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ISSUE 3   May 24, 2007

SUNFLOWER BEETLE EMERGING

Although populations of sunflower beetle have been low the past few years. It is always a good idea to check fields by scouting for adult sunflower beetles. Adults are ¼ to ½ inch long and look like a Colorado potato beetle except for a reddish-brown head and patch on thorax and exclamation point on the side of the wing cover (see photo). Adults overwinter in the upper 2 to 4 inches of soil and begin emerging from the previous sunflower fields in late May to early June. Beetles overwinter throughout the field and exhibit no preference for field edges or shelterbelts. Early planted sunflower fields will attract newly emerged adult sunflower beetles first and have higher populations than later planted sunflower fields (late May to June). In the seedling stage, one-two adults per seedling is the recommended economic threshold. A foliar broadcast insecticide spray will control the emerged adult sunflower beetles. Most sunflower insecticides can be tank mixed with sunflower herbicides. Check label for specifics. If Cruiser Seed Treatment was used, it will also control adult sunflower beetles early in the season.

Adult sunflower beetle
Adult sunflower beetle

 

SCOUT SOON FOR ALFALFA WEEVILS

Reports of high populations of alfalfa weevil are coming in from southern Minnesota and other states. Alfalfa weevils overwinter as adults in plant debris, woodlots, and ditches. As temperatures warm up adults migrate to alfalfa field to lay eggs. By using degree days with a base of 48, the life stages and development of alfalfa weevil can be predicted (see degree day table).

Go to the insect section in the NDAWN website:

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

and select the degree day base of 48F to determine the accumulated degree days for your location. See map of North Dakota for alfalfa weevil degree day accumulations.

Stage of Development

Degree Days Required to Complete Indicate Life Stage

Accumulated Degree Days (base 48 )

Days

General Activity

egg

300

300

7 to 14

 

1st instar

71

371

21 to 28

Light leaf feeding

2nd instar

67

438

Light leaf feeding

3rd instar

66

504

Major leaf feeding

4th instar

91

595

Major leaf feeding

pupae-adult

219

814

10

Mating & egg laying

Field scouting for alfalfa weevil is initiated at 300 DD. Scout the south-facing slope or sandy knoll areas which warm up rapidly first. These areas will have early development of alfalfa weevil larvae. Scout fields by sampling 10 plants in 5 random locations (50 total plants) and walking in a M-shaped or similar pattern throughout the field. Small alfalfa weevil larvae are slate-colored. As larvae mature, their color changes to bright green with a white line running down their back and a black head capsule, and about th of an inch long (see photo). Alfalfa weevil larvae can be confused with the clover leaf weevil larvae. However, the clover leaf weevil is two times larger (5/16th inch long), with a brown head capsule, and the white stripe is usually bordered with smudges of pink. In alfalfa, the clover leaf weevil rarely causes economic yield loss.

Management of weevil infested alfalfa stands depends on when the infestation occurs. If the infestation occurs relatively late, when the alfalfa has reached 20 to 25 inches in height, consider taking an early harvest. Small alfalfa weevil, those less than ¼ inch in length, will drop to the soil and generally die if the soil is dry. If the infestation occurs early, when alfalfa is 10 to 15 inches in height, chemical treatment may be necessary. Insecticide treatment is recommended if two live larvae per stem occur at this stage and/or 35 to 40% of the plants are showing tip feeding. In general, if alfalfa is 7-10 days out from harvest and 35-40% tip feeding is present, an insecticide treatment is recommended. ND insecticide recommendations for alfalfa are listed at:

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

Remember to check the preharvest interval as these restrictions vary according to the insecticide used and the rate applied. Other factors to consider when selecting an insecticide is its price, potential hazards to honey bees and whether or not it is a restricted use insecticide.

If weevils infest an alfalfa field, be sure to scout the field following harvest for re-infestation of the second harvest. Major feeding by the alfalfa weevil will occur from 430 to 595 growing degree days (2nd - 4th instar). At greater than 600 growing degree days feeding normally stops and adult emerge. This will occur usually during the second harvest unless the first is taken late. If following the first harvest, your scouting shows 8 or more larvae per square foot or larvae are suppressing regrowth, chemical treatment is recommended.

Alfalfa weevil larva
Alfalfa weevil larva
(Univ. of Illinois Ext.)

Clover leaf weevil larva
Clover leaf weevil larva
(Univ. of Illinois Ext.)

Janet Knodel
Extension Entomologist
Janet.Knodel@ndsu.edu

 

SUGARBEET ROOT MAGGOT: EMERGENCE HAS BEGUN, SEVERAL DAYS ‘TIL PEAK

NDSU crews have been monitoring root maggot flies (Fig. 1) since May 16, and activity at all sites is very low thus far. The first sites at which flies were detected included Baker, MN, Bathgate, Hamilton, and St. Thomas. Root maggot emergence and adult fly activity are strongly correlated with air and soil temperature accumulations. Typically, peak (50%) emergence from old (last-year’s) beet fields occurs when air degree-day (DD) accumulations reach 450. Currently Baker, Grand Forks, and St. Thomas are at 373, 388, and 367 air DD, respectively. Thus, peak emergence is not likely to occur for another 5 to 8 days.

sugarbeet root maggot fly
Fig. 1. Female sugarbeet root maggot fly

Peak activity of root maggot flies occurs in current-year sugarbeet fields on the first warm (80 degrees Fahrenheit or above) day following the accumulation of 600 bare-soil DD units. As of May 22, DD accumulations in the Red River Valley were 320, 301, and 202 soil DD at Baker, Grand Forks, and St. Thomas, respectively. Given the current DD levels and anticipated daily accumulation rates, peak activity is not likely to occur for at least a couple more weeks and probably later, irrespective of location within the Valley. However, one should remember that RRV soils can warm up quickly and, therefore, maggot development could be accelerated if a warm trend were to occur in the near future.

Root maggot populations have been mostly restricted to northeastern ND during the past several years; however, as indicated in Figure 2, areas of moderate to high risk for root maggot infestations this year include Cass, Grand Forks, Walsh, and Pembina counties of ND, and Clay, Norman, and Polk counties of MN. Growers within or adjacent to these zones should monitor their fields closely and be ready to apply rescue insecticide applications if high fly activity occurs.

sugarbeet root maggot population levels
Fig. 2. Anticipated sugarbeet root maggot
population levels for 2007 in the Red River Valley

 

ROOT MAGGOT CONTROL FOR REPLANTING SUGARBEET FIELDS

Recent heavy rains, which were followed by warm, windy weather, have resulted in crusting over of some Red River Valley sugarbeet fields. Growers that choose to replant fields due to crusting are advised that replanted beets will be smaller and more vulnerable to attack during the root maggot larval feeding period than older, more established plants. Therefore, growers in areas where high maggot populations are anticipated are advised to consider two management options: 1) apply another granular material at replanting of the crop; or 2) apply a postemergence treatment of either a liquid or granular insecticide. It is critical to note that use of Counter 15G and Lorsban 15G is limited to one application per year. Therefore, if one of these products was applied on a particular field at initial planting, another material must be used for the succeeding application.

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


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