Soybeans

Bean Leaf Beetle Article

What is the Bean Leaf Beetle?

The Bean Leaf Beetle is tied for second among all pest species attacking soybeans. They feed on soybean foliage, pods and seeds. The feeding not only causes plant damage but the beetles can also transmit bean pod mottle virus (BPMV). 

What is the Life Cycle of the Bean leaf Beetle?

The Bean Leaf Beetle overwinters as adults under leaf litter at or near soybean fields.  Once spring temperatures reach 50 – 55 degrees F, adults become active and seek available host plants such as grasses, soybean plants, and other legumes. The coloration varies from red, orange, tan, or gray and the markings (dots, strips, or both) may vary among individual populations. However, all adults have a black triangle at the base of their forewings. Females are capable of producing 130 to 200 reddish colored eggs which they lay in the upper 5 inches of soil adjacent to plant stems. In 5 to 7 days the eggs will hatch and feed on underground plant parts. Depending on the soil temperature, bean leaf beetle larvae may feed 3 to 6 weeks before pupating into earthen cells. Two generations of bean leaf beetles usually occur in the North Central region. On average, the first adult generation peaks around the late vegetative or early reproductive soybean stage (mid July), and the second generation peaks at pod fill time (later August to early September).  The second generation becomes the overwintering adults and eventually leave soybeans and feed on alfalfa and other legume hosts before seeking overwintering sites under crop and leaf debris. 

What Kind of Damage Does Bean Leaf Beetle Do? 

Both the larvae and adults are soybean pests. Larvae attack the roots and root hairs but show a preference for root nodules. Adults feed on foliage and pods. Pod damage by adults is most crucial because it can also lead to secondary disease infections of the pods and seed which lower both seed quality and quantity. Pod feeding may also cause complete pod loss and or pod lesions. Even though soybean plants may sustain more than 50% leaf damage, the soybean plant can compensate, unless damage occurs during the reproductive growth stage. Overall, a reduction of 0.6 pounds per acre can occur when beetles number one or more per foot.

Determining Soybean Seeding Rates

The seeding rate for soybeans is determined by the desired harvest plant population to achieve high yield and the expected loss of plants (or seeds) between planting and harvest. Recent comparisons indicate that stand loss from V3 to R8 ranged from 10 to 15 percent. Additional stand losses due to germination and seedling diseases prior to stage V3 probably range from 5 to 15 percent. Therefore, the planting rate should be from 15 to 30 percent above the desired harvest plant population.

Table 1 shows the results of a three-year study at five locations in Iowa. Narrow rows (7.5 inches at one location, 10 inches at four locations) were compared with wide rows (30 inches). A soybean drill was used to plant the narrow rows and a planter was used for the wide rows. The target plant stand was the same for both row spacings; however, manufacturers' recommended machine settings for each stand resulted in different harvest stands, with the wide rows having fewer plants at harvest than narrow rows at each target stand. The highest target stands resulted in the largest gap between target stand and harvest stand for each row spacing.

Plant stand 
target

Narrow rows (7.5 & 10 inches)

Wide rows (30 inches)

Harvest stand

Grain yield (bu/acre)

Harvest stand

Grain yield (bu/acre)

80K* 94K 47.9 73K 46.3
120K 133K 50.2 110K 49.3
160K 157K 50.9 135K 49.8
200K 186K 51.6 165K 50.6
240K 221K 51.4 189K 49.8

Grain yield did not differ significantly for harvest stands above 100,000 plants per acre in either row spacing. The narrow-row harvest stand of 133,000 plants per acre produced 50.2 bu/acre compared with 51.6 bu/acre with a stand of 186,000 plants. In wide rows, the harvest stand of 110,000 plants per acre produced 49.3 bu/acre and the stand of 165,000 plants per acre produced 50.6 bu/acre. A harvest population of less than 100,000 plants per acre produced a significantly lower yield than stands of 110,000 plants or more. These studies indicate that the producer should plant enough soybean seed to have a harvest stand of at least 100,000 plants per acre. The cost of additional seed, however, must be taken into consideration before planting excessively high plant stands.

