2002 WEED SCIENCE PROGRAM FOR NORTHEAST NEBRASKA

I want to take the opportunity to provide you with a quick overview of this year's weed science projects that will be conducted at the Haskell Ag Lab and at a few other locations in our northeastern District.

The long-term goal of my research program is to develop biologically based weed control strategies using a multi-disciplinary approach that builds bridges among scientific disciplines, especially between weed/crop ecology and herbicide technology. My research interests include, but are not limited to, weed ecology, crop-weed interaction as affected by cropping practices and nutrient management, herbicide technology, and system simulation.

Specific program activities:

I. HERBICIDE SCREENING: Herbicide screening studies are done routinely every year. This year we will have about 18 trials in conventional and no-till systems with traditional, Roundup-Ready, Liberty-Link and Clearfield hybrids.

II. SOYBEAN AND CORN VARIETY TRIALS: Crop variety tests are done as a joint project with Dr. Charles Shapiro. This year we expect to test about 100 soybean varieties including Roundup Ready and conventional, and 60 corn hybrids at both Concord and a sandy irrigated site in Antelope County.

III. HIGHLIGHTS of CURRENT RESEARCH PROJECTS:

Feel free to call me for enquiries about any of the above studies (402-582-2808). The program will also be presented during the Weed Tour, which will be held on June 26, 1:00 PM at the Haskell Ag Lab in Concord. (SK)

EUROPEAN CORN BORER MOTHS ARE FLYING

Since 1998 European corn borer moth flights have been extremely low, however, it appears that populations are beginning to rebound. For the first time in four years our nightly black light trap catches for first flight corn borer cracked 100 moths/night at Concord, NE (just barely). Current information on black light trap catches for several sites in Nebraska can be found at http://entomol/fldcrops/fldcrops.htm. While not a bumper crop of moths, the increased catch this year indicates we should not be complacent about the European corn borer in non-Bt cornfields. Its time to review corn borer management and begin scouting for this insect.

Timely and accurate scouting is the key to managing European corn borer in standard (non-Bt) corn hybrids. Remember that conditions are localized and fields must be scouted on an individual basis to make accurate decisions.

Corn borer larval survival is dependent upon several factors. High humidity and warm temperatures are ideal for establishment of larvae in the whorl. Egg masses are white, with 5 to 40 eggs in each mass, and laid on the underside of leaves near the midrib. The masses look like fish scales flattened against the leaf. In 4 to 7 days the heads of the developing larvae will be visible, and the eggs will appear spotted. This is the "blackhead" stage, and these eggs normally hatch within 24 hours. As the larvae enter the whorl to feed on the developing tissue, the feeding scars (shot-holes) appear as the leaves emerge from the whorl. Larvae will remain within the whorl for 7 to 14 days before boring into the stalk.

Corn that is below about 16" extended leaf height (distance from the tip of the leaf pulled up vertically to ground, about six-leaf stage) is unlikely to support young larvae because of the presence of a substance known as DIMBOA, a natural resistance factor. As the plants grow the level of DIMBOA decreases, so plants above the 16" extended leaf height will generally support corn borers. First generation corn borers prefer taller plants for egg laying, therefore, the earliest planted fields are more likely to have higher populations. Scout these fields first, but do not neglect other fields because any cornfield is a potential target and should be scouted.

Now that Bt corn is being planted widely, be sure you know whether the field you are scouting was planted to Bt corn. In Bt corn, corn borer injury to whorl stage plants should be limited to a
few tiny pin holes where larvae initially fed before ingesting a lethal dose of Bt toxin. However, seed lots
may contain a small percentage of off-type seed (typically less than 4%) which does not produce
sufficient toxin levels to kill corn borer larvae. If greater than 4% of plants show significant leaf feeding damage in a Bt cornfield, check to confirm it is corn borer causing the injury (other caterpillars such as corn earworms, or common stalk borer are not controlled by B. t. corns currently available). If you believe that corn corer is causing the injury, contact a representative of the company who sold the seed to investigate the situation more completely.

To determine the need for treatment, scout at least 20-25 consecutive plants in at least 4-5 different places in the field (100 plants minimum per field). The scouting locations should be randomly selected and representative of the field as a whole. At each scouting location, randomly select the first plant that will be sampled. If you do not and always start sampling at an infested plant, the counts may be inflated by up to 5%. Count the number of plants showing shot-hole feeding and determine the percent of infested plants. Next, pull the whorls from at least two randomly selected infested plants in each set of 20-25 plants. Unroll the leaves and count the number of larvae in the whorl and determine the number of larvae per infested plant. Young corn borers usually suffer from 60 to 85% or higher mortality due to natural enemies, weather and disease, so try to wait to make treatment decisions until most of the borers are second instar to take advantage of natural larval mortality.

