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ISSUE 8   July 2, 2009

SCOUT FOR BARLEY THRIPS

Barley thrips are a pest problem in central North Dakota again! Barley thrips have been found in Kidder, Sheridan, Wells, Eddy, Foster and Pierce Counties. Some fields have high numbers of barley thrips (>20 thrips per stem) while other fields are still low in numbers (1-3 thrips/stem). It seems early for barley thrips with the cool spring. However, the recent hot weather has been pushing insect development as well as crop development. Typically, hot dry weather conditions favor barley thrips development that may result in crop losses.

Barley thrips are small dark brown to black insects about 1 to 1.8 mm long (Fig. 1). Females have feathery wings while males are wingless. Immature larvae are wingless, pale yellow, white or green with red eyespots. Larvae are difficult to see due to their light, almost transparent color and extremely small size. Adult and immature thrips have a long, narrow body shape.

Female thrips overwinter as adults in debris in fields and shelterbelts. Thrips emerge in late May and early June and move into winter wheat/rye and eventually to early seeded barley (preferred host). Occasional, barley thrips will feed on hard red spring wheat and durum as well. There is one generation per year.

Adult and immature thrips cause damage by feeding on succulent plant tissues (puncturing plant cells and sucking out the contents). Feeding injury symptoms are a whitened or bleached appearance (Fig. 2) with gooseneck-shaped stem and heads under severe pressures. Intensive feeding at the beginning of head formation produces small, shriveled grains. Often there is no seed development at the top and bottom of the head (Fig. 3) and intermediate grains are shriveled. When thrips feeding is severe on the flag leaf, kernels do not fill properly and seed weight is reduced.

Scout for barley thrips from flag leaf to heading. Barley thrips can be found by unrolling the flag leaf away from the stem. Remember, populations will probably be higher at the field edges.

Economic Threshold: [Cost of control ÷ Expected value per bushel ($)] ÷ 0.4

Using the cost of control as $8-12/acres (insecticide + application cost) and $3.85/bu of barley (low price range), the economic threshold is 5-8 barley thrips per plant before the crop is fully headed. Using the cost of control as $8-12/acres (insecticide + application cost) and $4.55/bu of barley (high price range), the economic threshold is 4-7 barley thrips per plant before the crop is fully headed.

One thrips per stem results in a 1.25 bushel per acre loss! Once the barley heads the insect damage is done and NO insecticide treatment is advised. The only registered insecticide for barley thrips control in North Dakota is methyl parathion 4 EC at 4-6 fl oz per acre (do not enter treated fields within 48 hours after application). Other insecticides approved for use on barley but do NOT have barley thrips listed on the label include: Warrior II (lambda-cyhalothrin), malathion, Lannate (methomyl), and Penncap-M (methyl parathion). It is legal to apply an insecticide if it is labeled for use in the crop; however, if the target pest is not listed for that crop, efficacy is not implied by the manufacturer and growers who choose to use the product assume their own liability for any unsatisfactory performance.

Although many growers want to wait to tank-mix the insecticide with a fungicide for scab control at Feekes 10.5 (head fully emerged), I do NOT recommend waiting for the optimal timing of a fungicide application for scab control in barley. This is too late for effective barley thrips control and the damage/yield loss is already done by then.

Please be aware of any bee hives located near your barley fields as insecticides, especially methyl parathion, are extremely toxic to honey bees. Notify your local beekeepers if you intend to spray, so the bee hive can be moved to another area before spraying. A list of honey beekeepers in North Dakota is available on the North Dakota Department of Agriculture website.

www.agdepartment.com/PDFFiles/2009BeekeepersList.pdf

 

WHEAT STEM SAWFLY POPULATIONS HIGH IN SOUTHWEST NORTH DAKOTA

Wheat stem sawfly populations are very high in southwest North Dakota. Some producers are justifiably nervous and are spraying insecticides for adult sawflies control. NDSU studied insecticides and spray timings for control of wheat stem sawfly at Hettinger and Makoti in 2008. The following treatments were evaluated: 1) untreated check, 2) foliar insecticide at the 4-6 leaf stage, 3) foliar insecticide at flag leaf stage, 4) low rate of insecticide seed treatment, 5) high rate of insecticide seed treatment, and 6) low rate of insecticide seed treatment + foliar spray at the 4-6 leaf stage. Cruiser 5FS was used as the insecticide seed treatment and Warrior II with Zeon Technology was used for all foliar insecticide applications. All seed treatments were applied commercially. There were no significant differences between the mean numbers of infested stems per treatment regardless of location (Table 1). The exception was the high rate of Cruiser seed treatment, which had lower infested stems than the untreated check at Hettinger. There were no significant yield differences at Makoti (Table 2). At Hettinger, the high rate of Cruiser 5FS had a significantly lower yield than any other treatment except the low rate of Cruiser 5FS + Warrior II at the 4-6 leaf stage. Severe drought and heat stress occurred at Hettinger during the 2008 growing season and yield differences among treatments may be more indicative of heat stress and available soil moisture. Results from 2008 indicate that insecticides were not effective in controlling wheat stem sawfly infestations. Some of the reasons why include: adult sawflies are non-feeding and take in little water, sawflies emerge over long 3 to 4 week period, and sawflies can move around and reinvade fields after spraying. This would require multiple applications of an insecticide for control. Personal communications with other sawfly researchers in Montana also confirm that insecticides were inconsistent with controlling wheat stem sawfly. Insecticide spraying will also kill the parasitoids that attack and kill sawfly larvae. Research efforts are continuing in 2009 with insecticides for control of wheat stem sawfly.

