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ISSUE 7   June 25, 2009

SCOUT FOR ALFALFA WEEVILS IN SECOND CUTTING

Alfalfa weevils overwinter as adults (Fig. 1) in plant debris, woodlots, and ditches. As temperatures warm up adults migrate to alfalfa fields to lay eggs. By using degree days with a base of 48 F, 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 48 F to determine the accumulated degree days (DD) for your location. See map (Fig. 2) of North Dakota for alfalfa weevil degree day accumulations.


Figure 1.
Alfalfa weevil adult (photo courtesy Clemson
Univ., USDA Coop. Ext. Slide Series, Bugwood.org
)


Figure 2.
Alfalfa weevil degree day accumulations

Stage of Development

Degree Days Required to Complete Indicate
Life Stage

Accumulated Degree Days
(base 48 F)

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. However, the cool spring has delayed alfalfa weevil development into the second cutting. Major feeding by the alfalfa weevil will occur from 430 to 595 DD (2nd - 4th instar). At greater than 600 growing degree days feeding normally stops and adult emerge.

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 an M-shaped or similar pattern throughout the field. Small alfalfa weevil larvae (Fig. 3) are slate-colored. As larvae mature, their color changes to bright green with a white line running down their back. Larvae have a black head capsule and are about th of an inch long.


Figure 3.
Alfalfa weevil larva (photo courtesy Clemson Univ.,
USDA Coop. Ext. Slide Series, Bugwood.org
)

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 larvae, those less than 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 to 40% tip feeding is present, an insecticide treatment is recommended. North Dakota insecticide recommendations for alfalfa are listed at the following website:

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.

 

INSECT SCOUTING UPDATES

Wireworms:

Some complaints have been reported with wireworm in no-till barley from Renville County near Mohall. Wireworms were tunneling up into stems and killing plants (appeared yellow and stunted, Fig. 4). If a plant is dissected, one will find a healthy wireworm larva inside stem. There was no insecticide applied as seed treatment in this case. The crop consultant indicated that some of the barley plants were recovering from injury and starting to tiller. On the North Central Region Extension Entomologist conference call this week, entomologists were concerned about the lack of control with insecticide seed treatments in general on many field crops including soybean, corn, canola and sugarbeet. This could be from the cool spring slowing plant development and uptake of insecticide active ingredients into the plant.

Soybean Aphids:

Low populations of soybean aphids (<25 aphids per plant) have been reported in Indiana, Ohio, Michigan, Wisconsin, Iowa, Minnesota, and South Dakota. In MN, reports are mainly from the St. Paul area where the majority of the overwintering host, buckthorn, occurs. There are no reports from northwest or southwest MN yet. The recent rains may provide some control of aphids by drowning them in areas with severe thunderstorms. However, do not expect much control of aphids from entomopathogens, since these insect-killing fungi require cooler temperatures (in 70s F) for aphid control.

Seed Corn Maggots:

Seed corn maggots on dry beans have been reported causing reduced plant stands and spindly seedlings near Finley in Steele County and on corn at Prosper in Cass County. Seed corn beetles were also reported on corn at Prosper. There are NO rescue insecticide treatments available for seed corn maggot. Insecticides must be applied as a seed treatment or at-plant to control seed corn maggots. Most of the fields with problems had no insecticide treatments and had manure applied. Seed corn maggots prefer fields with high organic matter (e.g., manure). Wet, cool springs also provide ideal conditions for high populations of seed corn maggots.

Wheat Stem Sawfly:

Adult wheat stem sawfly is emerging in large numbers in southwest North Dakota near Hettinger, Regent and Scranton. There are no reports of sawfly in north central or northwest regions of North Dakota yet. However, emergence should be underway soon. Adult sawflies can be monitored using sweep nets or yellow sticky cards. Adult wheat stem sawflies are dark and slender, with yellow markings on the abdomen. The adults are less than 1 inch long, and females are generally larger than males though size can vary greatly (Fig. 5).

