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ISSUE 11    July 13, 2005

OUTLOOK FOR BERTHA ARMYWORM IS GOOD NEWS FOR CANOLA PRODUCERS!

Pheromone trap catches for the bertha armyworm have been low. Cumulative trap catches are under 300 moths at all sites, which indicate a low risk for field infestations. The last year bertha armyworm was a major insect pest problem in North Dakota was 2001. Moths emerge from mid June through July. Moths will be attracted to flowering fields. 2005 trap data is posted on the following website:

http://www.ag.ndsu.nodak.edu/minot/pest/index.htm 

 

ADULT BANDED SUNFLOWER MOTH FLYING!

The first banded sunflower moths have been observed. The banded sunflower moth, Cochylis hospes Walsingham, is a small, straw-colored moth about 7mm (¼ inch) long with a wing span of about 14 mm (½ inch). It has a brown triangular area in the median portion of the forewing; the peak of the triangle is oriented toward the leading margin and the base is oriented along the rear margin of the wing. A smaller and less well-defined brown area is located near the wing tips. The hind wing is light gray brown and bears no distinctive markings.

Banded sunflower moth emerges form the soil about mid-July. Peak activity normally occurs about the last week of July or the first week of August. Moths fly from last year’s field to the current year's field. At this time moths congregate in the grassy areas around field margins. Moths move to fields during the bud stage, with a preference for the mid-bud stage. Eggs are laid on the back of the bud and the outside of the bracts.

The newly hatched larvae move from these sites to the face of the flower and begin feeding on bracts and florets. Third and later instars tunnel through the disk flowers and feed on young developing seeds. As the seeds mature and harden, larvae chew into the seeds to feed. Each larva penetrates and consumes the contents of several (six to seven) mature seeds in addition to the disk flowers consumed by the early instars. Small areas of silken webbing on mature sunflower heads indicate the presence of banded sunflower moth larvae within the head.

Threshold: When 1 moth for every 2 plants inspected can be found, treatments should be considered. Because the moths initially congregate around field margins prior to flowering, treatment of the field margins has reduced the adult populations.

 

WHEAT MIDGE UPDATE

Wheat Midge Update – Increasing Numbers of Adults Observed!

Continue to be vigilant with wheat midge scouting for wheat / durum crop in the susceptible stage - heading to early flowering! Most of the northern part of state is in the peak emergence period as the southern region starts to see the end of emergence. Field reports from the north central region indicate increasing numbers of wheat midge with some fields above threshold. Remember, wheat midge adults will continue to live about 3-7 days after emergence, depending on environmental conditions. The high temperatures this week will accelerate wheat midge emergence and crop development, which should shorten the interval of crop susceptibility to wheat midge damage. Several producers are tank-mixing a reduced rate of Lorsban insecticide with Folicur fungicide for midge-scab control this year.

Janet Knodel
Area Extension Specialist
North Central Research Extension Center
Minot, ND
jknodel@ndsuext.nodak.edu

 

SOYBEAN APHID

There are widespread reports of soybean aphid in North Dakota fields at this time. Soybean aphids damage plants by removing plant sap. They may also be important in transmission of viral diseases. If sufficient aphids are present during the plants early reproductive stages, pod and seed set may be reduced.

The soybean aphid is small 1/16" or less, pale green in color, and has two black tubular structures (cornicles) protruding from the top of the back end of the aphid. Other aphids can sometimes be found in soybean but they lack black-colored cornicles and do not develop high population levels.

Fortunately, the numbers of soybean aphids being reported at this time are generally low. However, we need to begin diligent scouting to know if populations are reaching economic levels. In our large North Dakota fields, soybean aphids are usually first found on field sides next to shelter belts. While plants are in the vegetative stage, look for aphids on the underside of young leaves, petioles and stems near the top of the plant. As the plants get older, new branches lower in the canopy become infested.

You should check from 20 to 30 plants per field and tally the numbers of aphids per plant. An average of 250 aphids per plant is an indicator that the field may be approaching economic levels. Check the field again in two or three days to see if the aphid population in increasing. If the population level is trending upward, treatment is probably warranted. However, the second count may reveal that the numbers of aphids is declining because of predators or for other reasons. In this case, the field is probably safe but you should continue to monitor the population.

The critical time for an insecticide application is from the start of bloom to early pod set. Insecticides will give you 7 to 10 days of protection. Early applications made before the onset of flowering may allow the aphid population to recover and cause damage. Late applications made after full bloom have not given a consistent return and aphid populations at this time are often in decline because of migration out of the field and from fungal disease.

Several insecticides are labeled as foliar applications to control the soybean aphid. Check the labels for proper usage. Some labels refer to the soybean aphid as the Chinese aphid. Because the aphids typically feed on the underside of the leaves and have the potential to double their population size in 3 to 4 days, they are difficult to control. Best control is with ground applications and the use of high water volumes and pressures. Tank mixing with herbicides is not recommended because of timing issues and the water volume needs for aphid control are not recommended for herbicide application.

Gary Brewer
Research Entomologist
North Dakota State University
gary.brewer@ndsu.edu

 

WHEAT STEM MAGGOTS IN WESTERN ND

Infestations of wheat stem maggot have been detected in many North Dakota wheat fields. Most reports of high incidence are from Morton, Oliver, and Stark counties, although fields in nearby counties could also be infested. Wheat stem maggots (Figure 1) are the tiny (0.25-inch long) pale green, narrow larvae of a small fly. Adults are yellowish-white, 0.2-inch long flies with green eyes and three black stripes running lengthwise down their backs (Figure 2).


Figure 1: Wheat stem maggot larva and damage.


Figure 2: Wheat stem maggot fly.

The larva is the damaging stage. Larvae injure plants by feeding as they tunnel through the stem. This severs the vascular tissue that supplies the head with nutrients. Most feeding in wheat occurs above the upper node of the stem, which causes the upper stem and head to turn white while the flag leaf, lower leaves, and stem remain green. This condition is referred to as "whiteheads" or "silvertop" (Figure 3).


Figure 3: Normal wheat head, and characteristic
"whitehead" symptom of wheat stem maggot injury.

Heads of affected plants appear as if they just dried down prematurely, but florets are actually barren (grainless), and the can be easily pulled out with a gentle tug. Other problems such as root rot infections, high temperatures, or drought stress can also produce white heads; however, root rot causes the entire plant to turn white, and drought stress symptoms will usually be apparent in a more general pattern throughout the entire field. To confirm that wheat stem maggot is the culprit, split the stem in the area immediately above the upper node of the plant and look for a tunneled out area that should contain a larva and maggot "frass", or droppings. The insect could also be present as a pupa (cocoon stage) at this time of year, but may also be absent if it has since emerged as an adult fly.

Infestations mostly range from 1 to 5% damaged heads, and rarely exceed 2%. Thus, the insect is usually not of major economic consequence. In addition to wheat, the maggot also can complete its life cycle on oats, barley, rye, and many other wild and cultivated grasses. It is too late for remedial action against wheat stem maggot this season. Crop rotation and stubble cultivation (especially if a significant amount of post-harvest volunteer grain is present), are the most effective methods of reducing maggot populations, and could be helpful next year in areas that were infested this season. Destruction of infested straw is also recommended. There are no resistant varieties for use against wheat stem maggot, and chemical control is not recommended.

Mark Boetel
Research & Extension Entomologist
mboetel@ndsuext.nodak.edu

Justin Knott
Plant Protection Specialist
ND Dept of Agriculture
jknott@state.nd.us


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