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ISSUE 13   August 13, 2009

SCOUTING FOR SOYBEAN APHIDS AND SPIDER MITES IN SOYBEANS STILL CRITICAL

Soybean aphid populations continue to build slowly in the Red River Valley region. Our test plots near Prosper/Amenia have 25-50 aphids per plant on 25% of the plants (Fig. 1). Aphid "hot spots" (sites of initial infestation) are uniformly distributed in the field. We noted several alates (winged aphids) surrounded by small nymphs, indicating that soybean aphids are still moving into the area and are reproducing.


Figure 1.
Soybean aphid observations.
(Source: Soybean IPM PIPE)

We have also noticed an increase in aphid predators (lady beetles, lacewings, syrphid fly larvae, etc.). The unseasonably cool temperatures during July and the first week of August have kept aphid populations in check. However, warm daytime temperatures (80-85 F) and moderate night temperatures (60-65 F) are forecast for the next two weeks and this may cause soybean aphid populations to build more rapidly. Continue to scout for soybean aphids and monitor existing populations closely. NDSU Extension Entomology advocates the economic threshold of 250 aphids/plant in 80% of the field with population increasing. Research in the North Central Region has shown that spraying below the economic threshold will not show economic returns. This practice also disrupts the natural control from biocontrol agents (predators, parasitoids, entomopathogenic fungi) in the field and increases selection pressures to for insect resistance to insecticides.

The recent rains are welcomed and may help subside the growing spider mite problem in soybeans. However, new field reports have come in from Fargo, Cooperstown and Valley City, in addition to the reports earlier in Dazey, Montpelier and Mayville-Portland areas. These fields are near or at treatable levels for spider mites. Please see last week’s report of Crop & Pest Report Issue 12 - July 29, 2009 for additional information on scouting, action thresholds and control of spider mites in soybeans.

Patrick Beauzay
Entomology Research Specialist
patrick.beauzay@ndsu.edu

 

SCOUT FOR PEA APHIDS IN LENTILS

High populations of pea aphids (Fig. 2) are moving out of pea fields and looking for other green crops to feed on, like lentils in the north central and northwestern regions of ND.


Figure 2.
Pea aphids on pea (K. McPhee, NDSU)

Most lentils are still in the flowering stage or just completing flowering, so they are in the susceptible crop stage to aphid injury.  Lentil fields need to be scouted to prevent yield loss to pea aphids. Fields that have not started to yellow and ripen, especially if they are still flowering, are a priority. Fields that are yellowing and ripening should be ahead of any aphid development.

One lentil field in McLean County had to be treated for pea aphids on August 6 due to high aphid numbers. (Source: K. McKay, Vision Research Park LLC.)

For pea aphids in lentils, a nominal threshold is 30 to 40 aphids per 180º sweep of a 38 cm (15 inch) diameter insect net, and few natural enemies are present, and when aphid numbers do not decline over a 2-day period. (Source: J. Gavloski, Manitoba Agriculture, Food and Rural Initiatives)

 

NEW INSECT PEST IN SOYBEANS?

While conducting soybean aphid counts in the Rag1 resistant soybean aphid study, pinholes were observed in the leaves (Fig. 3). The mystery insect causing the injury was identified as the red-headed flea beetle (Systena frontalis) (Fig. 4). The red-headed flea beetle is about 1/6 inch long and dark black with a reddish head. It feeds on over 40 different host plants including cabbage, beans, beets, corn, alfalfa, potatoes, nursery crops, cranberry and many weed species.


Figure 3.
Pinholes in soybean leaf caused by
red-headed flea beetle
(P. Beauzay)


Figure 4.
Adult red-headed flea beetle (P. Beauzay)

It overwinters in the egg stage in the soil. Eggs hatch in June and larvae feed on the roots. Larvae pupate and then adults emerge in July-August and feed on foliage until September. Adults deposit eggs in soil, which overwinter. There is only one generation per year. Extension reports from other states indicate that the adult stage is readily controlled by foliar insecticides registered in different crops. An action threshold for determining the need for a rescue treatment would be based on percent defoliation and the stage of soybean (Fig. 5): 30% in vegetative stages (prebloom), 15% in bloom to pod-fill, and 25% in pod-fill to maturity (unless pod feeding observed). While we don’t know if this flea beetle will become an occasional insect pest of soybean in the future, it is interesting to note.


Figure 5.
Soybean defoliation guide.

 

SUNFLOWER MIDGE RETURNS

Severe sunflower midge damage has been reported from the Carrington and other areas of Foster County this past week. Some fields were severely damaged and sunflower heads were reported to be 100% destroyed throughout the field (Fig. 6).