Janet Knodel
Extension Entomologist
janet.knodel@ndsu.edu

 

SUGARBEET ROOT MAGGOT HOT SPOTS ERUPTING

Major increases in sugarbeet root maggot (SBRM) fly activity have been observed at several sites in the NDSU trapping network during the past several days. Of the 43 monitoring sites, the most active hot spots as of June 22 include fields in the following areas: Manvel, Minto, Grafton, Voss, Nash, Auburn, St. Thomas, Glasston, and Hamilton in ND; and Ada, Borup, Crookston, and Euclid in MN. Although this monitoring program is a good indicator of general activity across the production area, it is no substitute for closer monitoring of individual fields.

Fly activity in the central and southern Red River Valley has probably peaked in the past 2-5 days. The root maggot model suggests that 3-5 more days of high activity are possible for the northern Valley. Weather conditions during the next few days will have a major influence on the exact date of peak activity in fields that have not yet peaked. Cold, windy, or rainy weather will delay the actual peak, even if adequate DD units have been accumulated for adults to emerge from soil.

Current DD accumulations and the updated forecast for peak fly activity at representative locations in the Red River Valley are presented in the following table:

Degree-day (DD) accumulations for sugarbeet root maggot development and peak fly forecast as of June 22, 2009

Site

Air DD1

Peak fly activity forecast

Fargo

655

Peaked about June 20

Hillsboro

617

Peaked about June 22

Grand Forks

590

Peaked about June 23

Grafton

503

June 27 + 1st 80 day

Cavalier

494

June 28 + 1st 80 day

Raw data provided by the North Dakota Agricultural Weather Network (NDAWN)
1Peak fly activity occurs in current-year beet after 600 air DD.

Postemergence additive insecticide applications are recommended in areas of moderate to high risk for damaging root maggot infestations this year. Fields at most risk are those where a moderate rate (10 lb product/ac or lower) of a granular at-plant insecticide was used or in those planted with seed coated with Poncho Beta insecticidal seed treatment. Fields treated with Mustang Max insecticide at planting time will also be at risk because the product is only considered to provide suppression of SBRM. If fly activity in a particular field exceeds about 1 fly per 2 plants, the field should be checked daily to determine if activity increases. Once activity reaches about 0.75 flies per plant on average across the field, a postemergence insecticide application should be made to protect the field.

The 2009 risk map for sugarbeet root maggot infestations in the Valley is presented in Figure 1. Growers in areas of moderate to high risk of damaging maggot infestations should be vigilant at monitoring fly activity in their fields, and plan on applying an additive postemergence insecticide to ensure adequate control.


Figure 1.
2009 Forecast map for sugarbeet root maggot populations
in the Red River Valley (based on fly activity and root maggot feeding
injury ratings at 40 monitoring sites during the preceding growing season)

Postemergence Control Tips: At this late date in fly development/activity, granular insecticides are no longer a viable option. Thus, liquid insecticides will be the tools of choice. Liquids work best if applied between 3-4 days before or within 3 days after peak fly. NDSU research indicates that control can be optimized by splitting full rates of Lorsban 4E (and other chlorpyrifos-containing liquid materials labeled for use in sugarbeet) in to two applications: make one application a 3 or 4 days before anticipated peak fly and repeat it about 7 days later. This strategy should work well this year with such high fly activity spread over several days of warm weather surrounding the actual peak. Rates less than 1 pt/ac should be avoided if applying as a broadcast treatment.

Be careful of label restrictions! The North Dakota Department of Agriculture will be monitoring pesticide applications closely. The following restrictions apply to Lorsban 4E most other materials containing chlorpyrifos as the active ingredient:

1. Restricted-use pesticide: treated fields must be posted

2. Do not make more than 3 applications per season

3. Do not apply more than 6 pints of product (3 pounds a.i.) per season

4. Maximum single rate: 2 pints product (1 lb a.i.) / acre

5. Wear personal protective equipment (see label)

6. Do not re-enter or allow a worker to enter treated field within 24 hours unless required personal protective equipment is worn.

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


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