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ISSUE 12   July 29, 2009

SOYBEAN APHIDS SLOWLY INCREASING

With the cool weather, soybean aphid development has been slow. Temperatures in the low 70s are not optimal for development. Optimal temperatures for soybean aphid development are the high 70s to low 80s. Laboratory research has also shown that aphid reproduction is almost nil when temperatures are over 95 F. However, it is important to remember that this research was conducted in the laboratory where aphids were exposed to a constant temperature. In the field, temperatures fluctuate and moderate the negative affects on aphid reproduction. Also, aphids can move to the lower crop canopy where temperatures are cooler. Soybean aphid counts have increased slightly from last week. Low population levels (about <50 aphids per plant) can be found in most fields now. The good news is that most fields also have many beneficial insects present. A new county record from Renville County near Lansford indicates that soybean aphid is moving into the north central region of North Dakota (Source: B. & C. Michels). Producers in the western regions of North Dakota should scout for soybean aphids, since it is becoming more common in soybeans grown further west. Another producer had to spray a field near the river bottom in Mayville-Portland. Spider mites were present in the same area at low densities and only on field edges. For more information about making spray decisions for soybean aphid and spider mites, see the weblink to the University of Minnesota publication:

http://swroc.cfans.umn.edu/SWMNPEST/09publications/spidermite.pdf.

Research results have indicated that early-season application of insecticides for control of sub-economic populations of soybean aphids have caused more harm than good. NDSU Extension Entomology recommends no early-season insecticide applications prior to the economic threshold level of 250 aphids per plants on 80% of the plant. Here’s four good reasons to avoid spraying early: 1) kills beneficial insects like lady beetles, lacewings, nabids that keep aphid populations low and below economic threshold levels; 2) increases secondary insect pest outbreaks, like spider mites. Early spraying of pyrethroids for aphids causes mite populations to flare; 3) requires second insecticide application to control aphids and/or mites increasing input costs; and 4) increases the risk of aphids developing insecticide resistance due to unnecessary applications of insecticides.

 

SPIDER MITES STARTING TO SHOW UP IN SOYBEANS

With the recent dry conditions, spider mites have started to show up in treatable numbers on field edges. Reports have come in from the Valley City, Dazey, Montpelier and Mayville-Portland areas. An excellent publication on spider mites was just published by the University of Minnesota, which summarizes everything that you want to know about spider mites on soybeans and corn:

http://swroc.cfans.umn.edu/SWMNPEST/09publications/spidermite.pdf

Mites can be found on the undersides of leaves of the lower half of plant. The life cycle of spider mites can be completed in only 5-19 days with fastest development rates occurring at >90 F. Each female lives for 30 days and she produces about 100 eggs during her lifetime. In hot, dry weather, natural fungal diseases of mites are slowed and populations can increase from a few individuals to millions within a few generations. Remember, spider mites thrive on stressed plants that are nutrient rich!

Leaf injury symptoms appear as stippling first and then progresses to yellowing, browning or bronzing as feeding injury increases (Fig. 1). Leaf drop eventually occurs. Feeding injury causes water loss from the plant and reduces the photosynthetic ability of the plant. In severe cases, premature leaf senescence and pod shattering will occur and even plant death. When severe mite infestations occur during late vegetative and early reproductive growth, a 40 to 60% yield loss between treated and untreated soybean has been demonstrated in other north central states. Spider mites can cause yield reduction as long as green pods are present.

When scouting for spider mites, look on the underside of leaves and lower foliage for a tiny mite and fine spider-like webbing. Adult spider mites are small (< 0.2 inch), greenish-white to orange-red in color, and have 2 dorsal spots and 4 pairs of legs (Fig. 2).

