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It's “Miller” Time

If you have been outside near your porch light the last week or so, you may have noticed a great number of moths flying around. Many of these are simply a nuisance, but one, the armyworm moth, may have implications for Nebraska farmers.

University of Nebraska operated light traps at Hastings, Clay Center, and Concord, Nebraska have been capturing increasing numbers of armyworm moths. Although there have been no reports of armyworm larvae yet, farmers should be on the lookout for developing caterpillar populations.

Armyworm moths migrate from the south and lay eggs on grasses, often in pastures, field edges, or on grassy weeds in cornfields. After eating the grass, or after the grass is killed by an herbicide, the larvae move onto the corn to feed. They feed at night, usually on the leaf margins, but can consume the entire leaf except for the tough midrib when populations are high. Often they will start on the lower leaves and work up the plant. The armyworm larvae may also injure wheat or other small grains.

Armyworm larvae are smooth with two orange stripes along each side and two dark stripes on the back. A distinguishing characteristic is a yellow-brown head with a brown netlike pattern of dark lines.

Cornfields that have grassy weeds should be scouted regularly, particularly after the weeds begin to die following an herbicide application. Corn plants can tolerate quite a bit of early season defoliation, so do not treat the field with an insecticide simply because you see some armyworm injury.

Treat when more than 50% of the plants have armyworm injury and the larvae are numerous and less than 1.25 inches long. Larvae that are 1.25 inch or more long are nearly done feeding, so little additional injury will accumulate and treatment would not be of benefit.

Updated information on Nebraska light trap insect capture, armyworm management, and insecticide choice can be found at www.entomology.unl.edu . Information also is available in the NebGuide, “Insects that Feed on Corn Ears”, at http://www.ianrpubs.unl.edu/epublic/pages/publicationD.jsp?publicationId=524 (TH&KJ)

Control of Problem Weeds in Roundup-Ready Soybean with Glyphosate Used Alone

More than 90% of soybean fields in Nebraska are planted with glyphosate-tolerant varieties (e.g. Roundup-Ready varieties). Widespread and repeated use of glyphosate-based herbicides (brand names and generics) raises several concerns from the practical standpoint, such as (1) potential for weed resistance and (2) shifts in weed species. In the last three 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, it has happened basically since man started cultivating crops. Weedy and invasive species can adapt to changes in production practices in order to take advantage of the newly formed “niche”. Species that do not adapt to those changes become less frequent compared to those that do adapt to the system. 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. This list includes: marestail (horseweed, now considered glyphosate resistant in Nebraska), morning-glory (common and ivyleaf), wild buckwheat, Pennsylvania smartweed, lady's thumb, venice mallow, yellow sweet clover, 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.

In 2004 studies were done at Concord and North Platte with the objective of determining the appropriate dose of glyphosate used alone to control above mentioned species.

Seven rates of Roundup WeatherMax were tested, 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 were conducted approximately 21 days after treatment, based on a scale from 0 to 100 (where 0 = no injury and 100 = plant death).

Most weeds were able to survive the labeled 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 (Table 1). Ivyleaf morning-glory and sweet clover were the hardest species to control. For example, the 22 oz rate provided only 50% control of ivyleaf morning-glory that was 4" tall. The control level was further reduced with the increase in morning-glory size, resulting in 30% and 21% control for 8" and 12" tall plants, respectively. Similar trends were observed for other weed species. Labeled rate of Roundup WeatherMax provided good control (>85%) of kochia and Russian thistle regardless of the plant height.

Since the label rate of WeatherMax was not able to provide adequate control of most species over 3" tall, dose response curves (not shown) were developed to determine how much glyphosate is needed to achieve at least 90% control of taller plants (6"-20" tall). Based on data from the dose response curves, in order to achieve at least 90% control of taller weeds, it requires much higher rates of Roundup WeatherMax ranging from 1.5 - 4X of the labeled rate. About 1.5-2X rate was needed to control 3-6" tall wild buckwheat, venice mallow, velvetleaf, waterhemp, sweet clover, ivyleaf morning-glory and field bindweed. About 3-4X rate was needed to control 12"-15" tall ivyleaf morning-glory and yellow sweet clover. 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 trends in weed shifts continue to occur, glyphosate used alone will no longer a viable tool for weed control in Roundup-Ready systems. Mixing glyphosate with other post-emergence broadleaf herbicides, or using soil applied herbicides after planting, indicates potential to effectively control most of these species (see last weeks newsletter).

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).

David P. Shelton
Extension Agricultural Engineer

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