Use the information gathered from field scouting to complete the accompanying worksheet. This takes you through the calculations needed to estimate the preventable loss if an insecticide is used. Compare the preventable loss to the total cost of insecticide application. An insecticide application is economically justified if preventable loss exceeds the total cost of insecticide application. An interactive version of the worksheet is available at http://ianrwww.unl.edu/forms/forms.skp/ecb_1st.html

Treatments will be effective only if borers are still feeding in the whorl. Treatments made after corn borers begin to bore into the stalk (when they are about half grown) will not be effective. Based on research data, the best control is achieved with aerial or ground applied granular formulations or liquid applications through sprinkler irrigation systems, which provide the best penetration of insecticide into the whorl where the corn borer larvae feed.

Worksheet for First Generation European Corn Borer

To estimate the cost/benefits of applying an insecticide for European corn borers, you also need to know the cost per acre of the insecticide application ($/acre), the anticipated price of grain ($/bu), and yield potential (bu/acre) of your hybrid. Assume 5% yield loss/borer/plant and a proportion of larval population reduction by insecticide application of 0.75.

Average number of larvae/plant (percent of injured plants X number of larvae/injured plant)

Potential yield loss if all larvae survive (number of larvae/plant X 5% loss/borer/plant)

Potential bushel loss (potential yield loss X yield potential)

Potential dollar loss (potential bushel loss X estimated price of corn)

Preventable loss (potential dollar loss X proportion of larval population reduction)

All of the above numbers are variable and are unique to each field and farm management operation. Use the formula several times using different figures for yield, price, and cost of application to see how each one affects the outcome. Use the figures closest to your situation to make the final determination.

Many insecticides are registered for control of first generation European corn borers and most will do a good job if applied properly at the right time. The Bt-based insecticides Dipel, Condor, M-Peril and others are effective and do not reduce populations of corn borer natural enemies. Refer to http://entomology.unl.edu/instabls/ecb1st.htm for a list of suggested insecticides.

Additional information on first generation European corn borer management is available in First Generation European Corn Borer Scouting and Treatment Decisions, NebFact 98-364. This publication is available from your local cooperative Extension office or at http://entomol/ecb/ecb1.htm (TH & KJ)

AVOID CROP INJURY WHEN APPLYING LIQUID MANURE THROUGH
SPRINKLER IRRIGATION SYSTEMS

Applying liquid manure or lagoon effluent through a sprinkler irrigation system offers several benefits including being able to apply nutrients when the crop most needs them and when the potential for runoff is low. However the soluble salts in liquid manure can cause leaf burn and crop injury. Testing liquid manure for salts can help you determine application timing and procedures to reduce the potential for plant damage.

Soluble salts in manure consist of positively (ammonium, calcium, magnesium, potassium and sodium) and negatively (bicarbonate, chloride, sulfate and nitrate) charged ions. Soluble salt concentration can be estimated using electrical conductivity (EC) in millimho per cm (mmho/cm) or decisiemens per meter (dS/m); mmho/cm = (S/m). The soluble salt concentration (in parts per million) can be estimated as 640 * mmho/cm.

Liquid manure is considered safe for application to a growing crop when the EC is less than 3 mmho/cm, although manure with greater than EC of 5 has been applied without leaf burn. The thresholds for injury are not well established and vary with crop and stage of growth. Corn and wheat are less susceptible to leaf burn than legumes. Corn that is taller than 18" is less susceptible than younger corn.

When the EC of manure effluent is greater than 3, effluent can be diluted with fresh water to reduce the salt effect if appropriate check valves (required by law) are used to prevent contamination of the fresh water source. When diluting manure through the irrigation system, begin pumping fresh water first and then add the effluent to avoid application of the effluent at full concentration.

Wind, high air temperature and/or low humidity may increase potential leaf burn. Leaf burn is less likely if liquid manure is sprinkler applied in the evening or at night. However application at this time may increase odor problems since odors do not disperse as quickly when liquid manure is applied during the early evening and night compared to morning and early afternoon hours.

Soluble salt content can be determined by having manure liquid samples analyzed by a laboratory. Producers can also make their own EC measurements with pocket meters that are available for about $75.

In summary:

1. Test the electrical conductivity of liquid manure or effluent before applying through a sprinkler system to a growing crop.

2. If EC is greater than 3 mmho/cm, take one or more of the following precautions.

Electrical conductivity and ammonium content of liquid manure

Electrical conductivity is used to estimate the salt content of manure. As ammonium is generally the main ion in liquid manure, EC can be used to estimate ammonium-N concentration. If manure is less than 1% dry matter content, or if EC of manure is between 0 and 6 mmho/cm, ammonium-N concentration can be estimated as: NH4-N (ppm) = -9 + 77 * EC

 

Soil Fertility Extension Specialists:
Charles Shapiro and Charles Wortmann