NDSU Crop and Pest Report

Entomology


ISSUE 12  July 17, 2003

SOYBEAN APHID: EASIER TO FIND IN EASTERN COUNTIES, NUMBERS STILL LOW

The soybean aphid is becoming easier to find, but numbers have still been very low in the southeastern counties of North Dakota. The IPM survey scouts report finding infested plants in protected areas near shelterbelts or along field margins. Percent infested plant levels are low. When aphid colonies are found, they have been in the 4 to 5, some just starting to reach 10 or more, aphids per colony range. There were some reports from the Fergus Falls, MN area, where infestations were large last year, of infested plants having aphid counts of 100+, so keep an eye on this situation.

The NDSU Crop and Pest survey for this last week for soybean aphid found the following infested fields. There are other confirmed reports from Richland, Cass, Ransom, and Sargent that lack georeferenced points for insertion into the map.

Soybean aphid adult image
Adult

Soybeans are flowering in the majority of fields, so we have entered the high risk growth stages that require field scouting to determine if thresholds have been reached.

 

EUROPEAN CORN BORER FLIGHT CONTINUES

The moth flight of the univoltine corn borer is well underway in North Dakota. Richland county has about 90% of the moths emerged based on degree day accumulations. In the Carrington area, there are about 50% of the moths emerged. Field checks during the Carrington Research and Extension Field Day were able to detect egg masses on the undersides of leaves.

Field scouting corn to find eggs or small larvae should begin this week. Decisions to treat non-Bt corn will probably be made in the next 10 to 14 days. Corn is a little different growth stage than we are accustomed for when we see corn borer larvae from this univoltine flight. A lot of the ND corn is still in the whorl stage. Larvae hatching soon will move to that whorl, where they will be a lot easier to find than when we scout tasseling corn.

Thresholds for corn borer are summarized in the table below. The thresholds are based on value of the corn and the cost of control. We have such a range of yield potential from north to south and east to west, this chart should help define the practical treatment decisions for our range of conditions.

Phillip Glogoza
Extension Entomologist
pglogoza@ndsuext.nodak.edu

 

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)

 

ARE LYGUS BUGS CAUSING PROBLEMS IN YOUR PEAS OR LENTILS?

Lygus bugs are comprised of several species belonging to the genus Lygus. The tarnished plant bug, Lygus lineolaris, is one of the more common species and is known to feed on over 385 crop and weed plants. Adult Lygus bugs are about inch in length, and pale green, light brown, or dark brown with a distinctive triangular marking on its back.

Lygus bugs overwinter as adults in weedy areas under debris along fence rows, ditches and roadsides. Adults emerge in early spring, lay eggs in the stems, leaves and flowers of host plants, and then die. Immature nymphs hatch from these eggs and look like aphids. Several generations occur each year with the second generation occurring in mid-July to early August. As with many other insect pests, warm, dry weather favors the build up of Lygus populations and increases the potential for early season damage to peas or lentils. Both immature and adult Lygus bugs feed on developing pods and seeds of peas/lentils, and have been linked to "chalk spot."

Damage is caused by the piercing-sucking mouthpart, which punctures the pods and seed coats injecting a toxic substance into plant parts. Chalk spot is a pit or crater-like depression in the seed coat with or without a discolored chalky appearance. Damage seeds are smaller, deteriorate faster in storage, have poor germination, and produce abnormal seedlings as well as lower the grade and marketability.

It is important not to confuse damage caused by Lygus bug to damage caused by rough harvesting or handling. For example, peas harvested at high moisture levels are also susceptible to bruising when harvested or handled roughly, resulting in damage similar to chalk spot. There is increasing concern about Lygus bug damage on peas and lentils in north central and northwestern North Dakota due to the numerous truck loads of peas and lentils that were identified with chalk spot this past year.

Pest Management:

Monitor for Lygus bugs using a 15_inch sweep net during bloom to pod development (until seeds within the pod have become firm). Make ten, 180 degree sweeps at five sampling sites in a field during the warm sunny part of the day (2 to 6 PM). Lygus populations can increase suddenly. For example, when an alfalfa (preferred host) field is cut, Lygus adults will migrate quickly into nearby pulse crop fields, often in high numbers. No economic threshold has been determined for this region. However, in the Pacific Northwest, an insecticide treatment is recommended when "10 Lygus per 25 sweeps" are present. Current trapping and sweep net samples indicate that moderate numbers (average of 5 per 25 sweeps) are present near New Town, and low numbers (average of 1 per 25 sweeps) near Minot and Bowbells. Insecticides labeled for control of Lygus bug in peas and lentils include: carbaryl (Sevin), dimethoate, Asana, Mustang, and Warrior (recent label addition). Spray a blooming crop when there is minimal bee activity, preferably during the evening hours (after 8 PM).

Janet Knodel
Area Extension Specialist Crop Protection
North Central Research Extension Center
Minot, ND

 

U OF MINNESOTA APHID ALERT: APHID SITUATION, JULY 10

(the following information is from Dr Ted Radcliffe, Entomologist, U of MN)

Aphid flight activity remained light throughout the Northern Great Plains in the week ending July 10. Again, the species most commonly caught in our traps was bird cherry-oat aphid. Highest numbers of bird cherry-oat aphid were captured in traps at Karlstad and Williams. Flight activity of this species is expected to increase as small grains ripen. Bird cherry-oat aphid does not colonize potato, but is an efficient vector of PVY.

Turnip aphid was also commonly represented in the captures. Turnip aphid, like green peach aphid, reproduces abundantly on canola, but unlike green peach aphid, does not appear to be of importance in transmission of any potato virus.

Potato aphids have been abundant colonizers of potato this spring. To date we have only captured one green peach aphid, this week at Climax, MN, but green peach aphid apterae are found in low numbers on weed hosts, e.g., common mallow and redroot pigweed. Further to the south, at Rosemount, MN, green peach aphid are already well established on potato. We anticipate green peach aphid flight activity in the Northern Great Plains will remain light for the next 2-3 weeks and then peak in early August.

Adult potato leafhoppers remain abundant in potato fields throughout the region. All life stages, adults to small nymphs, are present in fields. Some growers have reported potatoes showing "hopperburn." Pay attention to leafhopper populations in potatoes, as well as dry edible beans and alfalfa, the other crops which are susceptible to leafhopper feeding.

http://ipmworld.umn.edu/alert.htm


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