Credits: Keith Whigham, extension agronomist, Department of Agronomy,
Iowa StateUniversity Extension, Integrated Crop Management issue IC-480(23), (10/12/1998)

How Much Do Skips Hurt Final Yield in Soybean?

Many of us agree that soybeans are normally over planted and that we have a high % more plants in the field than what we need for maximum yield. However our concern with lower germinating soybeans probably has two questions. What is the vigor of the seed that does germinate and what are my chances of having large skips.

The first question about vigor is very real. Because of the mechanical damage it is somewhat difficult to sort the abnormal seedlings from a low vigor seedling. Using the warm germination sand test gives us a better picture of the seeds ability to push through soil and therefore, while the germinations are lower, we feel confident that the seed that does grow in this test will have the ability to emerge. Field emergence in soybeans is never as high as germination, but here are some suggestions that can improve field emergence.

We would expect the following results if 5 out of 10 seeds emerge and we are planting 10 seeds per foot.

  1. Wait for near optimum soil temperature of 65° F to plant.
  2. Avoid cool damp soil conditions. Give the beans an environment that they can grow as quickly as possible in.
  3. Avoid planting before moderate to heavy rain. While the rain can cause crusting it can also cause these extremely dry soybeans to absorb water too fast.
  4. Plant as shallow as possible but maintain good seed to soil contact and cover.
  5. Handle every seed (bag) like it is an egg.
  6. If seed is not planted until air temperatures are constantly above 80° F, move the seed to storage that remains cooler with higher humidity. Don’t dry this seed out more than it already is.
  7. Remember that proper planting speed will increase accuracy of spacing for seeds.
  8. While this year’s seed is not the best seed, it is the best that we have to work with this year.

A second question relates to the skips in the field that one might expect to find. One can compute a statistical probability we can share, however field conditions will still cause variance, especially with uneven soils. We would expect the following results if 7 out of 10 seeds emerge and we are planting 10 seeds per foot.

  • We have a 9.0% chance of having two seeds in a row will not emerge.
  • We have a 2.7% chance of having three seeds in a row will not emerge.
  • We have a .8% chance of having four seeds in a row not will emerge.
  • We have a .2% chance of having five seeds in a row will not emerge.
  • We have a 25% chance of having two seeds in a row will not emerge.
  • We have a 12.5% chance of having three seeds in a row will not emerge.
  • We have a 6% chance of having four seeds in a row will not emerge.
  • We have a 3% chance of having five seeds in a row will not emerge.

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Soybean Aphids

Where Did the Soybean Aphid Come From?

The soybean aphid is a native of China and Japan.

What Does the Soybean Aphid Look Like and Where Can I Find It?

It is a small yellow insect with distinct black protrusions or “tailpipes” on the tip of its abdomen. Some generations will have wings. The aphid can be found on soybean stems, young leaves of growing soybeans and on the undersides of mature soybean leaves. No other aphid species colonizes on soybean plants so it is probably safe to assume if you find colonies of tiny yellow aphids on your soybean plants, you probably have soybean aphids.

What is the Life Cycle of the Soybean Aphid?

The life cycle of the soybean aphid is complex and can be as many as 15 to 18 generations per growing season. The aphid survives on two host plants during the year. Winter survival for over-wintering eggs is on the buckthorn plant. On the buckthorn, two generations of wingless females and one generation of winged females are produced. The winged females migrate from the buckthorn in search of soybean. The soybean aphid stays on the soybean for the rest of the summer in a repeated series of wingless generations followed by a winged generation. The winged generation is capable of leaving the crowded colony in search of other soybean plants where a new colony is started. In the fall, the winged soybean aphid migrates back to the buckthorn and produces a generation of wingless egg-laying females. The males develop on soybean and also migrate back to buckthorn where they mate with the wingless females which are responsible for laying the eggs that will over-winter on buckthorn twigs.

Are There Other Summer Hosts for the Soybean Aphid?

To date in North America, soybean is the only confirmed summer host for the soybean aphid.

When Do Soybean Aphids Migrate to Soybean Plants?

