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ISSUE 15    August 17, 2006

ERRATUM IN SOYBEAN HARVEST RESTRICTION:

The preharvest interval is 30 days for Warrior and Proaxis insecticides in soybeans (not 45 days as previously reported in Crop & Pest Report Issue 14).

 

SOYBEAN APHIDS MOVE WEST

Economic levels of soybean aphids were recently reported in soybeans from McLean County near Washburn. This is the first report of treatable levels of soybean aphids in the North Central Region. Winds were favorable for aphid migration into western North Dakota over the past weekend. In eastern North Dakota, the NDSU IPM Survey data indicate DECREASING populations of soybean aphids from previous weeks (see map). Last year, aphid populations rebounded in later planted soybean fields in late August and early September. So, continue scouting soybean fields for aphids until R6 or full seed!

soybean aphid map

 

SECOND GENERATION BEAN LEAF BEETLE IN SOYBEANS

A few soybean fields in southeastern North Dakota have reported with low levels of bean leaf beetles (BLB) in soybeans. This is the 2nd generation of BLB, and the most important in terms of potential yield impact due to its pod feeding. This causes pod scarring, pod clipping and decreased seed quality. Fungal pathogens can also infect injured pods causing discolored, shrunken or moldy pods. Adult BLB (see photo) are 5 mm long with yellow or red wing covers with four black spots and black margin around the edges of the wing covers. The main identification characteristic is the black triangle behind the head. Feeding injury symptoms include round holes between leaf veins. Economic threshold is 25% defoliation from reproductive pod fill to maturity. The following economic threshold table were developed for 2nd generation BLB by Dr. M. Rice and colleagues at Iowa State University.

bean leaf beetle
Bean Leaf Beetle

Table 3-6. Second generation bean leaf beetle economic thresholds in reproductive stage soybean*

Crop Value
($/bu)

Treatment cost / acre (insecticide plus application)

$7

$8

$9

$10

$11

$12

$13

$14

$15

 

---------------------------------------beetles / foot of row-----------------------------------

$5.00

5.5

6.3

7.1

7.9

8.7

9.5

10.3

11.0

11.8

$6.00

4.6

5.2

5.9

6.5

7.2

7.8

8.5

9.2

9.9

$7.00

3.9

4.4

5.0

5.6

6.1

6.7

7.3

7.8

8.4

$8.00

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

 

-----------------------------------------beetles / sweep-------------------------------------

$5.00

3.5

4.0

4.5

5.0

6.5

7.2

7.7

8.3

8.7

$6.00

2.9

3.3

3.7

4.1

5.4

6.0

6.4

6.9

7.3

$7.00

2.4

2.8

3.1

3.5

3.8

4.2

4.5

4.9

5.2

$8.00

2.2

2.5

2.8

3.2

4.1

4.5

4.8

5.2

5.5

* Economic thresholds are based on a row spacing of 30 inches and a plant population of eight plants per foot of row. For narrow-row soybeans (8-inch rows) and a plant population of three plants per foot of row, multiply the above economic thresholds by 0.70.  Source: M. Rice, Iowa State University, 2000.

 

TWO-SPOTTED SPIDER MITES CONTINUE TO THREATEN SOYBEANS

Field reports indicate that treatable levels of spider mites still exist in soybean fields over past weeks. However, the recent rains have improved conditions for soybeans and reduced the overall stress from spider mite injury. Unfortunately, the rains are not enough to control mites. There is no specific threshold that has been developed for two-spotted spider mite in soybean. As a result, several guidelines are available from different states to determine economic infestation of spider mites in soybeans. One of the more commonly used economic threshold is based on different crop stages:

Do not treat after early maturity stages. The discoloration is caused from the feeding injury (e.g., leaf stippling) of spider mites, and is difficult to estimate in the field. Be sure to scout during full pod (R4) through beginning seed (R5) stages since these crop stages are the most important contributors to soybean yield. If hot dry conditions exist, mites populations can quickly increase and sprays should not be delayed. Minnesota describes spray treatments level as when "there is heavy stippling on lower leaves with some stippling progressing into middle canopy; mites present in middle canopy with scattered colonies in upper canopy; lower leaf yellowing common; small areas with lower leaf loss" (Source: K. Ostlie). Remember to use an organophosphate insecticide (e.g. Lorsban, Dimethoate) over a pyrethroid insecticide to avoid flaring mite populations. See Crop & Pest Report Issue 13 for more information.

