Newsletters

Print this newsletter (pdf)
water use/GDD tables

SCOUT EMERGING SOYBEANS FOR BEAN LEAF BEETLES

Bean leaf beetles have not been a big problem the last couple of years, but we usually have at least a few areas each year that have problems with this insect, and early planted soybean fields always attract the most beetles. Delayed soybean planting caused by the weather has forced the beetles to wait for their preferred food, and now that fields are emerging, feeding is evident.

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

Bean leaf beetle

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 can be 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 from 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 to late August.

Bean leaf beetles vary in color, but are usually reddish to yellowish-tan. They are about ¼ 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, particularly if they are the only beans up and available for the beetles to move into. 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, soybeans planted during the normal soybean planting window in Nebraska are not colonized by enough beetles to cause economic injury.

The following table presents economic thresholds for bean leaf beetle on seedling soybean. Be aware that these thresholds are for defoliation of beans at VC - V1. 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 if the beetles appear to be significantly injuring or clipping the cotyledons and growing points, an insecticide treatment may be warranted. Research has indicated that early loss of both cotyledons can result in about a 5% yield loss. If control costs or crop values are lower or higher than those presented in the table, change the thresholds accordingly.

Remember that early-planted, temporally isolated 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. For those that plant early and regularly have economic levels of colonizing bean leaf beetles, insecticide seed treatments such as thiamethoxam or imidacloprid may be warranted.

We have also found that early season defoliation can affect weed management. Seedling defoliation can result in a need for earlier weed management. For example, with no defoliation, weeds can remain in the crop up to the V4 stage (third trifoliate) without significantly affecting the yields. However, at 30% and 60% defoliation, weeds require removal by the V3 and V1 stages, respectively.

Another reason some producers treat bean leaf beetle on seedling soybeans is to reduce the subsequent F1 and F2 generations; however, UNL 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 included later this summer.

CONTROL OF VOLUNTEER CORN IN SOYBEAN

It is not surprising to see volunteer corn in soybean since the two crops are used in rotation. However, volunteer corn is a weed and should be treated accordingly. It reduces light interception to soybean, interferes with harvesting, and makes the field look ‘messy'. Growth of corn is generally ‘faster' than soybean, therefore if it is left uncontrolled, soon it will overtop the soybean canopy. Control can be achieved by mechanical means (eg. inter-row cultivation) and herbicides.

Timing of inter-row cultivation should depend on the weed pressure. If volunteer corn is a predominant “weed”, the timing of cultivation should be around the 5-6 leaf stage of the corn. The growing point of corn remains in the ground until the 6 th leaf stage. Therefore, any cultivation done prior to that leaf stage may result in regrowth of the plants, and a second cultivation may be required. This is especially true with shallow cultivation.

If you have Roundup-Ready soybean, Roundup will control volunteer corn, unless you have had Roundup Ready corn in the previous year. Roundup will obviously not control volunteer RR-corn in RR-soybeans.

Herbicides can also be used effectively to control volunteer corn. There are several grass type herbicides (graminicides) that can be used at their lower rates post-emergence in both conventional and RR soybean. The list of herbicides and their lower rates per acre includes: Assure (4 oz), Fusilade (4 oz), Fusion (2 oz), Poast-Plus (10-16 oz), and Select (4 oz). Best control is achieved when herbicides are applied by the 3-4th leaf stage of corn. These herbicides used at full label rates will also control many grassy species, including barnyardgrass, green and yellow foxtail, fall panicum and sandbur.

CONTROL OF PROBLEM WEEDS IN ROUNDUP-READY SOYBEAN WITH GLYPHOSATE USED ALONE

Considering the fact that more than 90% of soybean fields in Nebraska are planted with glyphosate-tolerant varieties (eg. Roundup-Ready varieties), soybean producers must clearly realize the benefits from this technology. However, widespread and repeated use of glyphosate-based herbicides (brand names and generics) has raised several concerns from the practical standpoint, such as (1) potential for weed resistance and (2) shifts in weed species. In the last few years, university weed extension specialists have been receiving phone calls and complaints on glyphosate failing to control certain weed species, including some “new weeds”.

