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ISSUE 2   May 21, 2009

FIELD SCOUTING CRITICAL FOR CUTWORM CONTROL

Although it has been a slow start to the season with cool spring temperatures, some field reports of cutworm activity have been reported, especially in pulse crops in the northwestern and north central regions of North Dakota (pers. comm. - K. Brown, J. Pederson).

Cutworms are an early season pest that feed on most of the field crops grown in North Dakota. They become active when soil temperatures are above 40F. Crops with lower plant populations such as sunflower, peas and lentils are more susceptible to cutworm injury. Cutworm feeding activity usually extends from May through the end of June and young emerging plants are the most susceptible stage of crop development. Typical feeding injury symptoms include clipped off plants with the larvae (caterpillars) underneath in the soil, wilted plants or bare patches of ground in localized spots in a field. Another symptom of cutworm feeding activity in peas and lentils is finding 2, 3, or even 4 sprouts where the plant was cut off by a cutworm and then re-cut as the seedling tried to re-initiate growth. Excessive cool, wet soils tend to amplify stand reduction by slowing plant development relative to cutworm feeding.

The key to successful cutworm control is early detection, and knowing your plant population and proper spray timing. If the plant population is low, few or no plants can be lost to cutworm feeding. The greater the plant population the more damage can be tolerated without economic yield loss. Insecticides are ideally targeted at the young larvae, which are easier to kill than the larger larvae (>1 inch).

Cutworm larvae
Cutworm larvae (caterpillar)

Spraying is recommended at night when cutworms are actively feeding. There are questions about tank mixing insecticides with herbicides for early season weed control. Always, check labels for compatibility or do a simple ‘jar test’ mixing the insecticide and herbicide. If spraying for weed burn-down and early season cutworm activity, be sure to re-scout the field as cutworms will continue to emerge over a two to three week period. Cool temperatures will delay cutworm emergence and development. Wet soil conditions will also improve insecticide efficacy, as cutworms feed near the soil surface in these conditions. Rescue foliar treatments are warranted when cutworms exceed these treatment thresholds by crop:

  • Alfalfa – 4 to 5 or more cutworms per square foot (new stands – only 2/sq ft)
  • Canola – 1 cutworm per square foot
  • Corn - 3-6% of the plants are cut and small larvae (<3/4 inch) present
  • Peas / Lentils – 2 to 3 cutworms per square meter
  • Small grain – 4 to5 cutworms per square foot
  • Soybean - 1 cutworm per 3 feet of row or 20% of plants are cut
  • Sugarbeet - 4-5% cutting of seedlings or 3 to 5 larvae per square foot in late summer
  • Sunflower - 1 cutworm per square foot or 25-30% of plants cut
  • For insecticides registered in North Dakota for cutworm control, consult the 2009 Field Crop Insect Management Guide at:

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

     

    FLEA BEETLES EMERGING IN CANOLA

    The first spring flea beetles have been observed feeding on garden beets, volunteer canola and wild mustards.

    Based on the 2008 Canola Survey, flea beetles were found in 93% of the canola fields surveyed. Peak population densities occurred in the north central and northeastern regions of North Dakota (see map).

    Flea beetle map

    The average number of flea beetles per 4 sweeps was 32, with ranges between 0 and 660. These numbers represent an increase in flea beetle population levels in swathed canola in 2008 compared to 2007. Large populations of flea beetles in late summer (swath canola) indicate large overwintering populations and potentially large populations the next spring. Therefore, the risk forecast for flea beetle infestation this spring is moderate to high.

    Crucifer flea beetles overwinter as adults in the leaf litter of shelterbelts or grassy areas. Beetles emerge in large numbers when temperatures warm up to 57 F for several consecutive days and there is a rainfall event in early spring. Depending on the temperature and rainfall, it may take up to three weeks for the adults to leave their overwintering sites. Warm, dry, and calm weather promotes flea beetle flight (flying up to several miles) and feeding throughout the field. In contrast, cool, rainy, and windy conditions reduce flight activity, and flea beetles walk or hop, which leads to concentrations in the field margins.

    As the spring canola emerges, scout canola fields for flea beetle feeding injury (pitting) to ensure insecticide seed treatments are providing adequate protection.

    flea beetle injury
    Flea beetle feeding injury (pitting).

    The most susceptible growth stage to feeding injury is the first 14 to 21 days after emergence. The economic threshold for applying rescue foliar insecticide sprays is 25% defoliation on cotyledons and first true leaves. Adults are small ( in.), oval-shaped, black in color with a blue sheen on wing covers, and with enlarged hind legs for jumping.

    Adult flea beetle
    Adult crucifer flea beetle (photo by g. Fauske, NDSU)

    Funding for the canola survey was provided by the Northern Canola Growers Association and CSREES Canola Research fund.

    Janet Knodel
    Extension Entomologist
    janet.knodel@ndsu.edu


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