Nymphs are smaller than adults and have 3-4 pairs of legs. By shaking a leaf or plant over a white sheet of paper, you can see these tiny mites (yellow dots), which crawl slowly over the paper. When spider mites need to move due to diminishing food supply, they climb to the tops of plants, spin a silk ‘balloon’ and are dispersed by the wind (Fig. 3), so they can spread quickly within a field or to adjacent fields.

It is critical to scout early and check field edges where spider mites infest first. Scouting during during full pod (R4) through beginning seed (R5) stages is critical 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.

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. Minnesota is using this scale to assess mite damage:

0 = No spider mites or injury observed

1 = Minor stipling on lower leaves, no premature yellowing observed

2 = Stipling common on lower leaves, small areas or scattered plants with yellowing

3 = Heavy stipling on lower leaves with some stipling progressing into middle canopy. Mite present in middle canopy with scattered colonies in upper canopy. Lower leaf yellowing common. Small areas with lower leaf loss. Spray Threshold.

4 = Lower leaf yellowing readily apparent. Leaf drop common. Stipling , webbing and mites common in middle canopy. Mite and minor stipling present in upper canopy. Economic Loss.

5 = Lower leaf loss common, yellowing or browning moving up plant into middle canopy, stipling and distortion of upper leaves common. Mites present in high levels in middle and lower canopy.

Remember to use an organophosphate insecticide (e.g. Lorsban, Dimethoate) instead of a pyrethroid insecticide to avoid flaring mite populations. Reasons for the increase in mite populations include: disruption of the natural enemies that control spider mites (predatory mites); increased movement of mites out of fields, and increased reproductive rates of female mites. Early detection facilitates timely and effective rescue treatments. Current insecticides for soybeans provide short-term protection, maybe 7-10 days, from the pest. Fields will need to be re-monitored continually for resurging populations. The efficacy of an insecticide can be improved significantly with sufficient coverage (>10 GPA of water) and application at high pressure to penetrate foliage.

 

GRASSHOPPER ALERT

Grasshoppers (Fig. 4) have been increasing and several ‘hot spots’ have been observed on many field crops (barley, wheat, oats, peas), primarily in the western regions of North Dakota.


Figure 4.
Adult twostriped grasshopper,
Melanopus bivittatus (P. Beauzay)

As small grain crops start to dry and harvest begins, grasshoppers will start to move out into late season crops like flax, sunflowers, soybean, corn or lentils. Action thresholds for grasshoppers are based on counts per square yard (or four 180-degree sweeps). The threatening level when action should be taken:

 

Nymphs (young hoppers) per square yard

Adults per square yard

Rating

Margin

Field

Margin

Field

Light

25-35

15-23

10-20

3-7

Threatening

50-75

30-45

21-40

8-14

Severe

100-150

60-90

41-80

15-28

Very Severe

200+

120

80+

28+

Summit disease and small red mites that attack and kill grasshoppers have also been observed. This is biological control in action. Summit disease is a naturally occurring fungal disease called Entomophaga grylli (Fig. 5).


Figure 5.
Grasshopper infected with
Summit disease
(J. Knodel)

Insects infected with this disease exhibit characteristic symptoms which are easy to recognize in the field. Shortly before death, grasshoppers crawl to the tops of the plants and die with their heads pointing upward and their legs wrapped tightly around the stalks. These biocontrol agents are generally not effective in controlling high grasshopper populations.

 

BLISTER BEETLES ON CANOLA

Blister beetles have been reported in canola in the north central region of North Dakota (Ward and Renville Counties). Several species of blister beetle feed on canola, including: Lytta nuttalli, a large purplish green beetle (Fig. 6) and Epicauta ferruginea, a smaller rusty color, pubescent beetle.


Figure 6.
Blister beetle, Lytta nuttalli (J. Knodel)

Most species of blister beetle have one generation per year. Adults become active in early to mid summer and lay eggs in the soil. Eggs hatch in about two weeks into active larvae called triungulins, which actively prey on ground nesting grasshopper egg pods (genus Epicauta) and bee eggs (genus Lytta). Larvae overwinter. Adult blister beetles are attracted to blooming canola fields, where they are ravenous feeders devouring leaves, stems, flowers, and pods. These beetles are mobile and gregarious often congregate in certain spots in a field. In some instances, blister beetles feed for a short period of time and then migrate to other plants or fields.

