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ISSUE 3   May 29, 2008

SPRING FLEA BEETLE POPULATIONS IN CANOLA

Fall populations of flea beetles in 2007 are generally indicative of the spring population levels in 2008. If high populations are seen at harvest 2007, there will be high populations in spring 2008. This past fall a total of 156 canola fields were surveyed in the swath (mature) stage for flea beetles in 24 counties in ND. This number represents approximately one field surveyed per 6,000 acres of canola. The survey was initiated on July 30 and continued through August 15, 2007. Flea beetles were surveyed using 20 sweeps per field with a 15-inch sweep net in freshly swathed canola fields. Flea beetles were found in 99% of the fields surveyed. Peak population densities occurred in the north central and northeastern regions of North Dakota. The average number of flea beetles per 4 sweeps was 24, with ranges between 0 and 94 (see map). These numbers are similar to the flea beetle populations that were found in swathed canola last year and represent low to moderate levels of flea beetles.

Flea beetle map

Crucifer flea beetles overwinter as adults in leaf litter of shelterbelts or grassy areas and are rarely found in canola stubble. Beetles emerge in large numbers when temperatures warm up to 57 F (14 C) for several consecutive days and there is a rainfall event in early spring. They feed on volunteer canola and weeds, such as wild mustard, and move to newly planted canola as it emerges. 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 field margins. When population levels are high, significant damage can occur within a 24-72 hour period. The most susceptible growth stage to feeding injury is the first 14-21 days after emergence.

More than 90% of North Dakota canola fields receive commercially applied seed treatments, like Helix XTra or Prosper FX. These systemic seed treatments can provide protection against flea beetles for about 21-25 days after seedling emergence. With the canola seed sitting in cool, dry soil conditions this year, crop emergence and growth is delayed. Without actively growing plants, systemic seed treatments are not readily taken up into the plant and this may result in reduced insecticide toxicity, residue, and efficacy. The take home message is to regularly scout seedling to 6-leaf canola for feeding injury - pitting (see photograph).

Feeding injury - pitting image
Feeding injury - pitting

An action threshold of 25% injury would justify a foliar spray on top of the seed treatment. Recent research indicates that the best insecticide strategy for management of flea beetle was the high rate of insecticide seed treatment plus a foliar insecticide applied at 21 days after planting, regardless of planting date. The foliar spray on top of the seed treatment controlled later-emerging flea beetles as the seed treatment residual was diminishing and the crop became vulnerable to feeding injury. The next few weeks will be critical for protecting canola against significant flea beetle damage!

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

 

CAN WE CONTROL THE WHEAT CURL MITES THAT VECTOR WHEAT STREAK MOSIAC VIRUS?

Wheat curl mites are very tiny, 1/100 in long (see photo) and feed with piercing-sucking mouthparts on the leaves of wheat, corn, volunteer wheat and other weedy grass hosts. It is a primary vector of Wheat Streak Mosaic Virus (WSMV), which can cause crop losses of 50 to 100% when severe.

Wheat curl mite
Wheat curl mite (photo by F. Peairs, Colorado
State University, Bugwood.org)

Early symptoms of WSMV
Early symptoms of WSMV

Early symptoms of WSMV are a yellow mosaic pattern of lines on the youngest leaves, distinct chlorotic spots and plant stunting (see photo). Wheat curl mites have a high reproductive rate with egg to adult development occurring in 8-10 days at 77 F (25 C). They overwinter in all life stages and can survive the cold winter temperatures of North Dakota.

Breaking the green bridge and avoiding early planting are the best pest management strategies for reducing the incidence and severity of wheat curl mites and WSMV. Mites cannot survive without green plants and a two week break with only dead plants will starve and kill any mites. For winter wheat, planting late in the fall will also provide for a longer break in the green bridge period, and as a result, mite populations will not build up. Although wheat varieties have been developed that show resistance to the wheat curl mite, mite biotypes can overcome this resistance in certain regions. Resistance to WSMV has also been developed; however, much better sources of resistance are needed. The relative susceptibility of wheat varieties in North Dakota is not known. Insecticidal control has limited effectiveness against wheat curl mites, partially due to the lack of efficacy with insecticides against mites (arthropods, not insects) and to their feeding behavior in protected areas of the plant, such as curled leaves or deep in the leaf whorls.

Please see the NDSU bulletin on wheat streak mosaic virus for more information:

www.ag.ndsu.edu/pubs/plantsci/smgrains/pp646w.htm

Janet Knodel
Extension Entomologist
Janet.Knodel@ndsu.edu

 

SPRINGTAILS DAMAGE RED RIVER VALLEY SUGARBEET

Numerous reports of subterranean (soil-dwelling) springtail infestations in Red River Valley sugarbeet fields have been received this spring. Most reports have come from central and southern portions of the Valley. Sugarbeet seedlings are especially vulnerable this year because of the cool temperatures that have characterized much of the spring thus far. Symptoms of springtail infestations include failure of plants to emerge and dead/wilting plants. Areas affected by springtails usually occur in multiple patches of 1 to 5 acres within a field. These patches will likely increase in diameter as affected plants continue to wither and die. Unfortunately, there is no recommended rescue treatment for springtails in sugarbeet because postemergence-applied insecticides are not likely to penetrate deep enough into the soil profile to reach springtails feeding on developing seedling roots and germinating seeds.

Sometimes, the best management option is to replant the affected areas of a field. Beets should be replanted if plant population is likely to be reduced to 50 plants per 100 row feet or lower. Springtails will likely continue to feed for several weeks. Therefore, an at-plant insecticide should be used at replanting. Producers are advised to avoid granular materials containing chlorpyrifos (Lorsban 15G, Nufos 15G, Chorfos 15G, etc.), which are relatively ineffective at controlling subterranean springtails.

For more information about springtails, see extension circular no. E-1205, "Springtails in Sugarbeet: Identification, Biology, and Management." It is also located on the web at: www.ext.nodak.edu/extpubs/plantsci/rowcrops/e1205w.htm

 

CUTWORMS INFESTING AREA SUGARBEET FIELDS

Scattered reports of cutworm infestations in sugarbeet fields have been received from the Drayton sugar factory district all the way south to the Moorhead area. Early-season cutworms typically clip the stem of seedling plants as they feed. Most feeding activity occurs during late evening through early morning.

If cutworm activity is suspected in a field, further scouting should consist of sifting through soil in the upper 2 inches near the base of plants in affected areas of the field. Cutworm larvae will curl up into a C-shape similar to white grubs, but differ from grubs by having darker coloring, a variety of markings on their body, and several pairs of prolegs (false legs) on their abdomen.

Foliar insecticides are effective in managing cutworm infestations, and early detection is important for protection from major economic loss. A rescue insecticide should probably be applied if 4 to 5% of seedlings have been clipped by cutworm larvae.

For specific information on managing insect pests of sugarbeet, please refer to the "Insect Control" section of the 2008 Sugarbeet Production Guide or the "Sugarbeet Insects" section of the 2008 Field Crop Insect Management Recommendations. The respective WWW locations for online versions of these publications are:

www.sbreb.org/Production/production.htm

and

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

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


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