 

INSECT IDENTIFICATION QUIZ

Question: What is this caterpillar observed feeding alfalfa in north central North Dakota?

armyworm

Answer: This is the armyworm (Pseudaletia unipuncta 1809), which can be injurious to alfalfa. Larvae feed on leaf tissue, often stripping the leaf and leaving only the stalks and leaf midribs. It does not overwinter here and migrates into North Dakota from the southern states. It is a general feeder on many host plants including corn, millet, bluegrass, wheat, and some legumes. The full-grown larvae is 1.5 inches long, green-brown with black mottling and white flecks, and dark strips on back. The head is yellow-brown with a brown netlike pattern of dark lines. The adult moth is pale gray-brown to pale brown with a 1 inch wingspan with a white dot in center of each forewing.

 

FLEA BEETLE POPULATIONS CONTINUE TO BE LOW IN SWATH CANOLA FIELDS

The 2006 NDSU IPM Survey indicates similar numbers of flea beetles per sweep (average of 16 beetles per 4 sweeps) compared to last year data (average of 15 beetles per 4 sweeps). Overall, the flea beetle populations continue to be low (see map). The summer population levels of flea beetles in the swathed canola can be used as a general risk indicator for flea beetle infestations for the upcoming spring.

flea beetle map

Janet Knodel
Extension Entomologist
janet.knodel@ndsu.edu

 

POTATO LEAFHOPPER: DONíT IGNORE THIS PEST DURING LATE SEASON

Potato leafhopper (PLH) cannot survive northern winters and arrives in the Midwest on upper air currents from the Gulf States during spring and early summer. The initial immigrants infest forage crops and during years when they arrive in the spring, subsequent generations build to high population levels. By mid to late June or early July, these populations move to other host crops including soybean, dry edible beans and peas, and in particular late season potatoes. Potato leafhoppers pass through several generations during the potato production season and they are usually present in the crop from early summer until the first killing frost.

Potato leafhopper adults are approximately 1/8" long, wedge-shaped, pale yellow-green with inconspicuous white spots on and directly behind the head (Figure 1). Nymphs resemble adults, but are wingless and more yellowish. Larger nymphs possess bud-like wings; these wing buds develop into functional wings of the adults (Figure 1). Adults and nymphs run backwards or sideways when disturbed.

potato leafhopper
Figure 1.  Potato Leafhopper

Potato leafhopper females lay eggs on the undersurface of leaves of their host plants. Both adults and nymphs use their piercing-sucking mouth parts to remove plant juices from the vegetation. The nymphal stage is the most injurious, as nymphs feed on the underside of leaves where they hatch. The fist sign of feeding by PLH is "hopperburn", wedge-shape yellowing of the leaf tips (chlorotic leaf tips) (Figure 2).

hopperburn
Figure 2.  Hopperburn

"Hopperburn" is produced by the plant in response to a toxin in the saliva of the PLH and it begins on older foliage and moves upward. When leafy injury is present, overall vegetative plant growth is stunted. Potato leafhoppers need to be monitored for and controlled before "hopperburn" is observed, as yield losses occur before injury is noticeable in the field. Yield loss is not directly related to the level of "hopperburn" and is worst during periods of hot dry conditions. When not controlled, feeding by PLH can result in yield losses of 30-80%.

Because PLH is a season long pest, potato fields need to be scouted weekly until first frost by sweep net for adults and by leaf counts for nymphs. Start sampling 100 feet from the field edges and sample in 5 locations in the field walking an X pattern. Determine the average number of PLH per sweep. Treat infested fields only when thresholds are exceeded.

  1. If over 1.5 adults/sweep are present, treat immediately.
  2. If 1.0-1.5 adults/sweep, treat in 5-7 days or immediately if nymphs are found.
  3. If 0.5-1.0 adults/sweep are present, treat if PLH persist for 10-14 days or if nymphs are found.
  4. When adults are below 0.5 adults/sweep, treat only when more than 1 nymph/10 leaves is present.

Potato leafhoppers are easily controlled by most insecticide products labled for this pest. Refer to the 2006 North Dakota Field Crop Insect Management Guide for products, application rates and restrictions on product use. Potato leafhopper has a broad host range and insecticide resistance in this pest has not been a concern; however, be sure to use insecticides which will not select for resistance to Colorado potato beetle when this insect is also present in the potato crop.

Denise Olson
Research Entomologist
Denise.Olson@ndsu.edu


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