Table 1.

Treatment

Rate

Wheat stem sawfly Mean No. Infested Stems/Treatment

Hettinger REC

Makoti

Cruiser 5FS (low rate)

39 g AI/100 kg

8.0 a

18.5 a

Untreated Check

---

6.5 ab

16.8 a

Warrior II (4-6 leaf stage)

1.28 fl oz/acre

6.3 ab

17.3 a

Cruiser 5FS (low rate) + Warrior II (4-6 leaf stage)

39 g AI/100 kg
1.28 fl oz/acre

6.0 ab

16.0 a

Warrior II (flag leaf stage)

1.28 fl oz/acre

5.0 ab

17.0 a

Cruiser 5FS (high rate)

50 g AI/100 kg

3.5 b

15.8 a

Means followed by the same letter are not significantly different (Fisher’s LSD, P 0.05).

Table 2.

Treatment

Rate

Yield (bu/acre)

Hettinger REC

Makoti

Cruiser 5FS (low rate) + Warrior II (4-6 leaf stage)

39 g AI/100 kg
1.28 fl oz/acre

20.2 a

26.3 a

Warrior II (flag leaf stage)

1.28 fl oz/acre

19.7 ab

26.5 a

Cruiser 5FS (low rate)

39 g AI/100 kg

19.5 ab

26.9 a

Untreated Check

---

19.1 ab

25.2 a

Warrior II (4-6 leaf stage)

1.28 fl oz/acre

18.0 bc

26.3 a

Cruiser 5FS (high rate)

50 g AI/100 kg

17.1 c

26.4 a

Means followed by the same letter are not significantly different (Fisher’s LSD, P 0.05).

 

INSECT SCOUTING UPDATES

Wheat / Barley - Cereal aphids have made their way up into ND. No treatable populations are present yet. General recommendations are to scout fields through early flowering. If weather continues dry with moderate temperatures (mid-70s F to 80s F), aphid populations can explode quickly and reach economic threshold. To protect small grains from yield loss due to aphid feeding, use either economic threshold:

- 85% stems with at least one aphid present, prior to complete heading.

- 12-15 aphids per stem prior to complete heading

Crops are susceptible to yield loss through early flowering. Dr. Ian MacRae of University of Minnesota has also put out an Ag Alert for cereal aphids in northwestern Minnesota. These aphids transmit barley yellow dwarf virus. When aphid populations are high, the disease can spread quickly through small grain fields. At greatest risk are later planted fields which attract migrating aphids that are moving from more mature fields.

Soybean - No soybean aphids detected in soybean fields in North Dakota. General scouting efforts are underway.

Sunflower - Emergence of banded sunflower moth is underway as the first moth was trapped in a pheromone trap in Mapleton, Cass County, and Carrington, Foster County, ND.

Canola - Emergence of bertha armyworm is also underway as the first moth was trapped in Minot, Ward County and Carrington, Foster County.

Canola - Diamondback moth flights are increasing. Flights are especially high in northwestern Minnesota near Thief River Falls and Crookston, MN. As the canola approaches the flowering stage, scouting for the larvae of diamondback moth will be important.

Field Pea - Pea Aphids have been reported from McLean County. Pea aphids are small, about 1/8+ inch long, and pale green (Fig 4). Aphid feeding on peas in the flowering and early pod stage can result in lower yields due to less seed formation and smaller seed size. Protein content and other quality issues do not appear to be affected. The following table relates yield loss in peas for average aphid counts from 1 to 8 aphids per 20 cm pea stem tip when about 25% of the crop has begun to flower.


Figure 4.
Pea Aphid (photo by F. Peairs,
Colo. State University, Bugwood.org)

Aphids per tip

% yield loss

1

3.4

2

4.9

3

6.1

4

7.1

5

8.0

6

8.8

7

9.6

8

10.3

Pea Aphid Thresholds: At the beginning of flowering, an insecticide application would be economical when 9 to 12 aphids per sweep (or 90 to 120 aphids per 10 sweeps) are present. When 50% of the plants have young pods, an insecticide application would be economical when an average of 2 to 3 aphids per plant tip is present. Population estimates should be calculated by averaging counts taken from at least five separate areas of the field. One application per season should give satisfactory control.

Grasshopper populations are increasing! Scout fields for nymphs (young) grasshoppers in field margins and ditches. Action thresholds are 30-45 nymphs per square yard in field or 50-75 nymphs per square yards in margin.

Janet Knodel
Extension Entomologist
janet.knodel@ndsu.edu  


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