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ISSUE 9   June 30, 2005

ARE COLORADO POTATO BEETLES INVADING YOUR POTATO CROP??

Overwintered adult Colorado potato beetles (CPB) typically emerge from soil during May to June. They invade newly emerged potato fields where they deposit eggs in clusters of 10 to 30 on the undersides of leaves. First-generation larvae are usually present in potato fields from June through July. The cool weather that persisted into early June this year has delayed beetle movement into potato fields. Beetles have just recently been moving into fields. Adults and larvae feed on potato foliage; however, larvae cause more damage, with about 75% of defoliation being caused by the 4th (last) instar. Extensive defoliation can reduce or prevent tuber development. Therefore, treatment must be directed at early instars to reduce the extent of damage and subsequent yield loss. Potato fields established with treated seed tubers are protected against first-generation larvae.

Begin monitoring for the orange CPB egg masses as adult beetles migrate into the potato fields. Flag and monitor the first egg masses detected, and apply a foliar spray at 15 to 30% egg hatch to target newly emerging larvae. A clean-up application may be necessary 5 to 10 days later. When making multiple insecticide applications to control CPB, always alternate the class of insecticide used because this potato pest has the ability to develop resistance to insecticides over a short period. Therefore, rotating the mode of action is of utmost importance when making multiple insecticide applications. Fields protected with a neonicotinoid (thiamethoxam or imidacloprid) seed treatment insecticide should not receive a second neonicotinoid treatment for use against the same generation of CPB. As a resistance management practice, insecticides of the same mode of action should not be used against the spring generation and the summer adults and 2nd generation larvae within a growing season.

Refer to the ND Field Crop Insect Mgt. Guide for a list of several insecticides that are registered for CPB.

The Guide is available on the web at

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

REMEMBER, to always read and follow the label directions.

Denise Olson
Research Entomologist
Denise.Olson@ndsu.edu

 

SCOUT FOR DIAMONDBACK MOTH LARVAE IN CANOLA FIELDS

Reports of diamondback moth (DBM) larvae causing feeding injury to bud-to-flowering canola are fairly common in north central North Dakota (Minot west to Stanley and north to Kenmare to Mohall, and then east to Bottineau). All DBM life stages (adults, eggs, larvae, and pupae) can be found in the field now. Larvae are small (about ˝ inch long), light green, and have a forked posterior. They have the habit of spinning a strand of silk and sliding down it when disturbed. Larvae feed on leaves, buds, flowers, seed pods, the green outer layer of the stems, and occasionally, the developing seeds of canola. The extent of damage will depend on crop stage, as well as larval size and infestation level. Extensive feeding on the flowers will delay plant maturity, cause uneven crop development, and significantly reduce seed yield. Producers can scout for diamondback moth larvae by beating plants and dislodging larvae from plants into buckets or onto the roof of a vehicle. Early monitoring of larvae and judicious use of insecticides when fields reach thresholds are the best pest management practices for preventing losses from diamondback moth in canola. Economic thresholds are: 10-15 larvae per square foot (1_2 larvae per plant) during flower; or 20 larvae per square foot (2_3 larvae per plant) during podding. Many fields are close to the proper spray timing for Sclerotinia control (early flowering). If you are planning to spray for Sclerotinia, a spray application of an insecticide-fungicide tank mixture is recommended. The pyrethroid insecticides (i.e., Capture, Decis, Warrior, Proaxis) labeled in canola are compatible with canola fungicides, such as Ronilan. Late-planted (June) canola will also need to be scouted for second generation DBM that will be emerging soon. There are usually three generations (32 days for each generation) per year of DBM in North Dakota.

 

BERTHA ARMYWORM EMERGENCE UNDERWAY IN CANOLA

Pheromone trap catches indicate that the first emergence of bertha armyworm is underway in canola for the 2005 season. Pheromone traps will be monitored until the end of July. Future updates will include any needed pest alerts due to high trap catches.

 

WHEAT MIDGE EMERGENCE UNDERWAY IN WHEAT & DURUM - TIME TO SCOUT

Wheat midge degree days are moving along fast due to the hot summer days we had last week! Most of the southern half and central regions of North Dakota are between 1300 to 1600 degree days when the female wheat midge emerges.

