Bean leaf beetle In this week's CropWatch entomologists and plant pathologists discuss the potential impact of the bean leaf beetle from direct feeding damage and from vectoring the bean pod mottle virus.

April 18, 2003: Focus on Soybean

Soybean plant damaged by bean pod mottle virus.

Recent research from Iowa State University on managing bean leaf beetles to control Bean pod mottle virus was presented in the mid-March Soybean Digest. The article points out that by applying two sprays (one at soybean emergence and one when the F1 beetle population emerges in early July), impact of the Bean pod mottle virus was significantly reduced. The article also points out that planting date should be used as the first course of action in managing this disease in problem prone areas.

Our research at Nebraska supports the planting date information presented in this article. We have observed, as many of you have, that earlier planted soybean fields will have more bean leaf beetles, and in return, more Bean pod mottle virus. However, in the fist year that we conducted studies similar to those at Iowa State, we did not observe a yield effect with an insecticide treatment.

So why do the results differ? First, the Iowa studies were conducted in fields with a history of Bean pod mottle virus causing significant yield loss. Our research fields in Nebraska have tested positive for Bean pod mottle virus, but the impact does not appear as severe as in Iowa. We should point out that we have only one year of data on this trial, but have done attempts in other settings without showing an impact on yield with one early-spring insecticide application. Second, we did not include two insecticide applications in several of our studies because we did not feel the economics of soybean would support this strategy. Using the Iowa strategy (two sprays of Warrior), one would spend $5-$6 an acre on chemical and about $5 an acre in application costs for each treatment, for a total of $20-$22 per acre. This is a lot to spend on soybeans when the incidence and potential impact of this disease in your field is unknown.

There are several other points that make us uncomfortable with recommending treating soybeans with insecticides. First, we do not know the potential impact of Bean pod mottle virus in Nebraska soybean fields. Although numerous fields have tested positive for this disease, many exhibit few if any disease symptoms. Before producers implement a chemical control strategy, they should know that they are likely to incur an economic loss from the pest or resulting disease. Second, we know that treatment timing is critical to control. Any bean leaf beetle feeding will potentially result in some vectoring of the disease. It is essential that the overwintered bean leaf beetles be kept out to minimize introduction of the virus. This means that the first application needs to be applied right at soybean emergence. Also, timing of the F1 population treatment will be critical as these beetles can pick up and transmit the virus quickly. Being just a few days late for either treatment would significantly reduce its effectiveness.

Things to consider

Although yield reductions have been documented as high as 52%, this does not mean that we are seeing this kind of yield reduction in Nebraska. In Iowa, 10-20 bu/acre yield loss has been observed in some areas. However, it should be noted that several factors can affect the amount of yield loss, including:

• Time of infection (infection early in development will cause maximum yield impact)
• Soybean variety: varieties tend to respond differently to Bean pod mottle virus.
• Percentage of plants infected: research in Louisiana in the early 70s showed that there needed to be at least a 20-40% infection for a significant yield impact.
• Virus strain: there are differences in virus strain and strains can recombine to result in higher impacting strains.
• Other stresses: additional stresses, such as drought may further increase the yield impact.

Summary

At this time we cannot recommend treating soybean with insecticide to control the bean leaf beetle and thus the Bean pod mottle virus. The possible exception would be if you know that Bean pod mottle virus is causing economic loss on your farm. We will continue to evaluate the effectiveness of this strategy in Nebraska. In the Soybean Digest article a producer who uses the two-treatment strategy with good results indicated that on farms 35 miles away there was not enough virus for this to pay a return. If you do try an insecticide strategy, please leave a few check strips in the field and let us know how it worked for you.

Bean leaf beetle

Each year at least some areas will have a problem with the bean leaf beetle. Because bean leaf beetles can be a pest of seedling soybean, I’ll discuss it in some detail.

Bean leaf beetles have two generations a year in Nebraska; however, since they overwinter as adults, three periods of beetle activity occur in the growing season. These are the overwintering colonizers, the F1 generation (offspring of the colonizers and the true first generation), and the F2 generation.