Typically, only heads along the field edge are more severely infested. Unfortunately, there are no actions to take at this time. Pest management relies on cultural practices done prior to planting, since chemical control is not effective against sunflower midge. Next year, planting a tolerant variety of sunflower is recommended and planting at multiple dates to minimize the extent of midge infestation. New fields planted next year should be established away from fields damaged the previous season. One producer was also wondering if he should destroy the field to help mitigate sunflower midge population next year. However, it is best to wait until it is well into bloom to assess the total seed loss. A field can have few to no ray pedals and still produce a reasonable crop (Source: J. Swanson).


Figure 6.
Heads deformed by sunflower midge (G. Endres)

The sunflower midge overwinters in the soil as a cocooned larva and pupates during June and July in North Dakota and Minnesota. Typically, the initial peak of first generation adult emergence occurs in early to mid-July. A second peak occurs about 7 to 10 days later. They prefer to lay eggs on sunflower buds with a diameter greater than 1 inch. Larvae initially feed on margins of the head between the bracts surrounding the head. Larvae migrate to the base of the developing seeds and to the center of the head as it develops. Presence of larvae is determined by necrotic areas at the bases of or between the bracts. As midge larvae mature, they move to the surface of the head and drop to the ground (Fig. 7). A partial, second generation occurs in August. Second generation adults oviposit among the seeds.


Figure 7.
Larva of sunflower midge (P. Beauzay)

Damage to sunflower is a result of first generation larval feeding in developing heads. When populations are low, damage is restricted to the base of the bracts of the head and causes slight localized necrosis but little if any economic loss. When many larvae are present, feeding prevents ray petal formation and distorts the growth of the developing sunflower head. If the abnormal growth is severe, the back of the head overgrows the front and little or no seed production occurs. If an infestation occurs in the early bud stage, the bud may be killed.

 

CABBAGE MAGGOT ROOT INJURY OBSERVED IN CANOLA

Tammy Duchsherer of NCREC in Minot has been scouting canola fields for pests all summer! It is interesting to report that she is finding root-feeding injury (Fig. 8) from the cabbage maggot this year.


Figure 8.
Root injury and larva of cabbage maggot
(Canola Council of Canada)

It would be easy to miss unless the roots are pulled up and examined, as the canola plant will continue to grow. She reports that the cabbage maggot damage is fairly widespread across the northern canola growing regions of North Dakota.

Cabbage maggot (Fig. 9) infestations are typically more severe with cool wet springs, like 2009. Heavy maggot damage in canola can stop blooming and cause lodging and yield losses. The root injury also serves as a point of entry for root rot fungi, causing additional stress on plant. Yield losses of 20-50% have been observed in Canada from cabbage maggot. John Gavloski of Manitoba Agriculture, Food and Rural Initiatives also reports moderate to high levels of root maggot feeding in some fields near Snowflake and Foxwarren. Chemical control of cabbage maggot is difficult. There are no insecticides currently available as an in-furrow application for control of maggot infestations. Insecticide seed treatments are not effective.


Figure 9.
Adult cabbage maggot
(Canola Council of Canada)

 

GRASSHOPPER UPDATE

The USDA APHIS Rangeland Grasshopper & Mormon Cricket Survey indicates high populations of grasshoppers in the southwest and north central regions of North Dakota (Fig. 10). Grasshopper populations are also high and more widely dispersed in states further to the south (South Dakota, Nebraska, Oklahoma) and west (Montana). Continue to scout for grasshopper in ‘green’ late season crops until harvest or fall.


Figure 10.
Grasshopper survey map (USDA APHIS)

 

SCOUT FOR RED SUNFLOWER SEED WEEVIL

Red sunflower seed weevil are emerging and can be found in R4 and blooming sunflower fields in the major sunflower producing areas. There are some reports of economic levels of seed weevil.

Identification: Adults are small about inch and reddish-brown in color (Fig. 11). Newly emerged adults feed on the bracts, sunflower buds, and pollen. Peak emergence is usually in late July and early August. The female seed weevil must feed on pollen for fertile egg development.


Figure 11.
Adult red sunflower seed weevil

Scouting: Sunflower normally reaches the bud stage in late July at which time only about 30 percent of the weevils in the soil have pupated and emerged. Most weevils emerge from the soil by the first week of August. Field scouting for adults should begin when plants are showing yellow ray petals (R5.0) to 30% of the head shedding pollen (R5.3), and should continue until most of the plants have reached 70% pollen shed (R5.7). A plant that has reached R5.7 has few seeds still suitable for red seed weevil egg laying and should no longer be susceptible to further significant damage.