Weed species shift is not a new thing. Weedy and invasive species can adapt to any changes in production practices in order to take advantage of the available change. Species that do not adapt to those changes become “ less frequent” compared to those that do adapt to the system and become “more frequent”. Therefore, despite the fact that glyphosate controls many weed species, especially grasses, there are broadleaf species that are naturally tolerant to the label rates of glyphosate. Therefore, it appears that as a result of repeated use of glyphosate in Nebraska, there is a slow shift in weed species occurring from those easily controlled by glyphosate to those more tolerant of this herbicide.

Based on our extension phone calls and questions from producers, crop consultants and agronomists we compiled a list of such species, which includes: marestail (horseweed), morning-glory (common and ivyleaf), wild buckwheat, Pennsylvania smartweed, lady = s thumb, venice mallow, yellow sweetclover, field bindweed, waterhemp, kochia, Russian thistle, primrose species and volunteer Roundup-Ready corn. If these weeds are not controlled, their seeds will be a major problem in the future, especially in no-till systems, due to lack of tillage as a tool for weed control. Such shifts in weed populations to more tolerant weeds is already resulting in an increased weed control costs due to additional herbicide applications or increased glyphosate rates.

Therefore, the purpose of this article is to summarize preliminary data from studies conducted at Concord and North Platte in 2004, with the objective to determine the appropriate dose of glyphosate used alone to control above mentioned weed species.

We tested 7 rates of Roundup WeatherMax ranging from 4.6 oz B 60 oz with 2 % v/v AMS. Each glyphosate rate was applied at 3 growth stages of the weed, targeting (1) 2 - 5" tall weeds (early POST), (2) 6-12" weeds (mid POST) and (3) 12-20" weeds (late POST). Visual ratings of percent weed control was conducted at approximately 21 days after glyphosate treatment, and it was based on a scale from 0 to 100 (where 0 = no injury and 100 = plant death).

Most weeds were able to survive the label rates of Roundup WeatherMax (22 oz/acre), while the weed size was the most important factor that determined the level of control for each species (see Table 1). Ivyleaf morningglory and sweet clover were the hardest species to control. For example, the 22 oz rate provided only 50% control of ivyleaf morningglory that was 4" tall. The control level was further reduced with the increase in morningglory size, resulting in 30% and 21% control for 8" and 12" tall plants, respectively Similar trend was observed for other weed species Label rate of Roundup WeatherMax provided good control (>85%) of kochia, and Russian thistle regardless of the plant heights.

Since the label rate of WeatherMax was not able to provide adequate control of most species over 3" tall, we developed dose response curves (not shown) to determine how much glyphosate is needed to achieve at least 90% control of taller plants (6"-20" tall). This information will also help determine rates for those late applications of glyphosate in Roundup-Ready soybeans. Many soybean fields receive late applications of glyphosate for various reasons. Based on our data from the dose response curves, in order to achieve at least 90% control of taller weeds, it was required to use much higher rates of Roundup WeatherMax ranging from 1.5 - 4X of the label rate. About 1.5-2X rate was needed to control 3-6" tall wild buckwheat, Venice mallow, velvetleaf, waterhemp, sweet clover, ivyleaf morningglory and field bindweed. About 3-4X rate was needed to control 12"-15" tall ivyleaf morningglory and yellow sweetclover. For weed sizes and respective rates, see Table 1.

Data presented in this article reaffirms what many practitioners were observing, which is that glyphosate used alone does not work well today compared to 5-6 years ago. The labeled rate of glyphosate did not provide adequate control of most species we tested. If the trend in weed shifts continues to occur, glyphosate used alone may no longer a viable tool for weed control in Roundup-Ready systems. Mixing glyphosate with other post-emergence broadleaf herbicides, and/or using soil applied herbicides, will be needed to effectively control most of these species.

Using various weed control tools is not a new thing, we only “forgot” about it since the introduction of Roundup-Ready crops. Changing modes of actions in your herbicide program is also one of the basic ideas in an Integrated Weed Management (IWM) program (eg. integrating several tools for weed control), especially to combat weed resistance/tolerance issues. Roundup-Ready technology has a fit under the umbrella of an IWM system only, and the value of this technology can be preserved only by proper management, and reduced overuse. It is easy to fall into a trap of overusing glyphosate when one glyphosate-tolerant crop is grown after another. Therefore, proper use of this technology, as a component of IWM program, is the key to preserving the long-term benefits of this technology while avoiding many of the concerns about their use, or misuse (eg. overuse).