Table 1. VC Economic thresholds (beetles per plant) Crop value Pest management cost $/bu $/acre $6 $8 $10 $12 $5 3 4 4 6 $6 2 3 4 5 $7 2 3 3 5 $8 2 2 3 4 Table 2. V1 Economic thresholds (beetles per plant) Crop value Pest management cost $/bu $/acre $6 $8 $10 $12 $5 4 5 7 8 $6 3 4 6 7 $7 3 4 5 6 $8 3 3 4 5

Bean leaf beetles overwinter as adults in leaf litter (woodlots) and soybean residue. They become active fairly early in the year (April-May) and often are found in alfalfa prior to soybean emergence. As soybeans emerge, the beetles quickly move to the seedling plants, feeding on cotyledons and expanding leaf tissue. These overwintered beetles, called colonizers, mate and begin laying eggs. Females live about forty days and lay 125 to 250 eggs. After egg laying is complete, the colonizing population dwindles as the beetles die. A new generation of beetles (F1) will begin to emerge in late June to early July. The F1 beetles mate and produce a second generation of beetles (F2) that begin to emerge in mid August and feed on leaf and pod tissues. The pod-feeding F2 beetles are most likely to cause economic damage.

These beetles vary in color, but are usually reddish to yellowish-tan. They are about 1/4 inch long and commonly have two black spots and a black border on the outside of each wing cover. These spots may be missing, but in all cases there is a small black triangle at the base of the wings near the thorax. Because they move to soybean fields so soon after seedling emergence, early-planted fields will usually have more beetles and suffer the most injury. This has become more of a problem in recent years because planting dates seem to be getting earlier each year. Although the defoliation the beetles cause can appear quite severe, research in Nebraska and elsewhere has shown that it usually does not result in economic damage. Soybean plants can compensate for a large amount of early tissue loss, so it takes a considerable amount of beetle feeding to impact yield. Generally, unless insect populations are large enough to cause more than 50% to 60% defoliation of seedling soybeans, it is unlikely that treatment would be economically justified. Tables 1 and 2 show economic thresholds for bean leaf beetle on seedling soybean. Be aware that these thresholds are for defoliation of beans at the VC - V1 growth stages. If beetles enter the field right at or during seedling emergence, the thresholds will likely be lower because the beetles do not have leaf tissue to eat and will feed on the growing point, stem, and cotyledons. We do not have a good research base for bean leaf beetle injury to newly emerging soybean, but the thresholds are probably about 1.5 beetles lower than the VC thresholds.

Remember that early-planted soybeans are the most susceptible. If economic thresholds are reached, many insecticides are available for bean leaf beetle control. All will do an adequate job if applied according to label directions.

Another reason some producers treat bean leaf beetle on seedling soybeans is to reduce the pod-damaging F2 generation that emerges in August; however, NU Cooperative Extension does not recommend this practice. There are many environmental factors that can impact beetle populations throughout the growing season, making it impractical to use spring beetle numbers to accurately predict if beetle populations will reach economically damaging levels in August. Regular scouting and the use of the appropriate economic thresholds are the best way to manage late season bean leaf beetle in soybean. Late-season economic thresholds will be published in CropWatch later this summer.

Bean leaf beetles also vector Bean pod mottle virus. This aspect is discussed in greater detail in the cover story, Bean Pod Mottle Virus in soybean.

Soybean aphid

The soybean aphid is an Asian soybean pest that was first discovered in the United States in the summer of 2000 in Wisconsin. Since then it has spread throughout the north central United States and parts of Canada. We expect the aphid to spread to all soybean-producing areas of Nebraska.

The soybean aphid was found in 13 eastern Nebraska counties in 2002, starting in mid-summer. Almost all of the soybean aphid infested fields in Nebraska during 2002 had low numbers of aphids and did not cause economic damage. I expect to see soybean aphids this year, but I am not willing to predict their impact because they are so new to North America.

Heavy infestations of this insect can cause significant damage and yield loss. Yield losses exceeding 25% were observed in Minnesota and Iowa in 2000. In addition, soybean aphids can transmit viral diseases, such as alfalfa mosaic, soybean mosaic, bean yellow mosaic, peanut mottle, peanut smut, and peanut stripe. Soybean aphid populations can grow to extremely high levels under favorable environmental conditions.

