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ISSUE 8  June 21, 2001

 

WHEAT MIDGE EMERGENCE PREDICTIONS

Degree day accumulations have reached the point where male midge should be emerging in the southeast region of North Dakota. The males emerge slightly ahead of the females. In our monitoring of emergence traps in previous years, male emergence starts at about 1150 DD (Base 40 F). The females start emerging around 1300DD.

The following map breaks the state into three broad regions for when female midge emergence is predicted to get underway. To make predictions, DD accumulations of ~25 per day were used to estimate dates when 1300 DD would be accumulated. As real-time weather data is substituted for estimates, the emergence date will be more accurate.

One thing you might note about this years predicted emergence dates is some similarity to last yearís outlook, except for the southeast. The southeast area is behind in degree day accumulations, pushing things back about 5 days. Due to cool, wet weather in this area, degree day accumulations are similar between the southeast and central counties, differing by only about 130 total DD, or about five days.

As reported in an earlier issue, midge populations remain low through most of the region. Areas of greatest concern continue to be the northwest and areas in Pierce and Rolette counties. Because of cooler temperatures prevailing in May, wheat planted in most counties during May is within the 200 to 600 DD planting risk window. Be sure to monitor heading wheat between the 1300 to 1800 DD window. This has typically been a 2 to 3 week interval when degree day accumulations average about 27 per day.

Wheat midge treatment threshold for yield protection is when an average of 1 adult midge per 4 to 5 wheat heads. Recent research from Canada has resulted in an additional threshold recommendation when quality protection, such as with durum wheat, is desirable. The revised treatment threshold in this case is 1 midge per 8 to 10 wheat heads.

 

BARLEY THRIPS QUESTIONS

Barley thrips are in small grain fields. Some concerns have been expressed from central ND counties regarding their damage potential. The life cycle of barley thrips is as follows:

Early spring - overwintered females move into winter grasses or early spring grasses (e.g. wheat), they will begin egg laying;

Late spring - overwintered and first generation females migrate to barley;

Mid summer - summer generation of females migrate to overwintering sites;

Late summer - females can be found in roadside grasses and shelterbelt sod.

Though barley thrips can be found in wheat, their numbers have been low. It is barley where we have the greatest concern.

In barley, a severe infestation will appear as bleached or whitened plants. This color is due to the feeding injury inflicted by the thrips. The adult thrips are only 1 to 2 mm. long, very slender and dark brown or black. The immature thrips resemble the adults but are smaller and white or green in color.

Sampling for barley thrips should be initiated when the flag leaf is first visible, and continue until the head is completely emerged from the boot. Most barley thrips can be found under the top two leaf sheaths. To count the number of thrips on a stem, first break off the plant at the second node from the top. Run your thumbnail between the two edges of the sheaths at the collar and slowly unroll the sheath away from the stem.

Treatments, when warranted, are only effective if applied before heading is complete. Treatment after heading has not demonstrated a yield increase according to NDSU trials. The only insecticide labeled on barley which lists barley thrips is methyl parathion.

 

CEREAL APHIDS - - WHERE ARE THEY ?

The NDSU IPM small grain survey has not detected cereal aphids through stem inspection . . . yet. Sweep netting has produced a few English grain and Bird cherry oat aphids. The survey has confirmed some isolated incidences of Yellow dwarf virus which is transmitted by aphids. So aphids are here, just at low levels, making detection difficult at this time.

The greatest risk of yield loss from aphids feeding on grains is in the vegetative to boot stages. Significant yield reductions after the onset of flowering could not be demonstrated in research published from South Dakota in 1997 (Voss et al., 1997. J of Economic Entomology 90: 1346-1350). Reasons for these conclusions were that:

Other components of yield are determined earlier (number of spikelets - determined at jointing; number of seeds - determined at flowering).

Therefore, to protect yield loss due to aphid feeding, the treatment threshold is 85% stems with at least one aphid present, prior to complete heading.

 

BERTHA ARMYWORM ANTICIPATED IN CANOLA

We havenít mentioned Bertha armyworm as a serious threat in the region since 1996. That may be changing for this year. The canola insect monitoring program, coordinated by J. Knodel, NDSU Plant Protection Specialist, Minot REC, is reporting some significant captures of Bertha armyworm moths in the Bottineau county region.

Bertha armyworms feed primarily on canola, mustard, alfalfa, and to a lesser extent on flax and potato. Significant feeding injury to crops usually occurs within a three week period between mid July and mid August.

Moths emerge from mid June to mid July. Moths will be attracted to flowering fields. Honeycomb-like clusters of 50 to 150 eggs are laid on the undersides of leaves. Newly hatched larvae are 1/10 in., pale green with a yellowish stripe along each side. Older larvae vary in color, ranging from green to brown or velvety black. The head is light brown and there is a yellowish-orange stripe along each side.

When Bertha armyworm larvae reach 1/2 inch in size, they are in the second to last growth stage. At this point, 80 to 90% of the plant material consumed during the life of the larvae is eaten. If the canola drops its leaves before larvae are mature, they begin feeding on seed pods.

Early detection and regular monitoring are critical to minimize crop loss potential. In general, begin field scouting by mid July. For accurate estimates, sample at least three locations, avoiding headlands or areas not representative of the field. At each location, mark out one square yard and beat the plants growing within the area to dislodge larvae. Push the plants aside or remove them and count the number of larvae in the square yard, taking your time to find larvae that may have hidden in leaf litter or soil. Use the average number of larvae at the sites surveyed within each field to determine if the economic threshold has been exceeded and an insecticide is necessary.

Treat only if larval populations are capable of causing economic damage. Applications should be applied prior to the final two larval stages (1/2 inch lang larvae). Capture includes Bertha armyworm on the label.

Economic Thresholds for Bertha Armyworm in Canola

Value of Crop

Treatment Costs ($ per acre)

- $/lb-

5.00

6.00

7.00

8.00

 

- larvae per square yard-

0.07

21

24

28

32

0.08

18

22

25

29

0.09

16

19

22

26

0.10

14

17

20

23

0.11

13

16

18

21

0.12

12

14

17

19

0.13

11

13

16

18

As more information about moth captures and larval populations becomes available, we will pass it on.

Phillip Glogoza
Extension Entomologist
pglogoza@ndsuext.nodak.edu


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