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ISSUE 9   July 8, 2010

CONTINUE TO SCOUT FOR WHEAT MIDGE IN NORTHERN TIER OF NORTH DAKOTA

With the localized hot spots for the wheat midge this year, field scouting will be important. Please see the past issue no. 7 of the Crop & Pest Report for the Wheat Midge Larval Soil Survey risk map for 2010 and scouting protocol. The current DD map (Fig.1) indicates that wheat midge emergence is well underway and should be peaking in the northern tier.


Figure 1. Wheat midge degree day map (NDAWN)

Wheat is in the susceptible growth stage (heading to early-flowering) when the wheat midge were at peak emergence in many areas. An insecticide should be applied during heading and when the adult midge density reaches one midge per four to five wheat heads for hard red spring wheat or one midge per seven to eight heads for durum. A late insecticide application should be avoided to minimize negative impacts on the parasitoid. Wheat midge larvae feed on the kernel and negatively affect yield, grade and quality.

 

SOYBEAN APHID POPULATIONS STILL LOW

The hot weather (>90F) has not been optimal for soybean aphid reproduction and this has continued to delay any economic field infestations. So, continue scouting soybean fields for soybean aphids until the R6 (full seed) growth stage. Please see the past issue no. 8 (July 1st) of the Crop & Pest Report for the scouting protocol.

 

BANDED SUNFLOWER MOTH AND SUNFLOWER MOTH DETECTED IN PHEROMONE TRAPS IN NORTH DAKOTA

Adult moths of banded sunflower moth (Fig. 2) and sunflower moth (Fig. 3) were trapped last week in the NSA-NDSU cooperative moth trapping network. This is the first detection of sunflower moth in North Dakota. Sunflower moths migrate into North Dakota from the southern states and so far trap catches are very low. For banded sunflower moth, scouting for eggs will be critical once the sunflowers reach the R3 bud stage (immature bud elongated about 2 cm above the nearest leaf). Scouting for banded sunflower moth eggs will be discussed in the upcoming issue of the Crop & Pest Report, so stay tuned!


Figure 2. Banded sunflower moth trapping
network map (T. Mittelsteadt, NSA)


Figure 3. Sunflower moth trapping network
map (T. Mittelsteadt, NSA)

 

THISTLE CATERPILLARS REPORTED IN SUNFLOWERS

With the strong southernly wind patterns, the painted lady butterfly or thistle caterpillar [Vanessa cardui (L.): Lepidoptera: Nymphalidae] has been blown up into North Dakota. Thistle caterpillars have been observed causing defoliation in sunflowers located in the southwestern region of North Dakota. Larvae feed on over 100 plant species, including thistle, sunflower, and burdock, and have also been noted feeding on crops such as canola and soybean.

The body of the adult (Fig. 4) is about 1 inch long with a wingspan of about 2 inches. The upper wing surfaces are brown with red and orange mottling and white and black spots. The undersides of the wings are marble gray, buff and white. Each hind wing possesses a row of four distinct eyespots.


Figure 4. Painted lady butterfly (Ext. Entomology, NDSU)

Eggs are small, spherical and white. Larvae or caterpillars (Fig. 5) are brown to black and spiny, with a pale yellow stripe on each side. When mature, the larvae are 1.25 to 1.5 inches long. The chrysalis, or pupa, is molten gold and about 1 inch long.


Figure 5. Thistle caterpillar (J. Knodel, NDSU)

The painted lady butterfly is indigenous to the southern U.S. and migrates annually to the northern Great Plains. The painted lady breeds in the north-central states and Canada, migrates south for the winter and returns to the northern areas in early June. Eggs are laid on Canada thistle, wild and cultivated sunflower, and many other host plants. Hatching occurs in about one week. Larvae feed on sunflower until they reach maturity in late June or early July. Chrysalids are formed and hang from the leaves of the plant. Butterflies will emerge in about 10 days from chrysalis and have two or more generation a year.

Larvae feed on the leaves and, when numerous, may defoliate plants. Larvae produce loose silk webbing that covers them during their feeding activity. Black fecal pellets produced by the larvae are often found in proximity to the webbing.

Insecticide use generally has not been warranted for control of larvae. However, instances of high localized infestations can occur within certain fields and spot treating may be necessary. Sample at least 75 to 100 feet from the field margins when scouting fields. Infestations will frequently be concentrated in areas of a field where Canada thistle plants are abundant. Plants should be examined carefully for the presence of larvae and/or defoliation. The economic threshold for the thistle caterpillar is 25% defoliation, provided that most of the larvae are still less than 1.25 inch long. If the majority of the larvae are 1.25 to 1.5 inches long, most of the feeding damage will have already occurred and treatment is not advised. Larvae are susceptible to disease outbreaks, which are indicated by dying larvae (Fig. 6) present on leaves.


Figure 6. Virus infected caterpillar (J. Knodel, NDSU)

 

SCOUT FOR LYGUS BUG IN FLOWERING FIELD PEAS OR LENTILS

A pea field in Bowman County near Rhame had 25-30% pod injury from Lygus bug feeding (R. Ashley, DREC). Pods exhibiting Lygus bug feeding injury had aborted one or two seeds per pod. A field south of Minot also was above threshold (D. Waldstein, NCREC). Most of the field pea / lentil fields are flowering to early podding, so now is a critical time to scout for Lygus bug.

Lygus bugs are comprised of several species belonging to the genus Lygus. The tarnished plant bug (Lygus lineolaris (Palisot de Beauvois), Hemiptera: Miridae), is one of the more common species and is known to feed on over 385 crop and weed plants. Adult Lygus bugs (Fig. 7) 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. Nymphs hatch from these eggs and look like aphids (Fig. 7). Several generations occur each year with the second generation occurring in mid-July to early August.


Figure 7. Adult and nymph Lygus bug (S. Bauer,
http://www.ars.usda.gov/is/graphics/photos/insectimages.new.htm)

Both nymph 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 (Fig. 8) is a pit or crater-like depression in the seed coat with or without a discolored chalky appearance.


Figure 8. Chalk spot damage from Lygus bug
on field pea (J. Knodel, NDSU)

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, handling or moisture issues. 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.

Monitor for Lygus bugs using a 15-inch sweep net during bloom to pod development (until seeds within the pod have become firm). Make 25 180-degree sweeps at five sampling sites in a field during the warm sunny part of the day (2-6 PM). Lygus populations can increase suddenly. For example, when an alfalfa (preferred host) field is cut, Lygus will migrate quickly into nearby pulse crop fields and 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. If 7-10 Lygus per 25 sweeps is found continue sampling field. For insecticides labeled in peas and lentils, please see the NDSU 2010 Field Crop Insect Management Guide at:

http://www.ag.ndsu.edu/pubs/plantsci/pests/e1143w1.htm

Janet Knodel
Extension Entomologist
janet.knodel@ndsu.edu


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