Reproduction is fastest when temperatures are 72-77^oF. Developmental time slows when temperatures exceed 81^oF. When populations reach high levels during the summer (there were reports of up to 13,000 aphids per plant in Michigan), winged females are produced that migrate to other soybean fields. Like a number of other insect species (e.g. potato leafhoppers), these migrants can be caught up in weather patterns, moved great distances, and end up infesting fields far from their origin.

Soybean aphids injure soybeans by removing plant sap with their needle-like mouthparts. Plant symptoms may include yellowed, distorted leaves and stunted plants. A charcoal-colored residue also may be present. This is sooty mold that grows on the honeydew that aphids excrete. Soybean plants appear to be most vulnerable to aphid injury during the early reproductive stages. Heavy aphid infestations during these stages can cause reduced pod and seed counts.

Grasshoppers

Grasshoppers were a major concern to Nebraska farmers last year and are likely to be a problem this year, particularly if we have a warm and relatively dry spring. Droughty weather favors the grasshopper, whose development stages are influenced by weather.

There are over 100 grasshopper species in Nebraska, but only a few are of major importance in soybean: the migratory, differential, two-striped, and red-legged grasshoppers. These grasshopper species prefer habitats with a variety of host plants, including both grasses and broadleaf weeds. As a result, they prefer cropland settings with nearby undisturbed areas such as roadside ditches, crop borders, abandoned cropland, and over-grazed pastures or rangeland.

The primary injury caused by grasshoppers is defoliation, but they also feed on soybean pods. They consume and clip foliage from the plant as they feed. With favorable, warm-dry climatic conditions, grasshoppers can hatch and mature two to four weeks earlier than normal. The key to effective grasshopper management is to understand their life cycle and if chemical treatment becomes necessary, to treat them before they become adults.

Without wheat residue over the row, the soil warmed up enough for these early planted soybeans to emerge early, putting them at risk of frost. These soybeans, at the Rogers Memorial Farm, were planted on March 17 this year and emerged on April 13. The heavy wheat residue in the field is delaying the soybeans from emerging until later when the danger of frost should be passed. Photo by Paul Jasa

As a machinery management decision and to spread production risks, some producers are no-till planting some of their soybeans before planting corn. While there are concerns about cold soils with early planting, producers report that soybeans can actually handle cold soil stress better than corn. These producers start their planting season a week or two earlier than normal and no-till plant some of their soybeans before planting corn on their normal planting dates. Then they finish planting the rest of their soybeans after their corn. This allows them to get the soybeans in the ground before the yield potential declines because of late planting.

Planting too early, however, has risks involved with late spring frosts and seedling diseases because of slow crop emergence. By using no-till and seed-applied fungicides, these risks can be minimized. The soybeans are no-till planted into cool, wet soil with plenty of residue cover. Planting takes place without residue movers so as to leave as much residue protecting the seed as possible. The seed is planted about 1 3/4 to 2 inches deep so it won’t germinate as readily if there is an early warm spell. Tilled soils tend to warm up too fast early in the spring, allowing early planted soybeans to emerge too quickly, increasing the risk of being killed by frost. Under no-till conditions, when the soil warms up enough for the seeds to germinate, the threat of a killing frost is usually past. The residue acts as insulation and provides some protection if there is a frost. In addition, the soil moisture conserved with no-till increases the “thermal mass” of the soil, holding heat to reduce the chances of frost injury.

Research was conducted from 1999 to 2002 at the University of Nebraska Rogers Memorial Farm east of Lincoln to evaluate the potential of early planting soybeans. (See table for 2002 results and the Crop Watch archives for the 1999 to 2001 results.) These dryland trials were in no-till and used a seed-applied four-way fungicide to protect the seedlings. Usually, the highest yields were with April-planted soybeans and the lowest yields were from those planted in June, about a 15 bu/ac difference. It’s interesting to note that in 2001 at the Rogers Memorial Farm and on several other related trials, some early planted plots without fungicides had reduced stands yet still had yields similar to normal planted plots with full stands.