Economic Threshold:

OILSEED Sunflower - The threshold can be calculated using the following formula:

Threshold (Weevils per head) =

Cost of Insecticide Treatment

(Market Price x 21.5) (0.000022 x Plant Population + 0.18)

The following table lists the economic threshold values for control of seed weevil based on 0.10 to 025 cent per pound for oilseed sunflowers , $8.00 to $10.00 per acre for cost of insecticide, and 17,000 to 25,000 sunflower plants per acre. The table can also be found on the National Sunflower Association weblink:

http://www.sunflowernsa.com/growers/default.asp?contentID=376

2009 Economic Threshold (weevils per head) for red sunflower seed weevil

Cost of Spray

Market Price

Sunflower Plants per Acre (x 1,000)

($/acre)

($/lb)

17

18

19

20

21

22

23

24

25

8.00

0.10

7

6

6

6

6

6

5

5

5

0.11

6

6

6

5

5

5

5

5

5

0.12

6

5

5

5

5

5

5

4

4

0.13

5

5

5

5

4

4

4

4

4

0.14

5

5

4

4

4

4

4

4

4

0.15

4

4

4

4

4

4

4

4

3

0.16

4

4

4

4

4

4

3

3

3

0.17

4

4

4

4

3

3

3

3

3

0.18

4

4

3

3

3

3

3

3

3

0.19

4

3

3

3

3

3

3

3

3

0.20

3

3

3

3

3

3

3

3

3

0.21

3

3

3

3

3

3

3

3

2

0.22

3

3

3

3

3

3

2

2

2

0.23

3

3

3

3

3

2

2

2

2

0.24

3

3

3

3

2

2

2

2

2

0.25

3

3

2

2

2

2

2

2

2

10.00

0.10

8

8

8

8

7

7

7

7

6

0.11

8

7

7

7

7

6

6

6

6

0.12

7

7

6

6

6

6

6

5

5

0.13

6

6

6

6

6

5

5

5

5

0.14

6

6

6

5

5

5

5

5

5

0.15

6

5

5

5

5

5

5

4

4

0.16

5

5

5

5

5

4

4

4

4

0.17

5

5

5

4

4

4

4

4

4

0.18

5

4

4

4

4

4

4

4

4

0.19

4

4

4

4

4

4

4

3

3

0.20

4

4

4

4

4

4

3

3

3

0.21

4

4

4

4

3

3

3

3

3

0.22

4

4

4

3

3

3

3

3

3

0.23

4

4

3

3

3

3

3

3

3

0.24

3

3

3

3

3

3

3

3

3

0.25

3

3

3

3

3

3

3

3

3

12.00

0.10

10

10

9

9

9

8

8

8

8

0.11

9

9

8

8

8

8

7

7

7

0.12

8

8

8

8

7

7

7

7

6

0.13

8

7

7

7

7

6

6

6

6

0.14

7

7

7

6

6

6

6

6

5

0.15

7

6

6

6

6

6

5

5

5

0.16

6

6

6

6

5

5

5

5

5

0.17

6

6

5

5

5

5

5

5

4

0.18

6

5

5

5

5

5

5

4

4

0.19

5

5

5

5

5

4

4

4

4

0.20

5

5

5

5

4

4

4

4

4

0.21

5

5

4

4

4

4

4

4

4

0.22

5

4

4

4

4

4

4

4

3

0.23

4

4

4

4

4

4

4

3

3

0.24

4

4

4

4

4

4

3

3

3

0.25

4

4

4

4

3

3

3

3

3

CONFECTION sunflowers- The economic threshold is only ONE WEEVIL PER HEAD.

Insecticide Spray Timing: Insecticide spraying is targeted at the adult weevil to prevent egg laying. The best time to treat is when more than half of the plants in a field are beginning to show yellow ray petals (R5.0) to 30% of the head shedding pollen (R5.3) and the rest of the plants in the field are still in the late bud stage. Although insecticides applied to sunflower at the bud stage will kill weevils, treatments at that stage are not economical or effective because (1) seeds have not developed to a stage suitable for oviposition, (2) eggs within the weevil are not mature, and (3) adult weevil emergence is still continuing. Considering treatment at the early bloom stage is the optimal insecticide timing for efficacy and should allow growers a sufficient time to have their fields treated.

Janet Knodel
Extension Entomologist
janet.knodel@ndsu.edu


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