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ISSUE 11  July 11, 2002

 

SOYBEAN APHID ACTIVITY INCREASING

Soybean aphid has been found in increasing numbers, particularly soybean fields nearer the ND-MN state line. One field near Harwood, ND had aphid numbers go from none detected last week to where 200 or more aphids can now be found on some randomly selected plants. Infested plants were near the side of the field bordered by a shelterbelt. Anyone managing soybeans needs to watch fields closely to determine if aphids are present and what is the size of the population.

Infested plants this week have had small colonies. Many of the colonies are numbering 5 to 15 aphids with the adult female in the center surrounded by her offspring. Remember, these aphids are all females, they give birth to live young, and, within seven to ten days, the aphid is mature and having babies of her own. They have very high reproductive potential that can result in rapid increase in the size of these field populations. What is reported in this issue of the Crop and Pest Report may be very different by the first of next week.

 

SOYBEAN APHID vs POTATO LEAFHOPPER NYMPHS

While scouting soybeans for aphids, be sure to recognize the difference between these two insects. The key differences are summarized below.

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Soybean Aphid

Potato Leafhopper

yellow

pale green

pear-shaped

longer than wide

stationary

moves sideways rapidly

found in colonies

found separately

Be sure to recognize the difference between the two. Typically, we havenít seen problems with potato leafhopper in soybean. One published threshold for potato leafhopper in soybean suggests "... 9 leafhoppers per plant for R1-R2 (first bloom) stage soybean." Leafhopper concerns in soybean are most often associated with dry conditions.

 

POTATO LEAFHOPPER IN OTHER CROPS

Dry beans, alfalfa, and potato are affected more seriously by potato leafhopper. Feeding injury by potato leafhoppers results in a symptom called "hopperburn". The visual symptoms include leaves changing from green to yellow to brown as they deteriorate. When leaf injury is present, plant growth is impaired. Control of potentially damaging populations should not be delayed to the point where damage symptoms are visible.

In alfalfa, potato leafhoppers can cause problems with regrowth after a cutting is completed. Monitoring regrowth with the use of 15 inch sweep nets to detect adults is highly recommended.

Treatment Thresholds for Potato Leafhoppers on Alfalfa

Alfalfa Stem Height (inches)

Leafhoppers/net sweep (average)

3

0.2 adults

6

0.5 adults

8 - 11

1.0 adult or nymph

12 - 14

2.0 adults or nymphs

Insecticides approved for use to control leafhoppers in alfalfa include: permethrin*, Baythroid*, carbaryl, dimethoate EC, Furadan*, Lannate*, Lorsban, Malathion 57 EC, Mustang, Penncap-M*, and Warrior*.

In dry beans, the threshold for basing spray decisions is when an average of one leafhopper per trifoliate leaf is found. Insecticides approved for use to control leafhoppers in dry beans include: Asana XL*, Dimethoate EC, endosulfan (Thiodan, Phaser), Lannate*, Malathion 57 EC, Orthene 75S, and Penncap-M*.

In potatoes, treatments are recommended when leafhoppers can be found at a level of 1 nymph per 10 leaves. Sample 35 leaves from 5 locations in a field. Sample for the presence of adult leafhoppers by using a sweep net. Do not let infestations and damage progress to the point that yellowing of foliage is easily detected. Insecticides approved for use to control leafhoppers in potato include: Actara, permethrin*, Asana XL*, Baythroid*, carbaryl, Dimethoate EC, endosulfan, Furadan 4F*, Guthion, Imidan, Vydate*, and Penncap-M*.

 

FLAX AND GRASSHOPPERS

Grasshoppers have been the number one threat to North Dakota flax in recent years. Young grasshoppers may attack young plants and cause damage. However, more damage is done to the crop before harvest by the older, larger, adult grasshoppers migrating to the fields after small grains ripen.. Under these conditions, grasshoppers can quickly cause large numbers of bolls to drop from plants when they chew through the more succulent portions of the stem below the boll.

Flax growers need to be aware of grasshopper activity in the vicinity of flax fields well before adult migration begins in July. Because of the limited availability of insecticides to control insects in flax, attempts to reduce grasshopper populations in neighboring crops and non-crop areas is advisable.

The only insecticides currently labeled for use in flax include carbaryl (Sevin formulations), and some malathion labels. A search of the ND Department of Agriculture pesticide database shows that the only malathion formulation labeled for use in flax is the product sold by Micro Flo Company LLC.

 

ARMYWORM IN SMALL GRAINS

True armyworm have been found in some small grain fields in the valley areas. While using sweep nets to sample for grasshoppers in wheat, there has been about 1 to 2 armyworm for 10 sweeps. This is not an overwhelming number, but using a sweep net during the day to sample armyworms is likely to underestimate numbers.