It is not known exactly when aphids will migrate to soybean plants. Monitoring for aphids at specific growth stages rather than calendar date has been more successful. Populations build and peak starting at the late seedling stage known as V2 (two expanded trifoliate leaves) through the blooming stage known as R1 – R2. Colonies concentrate on new trifoliate leaves and new leaves on side branches. In late July, when the top growing point of the soybean stops growing, the aphids move from the top of the plant to middle or lower areas of the canopy and usually can be found on the undersides of leaves, petioles and pods.

What Symptoms Should I See on Plants Affected By Soybean Aphids?

Aphid infestations that peak at the R1 – R2 growth stage can cause stunted plants and reduced pod and seed counts. Leaves on these plants may be distorted and under very heavy infestations they may be yellowed. Charcoal colored residue can be seen on stems, leaves and pods. This residue is sooty mold which grows on the honeydew excreted by the feeding aphids.  

Can the Soybean Aphid Cause Other Problems?

Soybean aphids are capable of transmitting a number of viruses that naturally infect soybean plants. The viruses include alfalfa mosaic, soybean mosaic, bean yellow mosaic, peanut mottle, peanut stunt, and peanut stripe. (Note: It is not possible to prevent these viruses by controlling aphids with insecticides.) The sooty mold residue indirectly can also rob yield by reducing the photosynthetic capacity of the plant. Does the Soybean Aphid Have Any Natural Predators / Diseases? Some natural enemies of the soybean aphid identified so far include lacewings, lady beetles, minute pirate bug, parasitic wasps and some fungal pathogens which turn dead aphids a reddish brown color.

When Should a Soybean Grower Control Soybean Aphids?

Marlin Rice, Professor of Entomology at Iowa State University, thinks it is critical to scout soybeans for aphids starting in early July. A first field check in early to mid August is probably too late. Rice says, “The beginning to second week of August is when growers will get the most benefit out of spraying, because they will have caught the population before it starts to build up. At the end of August is when you get less benefit.” Rice notes that The University of Wisconsin has established thresholds of when spray. The per plant spray thresholds are 200 or more aphids for plants at the R1 or R2 stages and 1000 to 1500 aphids at the R3 and R4 stages respectively.

Suggested Formula for Estimating Soybean Population

SUGGESTED FORMULA FOR ESTIMATING SOYBEAN POPULATION

Seeds/lb Germ %Field
Emergence
100,000
plants/acre
120,000
plants/acre
140,000
plants/acre
2500 90% 80%

55lbs./acre
137,000spa

66lbs./acre
165,000spa

77lbs./acre
192,000spa

2500 85% 80%

58lbs./acre
145,000spa

71lbs./acre
177,500spa

82lbs./acre
205,000spa

2500 80% 80%

62lbs./acre
155,000spa

75lbs./acre
187,000spa

87lbs./acre
217,000spa

2500 75% 80%

66lbs./acre
165,000spa

80lbs./acre
200,000spa

93lbs/.acre
232,000spa

2800 90% 80% 50lbs./acre 60lbs./acre 70lbs./acre
2800 85% 80% 52.5lbs./acre 63lbs./acre 73lbs./acre
2800 80% 80% 55lbs./acre 66.5lbs./acre 77lbs./acre
2800 75% 80% 59lbs./acre 70lbs./acre 80lbs./acre
3000 90% 80% 46lbs./acre 55lbs./acre 64lbs./acre
3000 85% 80% 49lbs./acre 59lbs./acre 69lbs./acre
3000 80% 80% 52lbs./acre 62.5lbs./acre 73lbs./acre
3000 75% 80% 56lbs./acre 66.5lbs./acre 78lbs./acre
3300 90% 80% 41.5lbs./acre 50lbs./acre 58lbs./acre
3300 85% 80% 45lbs./acre 54.5lbs./acre 63.6lbs./acre
3300 80% 80% 47.6lbs./acre 57.1lbs./acre 66.6lbs./acre
3300 75% 80% 50lbs./acre 60lbs./acre 70lbs./acre

Seeds/lb  X  germ  X  field emergence = potential plants per pound
Desired final stand population ________+ potential plants/lb= ________ Lbs to plant

CREDITS: University of Nebraska-Lincoln Extention Publications