet plants will be especially vulnerable to attack by sugarbeet root maggot larvae. Fields in low-risk areas (central and southern portion of the Valley) should be adequately protected if a planting-time soil insecticide was used. Growers in areas expected to have high root maggot populations should be especially wary of the potential for major damage and yield loss. If less than the full labeled insecticide rate was used at planting, a postemergence application is advised for adequate protection in high-pressure areas.

Soil moisture and the severity of fly population levels should be considered in choosing whether to use a liquid or a granular formulation. Granular materials usually perform better if applied slightly before anticipated peak fly activity and liquid products provide the best activity if applied within three days (before or after) of peak. Postemergence granules are more effective under moist soil conditions or if applied within a few days prior to a rainy period. Liquid insecticides will perform better than granules if soil conditions are dry. A liquid organophosphate insecticide such as Lorsban 4E may provide some larval control in addition to killing adult flies, especially if rainfall is received 1 or 2 days afterward to incorporate it into the soil. Liquid insecticides also may be useful if an unusually high flare-up of fly activity occurs. Research suggests that adult fly control can also be achieved with Asana; however, neither it nor other pyrethroid products such as Mustang will be particularly active against larvae when applied postemergence. Refer to the "Insect Control" section of the 2005 Sugarbeet Production Guide or the "Sugarbeet Insects" section of the 2005 Field Crop Insect Management Guide for more detail and insecticide recommendations. Online versions are at:

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

and

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

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

 

COLORADO POTATO BEETLE INSECTICIDE RESISTANCE MANAGEMENT

Start building insecticide resistance management (IRM) into your Colorado potato beetle (CPB) control program. Although insecticide resistance management should be a component of all insect pest management programs, IRM is especially important to ensure the continued effectiveness of insecticides for CPB control.

The Colorado potato beetle is the most important foliage feeding pest of potato and this insect has the ability to develop resistance to insecticides over a short period of time. Prior to the introduction of potatoes, the CPB survived on plants of the nightshade family and later transitioned to cultivated potatoes which also belong to this plant family. Nightshades contain highly poisonous defensive chemicals, cholinesterase inhibitors, and CPB evolved the physiological mechanisms to breakdown these toxins into substances that are benign or even useful to the insect. The toxins, or active ingredients of many insecticides registered for CPB control are similar to the cholinesterase inhibitors found in the nightshade plants. Because CPB has overcome the cholinesterase inhibitors found in nightshades, this beetle is predisposed to overcome insecticides that have a chemistry, or mode of action, similar to these plant toxins. So, although insecticide resistance in CPB populations is not widespread across the potato production areas of North Dakota and Northwestern Minnesota, the potential for such resistance to develop does exist.

In other potato production regions in the US, CPB has developed resistance to multiple insecticides from major insecticide groups: organophosphates, carbamates, pythrethroids, and the neonicotinoids. In Michigan and New York, populations of the CPB have developed res

istance to the neonicotinoids, imidacloprid and thaimethoxam, in as little as five years after these insecticides were first introduced for CPB control. The increasing popularity of the seed treatment formulations of imidacloprid and thaimethoxam against overwintered CPB adults and first generation larvae raises the concern that CPB will develop resistance to these insecticides in our area.

An IRM program should incorporate several tactics that will reduce the selection pressure on CPB populations to develop resistance to insecticides and so, delay the development of resistance.

  • Rotate the potato crop at least 400 meters from last year’s crop.
  • Fields that are rotated are usually colonized later by CPB and at a lower population density than non-rotate fields.
  • Avoid seed treatments in fields that have a history of a low to moderate risk to CPB infestations.
  • When a potato crop is protected using a seed treatment, avoid applying its foliar equivalent in the same field.
  • Base foliar sprays on treatment thresholds to ensure treating a CPB infestation only when necessary.
  • Target foliar sprays at the vulnerable life stages (1st and 2nd instar larvae) to ensure good control.
  • Field edges can be treated to target the beetles as they are moving into the field.
  • When possible, select foliar insecticides (SpinTor®) to conserve natural enemies.
  • When multiple treatments are necessary, rotate the insecticide mode of action to delay the development of resistance. Always maintain good plant coverage when using foliar sprays to ensure effective control of a CPB infestation.
  • Remember, The implementation of IRM tactics is necessary to ensure the sustainability of the current and future insecticides for CPB control in potatoes.

    Denise Olson
    Research Entomologist
    denise.olson@ndsu.edu

     

    SPRINGTAILS IN SUGARBEET FIELDS

    Springtail population increases rapidly in cold, wet soils with high organic material. Springtails, when present in large numbers feed on sugarbeet roots that results in stand loss of seedlings. In the Sidney factory district, a few fields were irrigated to facilitate germination. These fields remained cold and wet for a long period. In addition, no Counter insecticide was used at planting. As such, a few fields suffered stand loss from high populations of springtails. Counter insecticide should be used when replanting fields damaged by springtails (see page 73 of the 2005 Sugarbeet Production Guide for insecticide recommendation).

    Mohamed Khan
    Extension Sugarbeet Specialist
    mkhan@ndsuext.nodak.edu


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