The presence of large numbers of blister beetles in spots of a canola field have often concerned growers. However, adult feeding is generally not significant enough to warrant an insecticide treatment. The "High Plains Integrated Pest Management Guide" recommends treatment when there are 10 adult blister beetles per plant feeding on the flowers or pods. However, there is no recommended economic threshold in North Dakota. Foliar insecticides registered in North Dakota will control blister beetles. Follow safe pesticide practices when spraying flowering canola to protect honey bees.

 

CEREAL GRAIN APHIDS CONTINUE

Cereal aphids continue to infest late planted wheat, durum and barley crops. To decide whether you should treat, continue to scout fields and use one of the following thresholds to protect small grains from yield loss:

- 85% stems with at least one aphid present, prior to complete heading.
- 12-15 aphids per stem prior to complete heading

The crop’s growth stage also affects its susceptibility to aphids. The vegetative to boot stages are the most susceptible stage to aphid feeding and subsequent yield loss. For example, in the 4-6 leaf stage, injury caused by aphid is stunting, decreased number of kernels per head, and decreased kernel weight. In the boot stage, only kernel size and weight are affected by aphid feeding (not number of kernels per head). By heading, only kernel weight is affected. After flowering, small grains are less susceptible and producers are discouraged from spraying.

Many beneficial insects (lady beetles, aphid lions, syrphid flies, and parasitic wasps) have been observed in fields reducing aphid populations. When natural enemies are present in large numbers and the crop is well developed, farmers are discouraged from spraying fields.

 

SCOUT FOR BANDED SUNFLOWER MOTH

Pheromone trap catches indicate that adult banded sunflower moth numbers are increasing. Egg laying has just started on sunflowers in R2-R3 crop stage. Scouting for eggs or adult moths should start on crops in the preferred egg laying stage (R2-R3).

Egg Scouting Method and Economic Threshold:

The potential for banded sunflower moth damage is determined by counting eggs on floral bracts in the field (Fig. 7).


Figure 7.
Banded sunflower moth eggs
on bracts
(K. Mundal)

Because the eggs are very small a magnifier is needed to accurately count the small eggs. We recommend using a head-mounted 3.5X magnifier (Fig. 8) to leave both hands free for manipulating the bud being observed. Egg counts should be made when most of the plants in the field are at plant stage R3 (distinct bud elongated ¾ inch above the nearest leaf, yellow ray petals not visible). However, to avoid sampling bias, buds should be randomly selected without regard to plant stage.


Figure 8.
Sampling for eggs (K. Mundal)

Sampling for banded sunflower moth egg populations in commercial fields should be conducted as follows:

1) Divide each side of the field into two sections.

2) Sample the center of each section at 20 feet into the field from the field edge.

3) Randomly select five buds.

4) From each bud, randomly select six bracts from the outer whorl and count the eggs on each bract.

5) Average the egg counts from the five buds and then map the average egg counts from each sample site to a diagram of the field.

Next, compare the average egg density at each sampling site to the calculated economic injury level.

Economic Injury Level (EIL). The economic injury level is the number of eggs per 6 bracts and considers treatment cost ($/acre), market price ($/lb), and plant population per acre.

EIL =

TC

V * PP * 0.00078

V = Market value per lb
PP = Plant population per acre
TC = Treatment cost

Example: TC = $10, V = $0.13, and PP = 18,000

The EIL is 5.5 eggs per six bracts. A spreadsheet is available at the NDSU Extension Entomology website to automatically calculate your egg EIL. Click on "Banded Sunflower Moth Calculator."

http://www.ndsu.nodak.edu/entomology/ext.htm

Adult Moth Sampling During the Day and Economic Threshold:

Sampling sites should be at least 75 to 100 feet from the field margins. In monitoring a field, use the X pattern and sample 5 sites, counting moths on 20 plants per sampling site to obtain the total number of moths per 100 plants. Sampling should be conducted in the late bud stage (R3), usually during mid- to late July.