Table 1. Weed species and their heights at the time of herbicide application, levels of weed control with 22 oz rate of Roundup WeatherMax (at 21 days after application), and the rate of Roundup WeatherMax rate needed to provide 90% control of respective species at Concord in 2004 (preliminary data).

COMMON MULLEIN CONTROL IN PASTURE

Common mullein ( Verbascim thapsus ) is a weed species on the increase in northeastern Nebraska 's rangeland, woodland, and pastures. It is a biennial plant that reproduces only by seeds, but it is a prolific seed producer. The taproot of this species can access soil moisture from a deeper profile than the fibrous roots of pasture grasses, giving common mullein the competitive advantage over grass, especially during dry years.

Common mullein usually starts growing sparsely as individual plants and then spreads further if not controlled. A cluster of leaves, commonly known as a rosette, with a thick “hair” cover is a distinct identifying feature of this species. The stem is also woolly, erect, 2-6 ft tall, with no branches. Leaves are opposite, elliptic to ovate. Like many other plant species, the overall growth and development depends on the amount and timing of rainfall. Common mullein, in Nebraska , can flower in June and July, with yellowish flowers, and it has no value to livestock because of its low palatability.

Ranchers need to control this species because heavy stands can reduce grass production as much as 50%, especially in dry years, and the common mullein plants have no value for livestock forage. This weed can be controlled by various means. The best strategy is to control while the density is low. Density of this species can easily expand from few to hundreds plants per acre just over couple of years due to prolific seed production. Sparse populations can be controlled by mechanical removal using a spade or shovel in late April and early May. Individual plants can be dug out or cut just at the soil surfaces as long as whole rosette is removed. Single mowing of new 1-2 feet tall plants can reduce population and seed production for the season, especially in dry years.

Herbicides can also be effective tools in providing season long control. However, one thing to note is that the thick coat of hairs on the leaves can reduce herbicide uptake and level of control. Herbicide application should be conducted when the rosette has 6-12 leaves, before the stem starts to grow. The list of effective herbicides and their rates per acre includes: Grazon P+D (3-4 pints/acre), Cimarron (0.75-1 oz/acre), and a 3-way-mix of Cimarron (0.5oz ) with Glean (0.5 oz) and RangeStar (32 oz). Make sure to use enough additives such are Crop Oil at 1-2 q/acre to help herbicide penetrate the thick hair coat.

HOARY VERVAIN CONTROL IN PASTURE

Hoary vervain (Verbena stricta), also known as wooly verbena or tall vervain, is a commonly found native weed in northeastern Nebraska's on over-grazed rangeland, prairies and disturbed sites in all soil types. There are several other types of vervain in Nebraska , (prostrate, white, and blue) most having similar growth forms and habits as hoary vervain. Hoary vervain is a perennial forb from the vervain family (Verbenaceae) that reproduces by seeds. The taproot produces individual erect plants. The stem is nearly round, simple or branched above and can be up to 5 ft tall, covered with soft white hairs. Leaves are opposite, leaf blades are ovate with many teeth. The lower surface is pubescent with highly visible veins. Like many other plan species, the overall growth and development depends on the amount and timing of rainfall. Hoary vervain can flower from May to September, with blue or purple flowers positioned on the top of the main stem and branches and producing a two seeded fruit.

Hoary vervain provides forage for deer while seeds are important food source for small mammals and upland birds. Native Americans also made a tea from the leaves to treat stomachache. Hoary vervain has no value to livestock because of its low palatability.

This weed can be controlled by various means. Mowing the plants when they are 3-5 inches tall can reduce vervain population considerably for the season. Mowing can be done one or two times per season depending on the amount of rainfall during the season. One mowing done in mid June can be effective (>75% control) if the season is dry, due to lack of moisture needed for weed regrowth. If the season is wet, an additional mowing is needed in July-August. Herbicides can provide season long control. Herbicide application should be conducted when vervain plants are 3-5 inches tall, which is usually in early part of June. The list of effective herbicides, their rates and cost per acre includes: Salvo (12 oz/acre, $4), Grazon P+D (32 oz/acre, $8), Weedmaster (32 oz/acre, $6), Ally (0.25 oz/acre, $8), and Vista (22 oz/acre, $8).

David P. Shelton
Extension Agricultural Engineer

Newsletters

ipm052808