Females begin emerging at 1300 degree days (DD), and emergence is 90% complete at about 1600 DD (base = 40 degrees F). Also, moist soil conditions are conducive to wheat midge emergence. It is always a good idea to scout any wheat/durum fields in the susceptible crop stage – heading to early flowering during female wheat midge emergence. The midge emergence and wheat growth degree day model allows for estimates to be made based on specific planting dates. It identifies the expected growth stage of wheat and emergence time, as well as associated risk for egg laying and infestation by wheat midge. It is available at NDAWN (North Dakota Agricultural Weather Network) on the web at the following url:

http://ndawn.ndsu.nodak.edu/wheat_midge_form.html

Scouting should be conducted at night (after 9:00 PM), during warm nights (>60 degrees F or greater), and under light winds (<6 mph). Economic thresholds are one wheat midge per 4-5 heads for wheat, and one midge per 7-8 heads for durum. Typically, the most significant flight period for the entire wheat midge population extends over a 14 to 18-day window of time within a region. Individual adults can survive from 3 to 7 days, depending on environmental conditions. Observations over the years in North Dakota indicate that by about 1800 DD, adult numbers decline to the point where field activity is below economic levels. However, significant adult activity has been reported up to about 1900 DD in areas where reduced or minimum tillage is common. Remember not to confuse the Lauxanid fly for the wheat midge fly. The Lauxanid fly is larger (2.5-4 mm long), has a more robust body, and is yellowish-brown in color. In contrast, the wheat midge is a smaller (2-3 mm), more delicate fly, and is orange in color. No field reports are available yet this season.

 

CAN INSECTICIDE RATES BE REDUCED FOR CONTROL OF WHEAT MIDGE?

Many producers are interested in reducing insecticide rates for controlling wheat midge. A 2001 study conducted on durum (cv. Monroe) near Plaza-Makoti compared reduced rates of Lorsban and Penncap-M with and without a fungicide for scab and leaf disease control (Knodel and McKay unpublished). Night visual observations for adult wheat midge averaged > 1 wheat midge per 7 wheat heads in 2001, which is the economic threshold level for durum.

All insecticides and insecticide-fungicide treatments resulted in lower wheat midge infestations than the untreated check and fungicide-alone treatments (Table 1).

Insecticide treatments ranked in ascending order for the number of wheat midge larvae per head include:

Average wheat scab severity was 37% in the check, 37% in insecticide-treated plots, 28% in insecticide-fungicide plots, and 28% in fungicide-treated plots. Overall, field severity of scab was lower in fungicide and insecticide-fungicide plots than insecticide-treated plots and the untreated check. Folicur-only plots had lower field severity for scab than Penncozeb plots. Again, the fungicide and insecticide-fungicide treatments had lower severity of leaf diseases than the insecticide alone and the untreated check. Average severity of leaf diseases was 41% in the untreated check, 42% in insecticide-treated plots, 22.5% in the insecticide-fungicide plots, and 27% in the fungicide-only plots.

Table 1. Results of wheat midge, scab, and leaf disease counts.

Treatment

Rate

Avg. # Larvae per Wheat Head

% Kernels Infested

Scab Field

Leaf Diseases

Formulation

Fl. oz per acre

Severity*

Severity

Untreated

 

17.6

96

37.1

40.6

Lorsban 4E-SG

8

10.5

45

36.8

50.2

Lorsban 4E-SG +
Folicur 3.6F +
Induce

8
4
0.125 V/V

9.7

40

21.7

14.5

Lorsban 4E-SG

16

7.1

35

35.7

44.1

Lorsban 4E-SG +
Folicur 3.6F +
Induce

16
4
0.125 V/V

6.9

30

20.4

13.2

Folicur 3.6F +
Induce

4
0.125 V/V

12.1

56

26.2

21.7

Penncap-M 2EC +
Induce

16
0.125 V/V

8.4

37

39.4

41.6

Penncap-M 2EC +
Penncozeb 75DF +
Induce

16
1 lb/acre
0.125 V/V

13.5

62

36.6

35.8

Penncap-M 2EC +
Induce

32
0.125 V/V

12.9

53

35.8

31.9

Penncap-M 2EC +
Penncozeb 75DF +
Induce

32
1 lb/acre
0.125 V/V

9.8

43

33.2

26.9

Penncozeb 75DF +
Induce

1 lb/acre
0.125 V/V

14.1

66

30.8

31.5

C.V.%

 

64.6

67.3

29.5

33.7

LSD 5%

 

8.4

40

11.1

12.5

Means within the same column are separated by Anova and Fisher’s PLSD at 5% significance level.
*FHB field severity = incidence (% tillers with scab symptoms) x head severity.