The adult has a dark band across the buff or yellowish-tan forewings (Fig. 9).


Figure 9.
Adult banded sunflower moth (K. Mundal)

The wingspan is about 0.5 inch. During the day (late morning to early afternoon) the moths remain quiet, resting on upper or lower surfaces of the leaves of sunflower plants. When disturbed, they flutter from plant to plant. When sampling for moths during the day, the decision to treat or not is based on comparing the average number of adult moths in the field to the EIL for moths. The EIL is the number of moths per head that will, if not managed, result in seed damage with a value equal to the cost of treatment. Use the following formula based on treatment costs, plant population and market price to determine the adult moth EIL for day sampling.

EIL =

(TC *V)

(PP * 582.9) - 0.7

V = Market value per lb
PP = Plant population per acre
TC = Treatment cost

The following table can be used to determine the EIL for a variety of market prices and for a treatment cost of $10 per acre.

Insecticides registered for control of banded sunflower moth are listed in the sunflower section of the NDSU E-1143 2009 North Dakota Field Crop Insect Management Guide.

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

 

SUNFLOWER MOTH ALERT

Sunflower moth was just reported in large numbers (2-3 adult moths per head) near Sidney-Fairview in Montana on irrigated flowering sunflowers. These irrigated sunflowers are ahead of the dryland sunflower in terms of crop stage, so the sunflower moths are probably concentrating on these flowering sunflowers. Nevertheless, sunflower moths can do significant injury to sunflower and this report indicates that they are likely here in North Dakota. Unfortunately, the pheromone lures used in the sunflower trapping network are not capturing moths. We are working with the distributor and company on this issue to identify the problem. The bottom line is to get out and scout any blooming sunflowers for sunflower moth.

Biology: Sunflower moth migrates to North Dakota from southern states. This grayish-tan moth (Fig. 10) moves into fields in early bloom.


Figure 10.
Adult sunflower moth (L. Charlet)

Individual female moths will deposit up to 30 eggs per day on the face of open sunflower heads. The eggs hatch within 48 to 72 hours and the newly emerged larvae feed on pollen and florets. The larvae begin tunneling into seeds upon reaching the third instar larval growth stage. Tunneling continues throughout the remainder of larval development. Larval development from hatching to full maturity takes about 15 to 19 days.

Damage: Young larvae of the sunflower moth feed primarily on florets and pollen. Older larvae tunnel through immature seeds and other parts of the head. A single larva may feed on three to 12 seeds and form tunnels in both the seeds and head tissue. Larvae spin silken threads, which bind with dying florets and frass to give the head a trashy appearance. Severe larval infestations can cause 30 to 60 percent loss, and in some cases, the entire head can be destroyed. Sunflower infested with sunflower moth has an increased incidence or risk of Rhizopus head rot.

Sampling and Economic threshold: Sampling sites should be at least 75 to 100 feet from field margins. The X pattern should be used in monitoring a field, counting moths on 20 heads per sampling site for a total of 100 heads. Scouting is most accurate in the early morning or late evening, when moths are active.

The economic threshold for sunflower moth is one to two adults per five plants at the onset of bloom or within seven days of the adult moth’s first appearance. Since female moths lay eggs on the face of sunflower heads, insecticide should be applied in early flowering (R5.1 - R5.3). Insecticides registered for control of sunflower moth are listed in the sunflower section of the NDSU E-1143 2009 North Dakota Field Crop Insect Management Guide.

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

Janet Knodel
Extension Entomologist
janet.knodel@ndsu.edu


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