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ISSUE 4   June 5, 2008

SUGARBEET ROOT MAGGOT: COOL WEATHER DELAYS DEVELOPMENT AND FLY EMERGENCE

The cool temperatures that predominated the past several weeks have resulted in slow development of overwintered sugarbeet root maggot (SBRM) populations. The NDSU Root Maggot Developmental 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 an 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 Red River Valley are presented in the following table:

Degree-day (DD) accumulations for sugarbeet root maggot development at selected sites as of June 3, 2008

Site

Soil temperature

Soil DD1

Air DD2

Baker, MN

56

237

391

Ada, MN

56

208

426

Grand Forks, ND

60

270

432

St. Thomas, ND

59

211

380

1Target soil DD for peak emergence = 450
2Target air DD for peak fly activity = 600

Anticipated DD accumulations, based on the extended weather forecast, suggest that peak activity is not likely to occur before the third week of June, and could be delayed as late as the end of the month. Temperatures during the next couple of weeks and wind conditions near peak fly will dictate when the exact peak occurs. Bottom line: growers in high-risk areas should not rush to apply postemergence insecticides. Those planning on applying a granular material should wait until at least the second week in June. Granules should provide excellent results if applied between June 16 and 21. Liquid insecticide applications should be targeted for 2 to 5 days before peak or within 3 days afterward.

WHAT ABOUT REPLANTED BEETS? Over 12,000 acres of beets in the Valley required replanting. Replanted beets will be smaller and more vulnerable to attack when root maggots begin damaging plants. Beet fields in areas of moderate to high risk of root maggot infestation will likely need added insecticide protection. NDSU research has shown gross economic return benefits of $80 to 100 per acre from supplemental insecticide applications to replanted sugarbeets. If no insecticide was used at replanting, consider applying a postemergence material. If an insecticide was used at replanting, a postemergence application may not be necessary. Continued updates on the root maggot fly forecast will appear in future issues of the NDSU Crop & Pest Report. For guidance on postemergence control strategies, consult the "Insect Control" section of the 2008 Sugarbeet Production Guide or the "Sugarbeet Insects" section of 2008 Field Crop Insect Management Recommendations. Online versions of these publications are located at:

www.sbreb.org/Production/production.htm  &

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

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

 

A NEW CUTWORM SPECIES IN NORTH DAKOTA

The winter cutworm, Noctua pronuba (Lepidoptera: Noctuidae) was recently reported causing damage on crops in McKenzie County near Alexander this past week. It has also been reported in Divide, Cass and Richland Counties of North Dakota. However, entomologists believe that it is distributed across North Dakota. Noctua pronuba is native to Europe where it is one of the most common Noctuid moths. It was first found in the North America continent in Nova Scotia in 1979. It has spread rapidly across the northern U.S. Like other Noctuid moths, it is a strong flyer and active at night. It is called the winter cutworm, because it is very cold tolerant and often emerges to feed during warm periods in the winter. It has a wide host range feeding on many agricultural crops (alfalfa, grass hay, potato, sugarbeets, cabbage, carrots, grapes) and ornamental crops. Although there are no specific threshold developed for winter cutworm, NDSU Extension recommends that you follow the guidelines developed for cutworm management.

Noctua pronuba is a large cutworm with moths having a 3-inch wingspan and a bright orange-yellow hindwing with a black sub-terminal band (see figure). Caterpillars are up to 2.5-3 inches long when mature.

winter cutworm moth
Winter cutworm moth

identification of caterpillars graph
Source: H. Russel and C. DiFonzo, Ent. Dept., MSU, CCD #2007-06

 

REVIEW OF WHEAT STEM MAGGOT - WHAT LITTLE WE KNOW

Last year, there were a lot of complaints from wheat producers about white wheat heads caused by wheat stem maggot. The 2007 NDSU IPM Survey found maggots in 46% of fields surveyed from heading to maturity. White heads were observed from late June to the end of July. The average incidence was 19% white heads with a range from 0% to 50% (see map). Historically, infestation levels have been lower and rarely exceeded 5% in North Dakota.

wheat stem maggot map

Identification: Wheat stem maggot (Meromyza americana Fitch) is a small fly (about 1/5 inch long) in the family Chloropidae. Wheat stem maggot can be recognized by the following combination of traits: a mostly yellow body with three dorsal black stripes, swollen hind femurs, the head protruding forward between the eyes, and the corner of the mouth forming a nearly right angle (see figure). Wheat stem maggot is the most common chloropid fly infesting wheat in ND, though other chloropid species are found occasionally in wheat.

Adult wheat stem maggot fly
Adult wheat stem maggot fly

Biology: Wheat stem maggots overwinter in the larval stage, inside the lower parts of grass stems. In the spring, the larvae pupate and adults emerge in June. After mating, females deposit their eggs on the leaves or stems of grasses. The young maggot crawls down beneath a leaf sheath and tunnels into the stem. The stem is partially severed causing the head to turn white. The head and terminal straw of maggot-infested wheat will pull out easily due to the internal chewing damage by the larvae. The larva pupates within a cigar-shaped, pale green puparium. The adults emerge about midsummer and lay their eggs on wild grasses or volunteer grain. The resulting larvae overwinter in the stems of the wild grasses and volunteer grain. Host plants include HRSW, durum wheat, barley, rye, timothy, and several grassy weed hosts.

Integrated Pest Management: There are no scouting techniques or thresholds that have been developed for monitoring wheat stem maggot. However, flies can be collected using a standard sweep net and sweeping the wheat canopy. Research conducted in the 1930s found that the number of white heads was dependant on crop maturity (dates of first heading), variety, and planting date in relation to peak fly emergence. Early planting and early maturing varieties generally had a lower incidence of white heads from the spring generation of wheat stem maggot. At this point, it is not clear if variety differences are due to actual host plant resistance or differences in crop maturity and timing with peak fly populations.

Preliminary research data from NDSU suggests that tank mixing insecticides with the early season herbicides during 5-leaf to jointing wheat helped reduce the incidence of white heads and increased yields. However, it is not clear whether this was due to random chance of peak fly emergence coinciding with the timing of the herbicide-insecticide application. Research is continuing in 2008 to help determine the relationship between fly biology, crop development and crop damage (or yield loss). At this point, we do not know if this strategy will control wheat stem maggots, and therefore we cannot make any recommendations.

Wheat stem maggot chart

 

CUTWORM ACTIVITY CONTINUING

Field reports are still coming in on cutworm damage to crops - pulse crops, sugarbeets, sunflowers, and crops in CRP. Remember to be vigilant scouting for cutworms on agricultural crops all the way through June in North Dakota. Late season cutworms are just starting to become active and still small (<˝ inch). These small caterpillars have about 2-3 weeks of feeding before they become mature. The recent cool weather will delay cutworm development. If cutworms are above action threshold or significantly reducing plant stands, insecticide also should be applied in the evening, when cutworms are active feeding. There are many weeds starting to popup in fields now and early season weed spraying will be in full swing as soon as fields dry out. Many of the pyrethroid insecticides used for cutworm control are compatible with tank mixing with herbicides. However, check labels for compatibility or do a simple ‘jar test’ mixing the insecticide and herbicide. With the recent rains, insecticides need a minimum of 3-4 hours of drying time to be rain-fast and provide effective residue for insect control.

Janet Knodel
Extension Entomologist
janet.knodel@ndsu.edu


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