In 2002, the plots explored the interaction of population and early planting dates. A mid-season soybean variety for the area (2.7 relative maturity) and a full season variety (3.4 maturity), both with a four-way fungicide treatment, were planted at two populations (100,000 and 177,000 seed drop per acre). The rainfall pattern for the area was wet in May and September, and droughty in June, July, and the first half of August. The last planting date of the mid-season variety had a yield response to the late season rains at the higher populations, reinforcing why population should be increased with late planting (see table). However, notice for both maturities that the lower population tended to yield more on the early planting dates and the higher population did more on the later planting dates.

Yields for various planting dates for two varieties and two plant populations at the Rogers Memorial Farm near Lincoln, 2002. Planting dates 2002 Soybean Yields, bu/ac Maturity 2.7 2.7 3.4 3.4 Seeds dropped 100K 177K 100K 177K March 14 26.3 23.1 31.7 28.5 April 9 25.9 16.4 33.9 35.5 May 2 28.6 21.5 36.4 35.2 May 31 31.7 32.3 31.2 35.2 June 20 13.7 36.0 * 15.9 * Treatment not included due to limited plot space available

While yields from early planting dates look good, producers need to improve their crop scouting to ensure success. Careful scouting for bean leaf beetles and properly timed spraying may be necessary as later planting dates are a cultural practice to avoid seedling damage. Also, the bean leaf beetle has been identified as the vector for the bean pod mottle virus; beetle control may be needed even if beetle populations are not above economic thresholds for feeding damage. Also, the choice of herbicides and the timing of applications may have to change as the relative growth stage of the soybeans versus the weeds will change with different planting dates.

Even considering the risks, for machinery management purposes, properly managing and planting some soybeans a week or two before corn makes sense for producers who typically finish planting soybeans in June or for those who want to spread their planting workload and risks. By spreading out the planting dates, harvest dates are also spread, allowing producers to combine their soybeans at more favorable grain moisture contents, reducing losses from harvesting and marketing over-dried beans.

Knowing this aspect of soybean growth helps producers select maturities and adjust planting populations with respect to planting date. Early planting allows more vegetative growth, a larger “factory” to produce seeds. Depending on available inputs for pod formation and pod fill, more seeds can be produced. Conversely, late planting may not produce a very large plant so more plants may be needed to produce the same yield.

Adjusting population is a common practice for those double-cropping soybeans after wheat. They may drop 200,000 to 300,000 seeds because of the shortened growing season, compared to 125,000 to 175,000 for full season production. Even with full season production, some producers increase plant populations with later planting dates to maintain yields.

The one-day training session will include trainers from the University of Nebraska Institute of Agriculture and Natural Resources and private industry. It will be held at the University of Nebraska Agricultural Research and Development Center near Mead on Monday, May 12, from 8:30 a.m. to 5 p.m. Topics will include: how to stage corn and soybean growth and the importance of correct staging on pest management; corn and soybean insect pests – identification, damage, and life cycles; natural enemies – predators, parasites and pathogens; weed identification; crop diseases; nutrient deficiencies; and sampling methods – scouting do’s and don’ts.

Past program participants have given the training high marks. In fact, 100% of last year’s participants rated the program as above average or one of the best programs of its type available. Program participants said it provided improved confidence in scouting and working with growers and improved identification skills. Participants also said they appreciated the hands-on, practical format.

Cutworms feed on newly emerging leaves near the crown of alfalfa, slowing or delaying green-up. If your alfalfa seems slow to get started this spring, examine fields closely for cutworms. Alfalfa can die if enough cutworms are present and they are allowed to feed too long.

Look for army cutworms near the crowns of alfalfa plants. During daylight they often are found in the loose soil surrounding the plant, so scratch around a little to find them if you don’t see them right away. Count the number of cutworm larvae per square foot in several areas. The economic threshold for spraying is four or more army cutworms per square foot on established alfalfa, but just two larvae per square foot in fields seeded last year. Once your alfalfa gets four to six inches tall, spraying is unlikely to be beneficial unless you see a lot of active leaf feeding.

The best insecticides for controlling army cutworms in alfalfa are the synthetic pyrethroids such as Ambush, Pounce, Baythroid, and Warrior. Lorsban also works well. Before spraying, read and follow label directions to safely apply the correct rate.

Most alfalfa fields should start greening up soon. If yours doesn’t, check for army cutworms.