Harvest results are summarized in Table 2. In general, treatments with both insecticide+fungicide had higher yields than the other treatments. This suggests that the pressures from both wheat midge and diseases reduced the overall yield. The untreated check (33.4 bu per acre) had 9.3 fewer bushels per acre than the treatment with the highest yield, 8 oz per acre of Lorsban + 4 oz per acre of Folicur + Induce (42.7 bu per acre). Insecticide-only treatments averaged a 3.5 bu per acre increase, the insecticide-fungicide treatments averaged a 6.5 bu per acre increase, and the fungicide-alone treatments averaged a 1.5 bu per acre increase. Local environmental conditions may have impacted the level of disease infection. For test weight, only three treatments had a significantly higher test weight than the untreated control: 16 oz per acre of Lorsban + 4 oz per acre of Folicur + Induce; 8 oz per acre of Lorsban + 4 oz per acre of Folicur + Induce; and 32 oz per acre of Penncap-M + 1 lb per acre of Penncozeb + Induce. No significant differences were recorded in grain protein content among treatments. For percent damaged seed, the untreated check had 16% damage, and was significantly higher than all of treatments except 32 oz per acre of Penncap-M + Induce.  Lorsban + Folicur + Induce treatments (both low and high rates of Lorsban) had the lowest percent damaged seed, and the highest percent gain.

The estimated cost of Lorsban is about $2.50/acre for 8 oz/acre and $5.00/acre for 16 oz/acre, whereas Penncap-M costs about $3.50/acre for 16 oz per acre, and $7.00/acre for 32 oz/acre. Application cost for Folicur at 4 oz/acre is about $10.36/acre, and $2.62/acre when combined with Penncozeb at 1 lb/acre. Induce (the adjuvant), is about $0.50/acre. As a result, an application of Lorsban + Folicur + Induce ranges from $13.36 to $15.86/acre; in contrast, Penncap-M + Penncozeb + Induce ranges from $6.62 to $10.12/acre.

Summary: If reduced rates of insecticides are timed to coincide with peak wheat midge emergence, they will control wheat midge. However, the reduced rates will have a shorter residual (<3 days), which makes proper timing of the sprays more important for effective wheat midge control.

Table 2. Summary of harvest results.

Treatment

Rate

Avg. Yield

Avg. Test Weight

% Protein

% Seed Damaged

Yield Increase

% Gain

Formulation

fl. oz/acre

bu/acre

lb/acre

bu/acre

Untreated

 

33.4

53.9

14.7

16.0

0

--

Lorsban 4E-SG

8

38.0

55.0

14.7

10.8

4.6

12.1

Lorsban 4E-SG +
Folicur 3.6F +
Induce

8
4
0.125 V/V

42.7

55.8

14.6

8.5

9.3

21.8

Lorsban 4E-SG

16

36.9

54.5

14.7

11.2

3.5

9.5

Lorsban 4E-SG +
Folicur 3.6F +
Induce

16
4
0.125 V/V

41.1

56.2

14.6

8.6

7.7

18.7

Folicur 3.6F +
Induce

4
0.125 V/V

35.9

54.9

14.8

8.9

2.5

7.0

Penncap-M 2 EC +
Induce

16
0.125 V/V

37.3

54.4

14.8

10.7

3.9

10.5

Penncap-M 2EC +
Penncozeb 75DF +
Induce

16
1 lb/acre
0.125 V/V

37.6

54.9

14.7

10.0

4.2

11.2

Penncap-M 2EC +
Induce

32
0.125 V/V

35.5

53.8

14.6

13.2

2.1

5.9

Penncap-M 2EC +
Penncozeb 75DF +
Induce

32
1 lb/acre
0.125 V/V

38.1

55.2

14.6

10.3

4.7

12.3

Penncozeb 75DF +
Induce

1 lb/acre
0.125 V/V

33.8

54.7

14.8

10.6

0.4

1.2

C.V.%

 

12.3

1.9

2.2

34.9

 

 

LSD 5%

 

5.4

1.2

NS

4.4

   

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


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