Some field inspection for the presence of armyworm would be recommended. If feeding damage is found in the foliage and no other responsible insects pest can be found, scout for armyworms by parting foliage on the plant and inspecting the plant and the soil below for small fecal pellets. If pellets are found then look for larvae under plant trash, soil clods, or in soil cracks. If 4_5 worms or more are found per sq. foot, treatment is recommended. Because populations are higher in lodged areas, be sure to average the population over the entire field.

 

WHEAT MIDGE EMERGENCE THROUGHOUT ND

Emergence of wheat midge females is underway throughout ND based on degree day accumulations. Emergence cages in NE Williams county had male midge in them earlier this week, just prior to expected female emergence. Peak female emergence occurs at 1450 DD. Female midge are most active from 1300 to 1800 DD. After 1800 DD have accumulated, the activity generally declines. However, it appears that in areas where reduced or minimum till is used, some emergence is delayed. Therefore, it is advisable to continue scouting heading wheat until 50% flowering or there is no indication of midge activity.

 

EUROPEAN CORN BORER EMERGING

European corn borer moths have been emerging over the past couple of weeks. The numbers of moths have been very low; black light traps in three areas have caught lots of other insects, though..

Currently, the moths we are catching represent the univoltine, or single generation, ECB. The univoltine ECB has been the dominant type infesting our regionís corn during the past 10 years. We have used a degree day model to help identify when emergence is underway, and when peak activity occurs.

 

Degree Day Accumulations for Univoltine European Corn Borer Moth Emergence

Percent of Moths Emerged

Degree Day Accumulation (modified Base 50 F)

10

910

25

986

50

1078

75

1177

90

1274

Currently, 1100+ DD have accumulated in the southern most ND counties. In the central part of the valley and westward, DD have reached the 1050+ level, or peak emergence.

The degree day accumulations are determined using a formula we refer to a as a modified base 50EF. This is the same method of calculating growing degree days

for corn. Therefore, one model for monitoring corn growth also can be used for its key insect pest in the region.

Field scouting for corn borers:

Whorl stage corn . . . . Pull the whorls from 10 plants at 5 locations across the field. Select whorls at random, avoiding damaged plants. Unwrap the whorl leaves; count and record the number of live larvae found.

Use the corn borer worksheet to help make decisions about the profitability of treating an individual field.

Worksheet for Corn borer in whorl stage corn . . . You fill in the blanks

1. ___% of plants infested

x ___Avg no. borers/plant

= ___Borers per plant

2. ___borers per plant

x ___% yield loss per borer*

= ___percent yield loss

3. ___percent yield loss

x ___expected yield (bu/acre)

= ___bushels/a loss

4. ___bushel loss per acre

x ___price per bushel

= $ ___loss per acre

5. ___loss per acre

x ___percent control**

= $ ___preventable loss/a

6. ___preventable loss/acre

- ___cost of control per acre

= $ ___profit (loss)/acre

*5% for corn in the early whorl stage; 4% for late whorl; 6% for pretassel
**80% for granules; 70% for sprays.

Economic Threshold (Corn borer/plant) when factoring Crop Value and Control Costs

Control Costs2 ($/acre)

Value of Corn Crop1 ($/acre)

200

250

300

350

400

450

500

550

600

6

0.75

0.60

0.50

0.43

0.38

0.34

0.30

0.27

0.25

7

0.88

0.70

0.58

0.50

0.44

0.39

0.35

0.32

0.29

8

1.00

0.80

0.67

0.57

0.50

0.45

0.40

0.37

0.34

9

1.12

0.90

0.75

0.64

0.56

0.50

0.45

0.41

0.38

10

1.25

1.00

0.83

0.71

0.63

0.56

0.50

0.46

0.42

11

1.38

1.10

0.92

0.79

0.69

0.61

0.55

0.50

0.46

12

1.50

1.20

1.00

0.86

0.75

0.67

0.60

0.55

0.50

13

1.63

1.30

1.08

0.93

0.81

0.72

0.65

0.59

0.54

14

1.75

1.40

1.17

1.00

0.88

0.78

0.70

0.64

0.59

15

1.88

1.50

1.25

1.07

0.94

0.84

0.75

0.68

0.63

16

2.00

1.60

1.33

1.14

1.00

0.89

0.80

0.73

0.68

1 Crop value = expected yield (bu/acre) X projected price ($/bu)
2 Control costs = insecticide price ($/acre) + application costs ($/acre)

Phillip Glogoza
Extension Entomologist

pglogoza@ndsuext.nodak.edu


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