Paul Hay, Extension Educator in Gage County in southeast Nebraska: The wheat in southeast Nebraska looks quite good. This includes several fields I examined that were planted in 15-inch row spacings. It is nearing joint stage in many fields. We were later than desired in applying fertilizer and weed control due to winter frost damage which browned the leaves of wheat and winter annual weeds. Most fields were fertilized by April 10.

Bob Klein, Extension Crops Specialist, West Central REC: Most wheat stands look pretty good in the west central area, but the critical factor for this year’s production will be moisture. On fallow wheat in the Ogallala region, some areas have up to four feet of subsoil moisture while southwest areas have as little as fifteen inches. Before last week’s rain, wheat planted after corn, sorghum or sunflowers last fall had as little as six inches of soil moisture and now has a foot.

We’re finding wheat streak mosaic, mostly in areas where summer rains and/or hail caused volunteer wheat to emerge. It serves as an overwintering host for the disease and the vector, the wheat curl mite. In some fields, the disease level is high. With the long fall and wheat growing through much of the winter, there was a longer period during which infection could occur.

The USDA Nebraska Agricultural Statistics Service reported Monday that wheat condition had improved and was rated 5% very poor, 15% poor, 37% fair, 40% good, and 3% excellent. Plants have begun to joint in fields in the east, southwest, and southeast districts.

The stripe rust situation in Texas is worsening with fields in central and southern Texas showing high rust severities. The severity in northern Texas was light as of early April. We will watch the development of stripe rust in the southern plains closely and forewarn our growers if it is likely to be a problem in Nebraska.

Leaf rust, as of early April, was found only in trace amounts; however, it could increase rapidly with rain and warmer weather.

Pesticides that can be turned in include all types of herbicides, insecticides and fungicides; and all types of agricultural, livestock, home, lawn and garden, structural and commercial pesticides (including those in aerosol containers).

Farmers also can dispose of old electrical transformers from irrigation system renovations. These transformers can contain PCBs, which have been linked to certain cancers and other health problems.

Items such as oil, antifreeze, paint, varnish, thinners, cleaners and solvents are not accepted. Neither are pesticide products in pressurized cylinders.

Preregistration is not necessary and there are no fees for quantities of pesticides up to 1,000 pounds. If you’re planning to turn in more than a half ton of waste products, notify Rich Reiman at the NDA, in advance, by phoning (402) 471-2394. Products totaling over 1,000 pounds require a $1 fee for each pound over 1,000 pounds.

Persons turning in pesticides or transformers are encouraged to:

• Leave pesticide labels on containers.
• Handle containers with chemical resistant gloves and in a way to prevent them from spilling.
• Wash hands with soap and water after handling.
• Take pesticide materials in for identification and disposal if containers’ labels have been removed or are not legible.
• Transport smaller quantities of pesticides in fragile containers in a plastic bucket or other container that will contain the pesticide if it begins leaking.
• Do not transport pesticides in the passenger compartment of vehicles.

Whenever pastures are burned, about one inch of effective soil moisture is lost. Not only do you lose potential growth for the grass this year, you also lose the carryover feed value from last year’s residue. Green-up and rate of growth right after the fire also is much slower when soils are dry at time of burning. This increases the chance of wind and water erosion, and delays when these pastures will be ready for grazing.

Burning still might be appropriate on CRP, though. The loss of production isn’t nearly as important as improving stands. In this dry weather, be extremely careful. Fire is valuable tool. But like any other tool, in the wrong hands it can be dangerous.

Temperatures averaged from three degrees below normal to five degrees above normal. Oat seedings progressed to 47% complete but behind both last year at 57% and average at 49%. Five percent had emerged.

Sugar beet planting progressed in the Panhandle and Southwest districts to 9% complete. A year ago 25% had been planted.

Alfalfa condition rated 3% very poor, 13% poor, 44% fair, 38% good, and 2% excellent.

Pasture and range condition rated 31% very poor, 31% poor, 25% fair, and 13% good. This is significantly below a year ago. Cattle and calf condition rated 1% very poor, 3% poor, 24% fair, 59% good, and 13% excellent. Calving progressed to 85% complete with calf losses mostly average to below average.