August 9, 2002

Drought-stricken corn field in southeastern Nebraska, typical of many of the dryland fields across the state. All of the state is now considered to be in a drought. For further information about the drought and responding to its effects, see this and previous issues of CropWatch or visit the NU IANR Drought Information Web site. (Photo by Paul Jasa)

In this week's Crop Watch

1. Select varieties well adapted to their location.
2. Plant at the proper time.
3. Don't plant short-coleoptile wheats too deep.
4. Treat wheat seed with a fungicide that is active against root rot.
5. Plant into a firm, mellow seedbed (very important in a dry year).

These products are most effective when applied with a commercial-type seed treater. Good uniform coverage of the seed is important. Seed treatments will provide a two to three week window of protection following germination.

In addition to viewing various plots, producers will have an opportunity to hear USDA Farm Services Agency representatives discuss the new farm bill. Roy Frederick, NU extension public policy analyst, also will be on hand to visit about some of the economics.

A field tour of one of the 12 sites where research is being conducted is scheduled to begin at 1:30 p.m. Aug 19 south of Brunswick. The site is 3 miles south of Brunswick and 1.5 miles east on 860 Road.

The research is designed to determine the need for nitrogen, phosphorus and potassium in the same study, rather than looking at the need for a single nutrient. Speakers will include Achim Dobberman, Extension soils specialist, and Charles Shapiro, Extension soils specialist at Northeast REC Haskell Ag Lab.

In addition, a second site will be visited near Page. This site will demonstrate variable rate nitrogen application based on multiple years of yield data and is conducted by the Central Farmers Coop. Jason Steffen will lead the discussion.

Cowpea aphid (University of California IPM Project)

During the past two years, this insect, Aphis craccivora Koch, has been extremely abundant in alfalfa fields throughout arid parts of the Southwest, including Arizona, California, and Texas. Commonly referred to as the "black aphid", it has been around for many years in the south, usually present in low numbers on cotton, alfalfa, and weeds. In addition, outbreaks of cowpea aphid have been reported sporadically throughout Kansas and Oklahoma, all within the past year.

Infestation levels in these areas were reported to be from 50 to 125 aphids per stem and there was noticeable yellowing and stunting. In Nebraska the aphids were noticeable but were not quite at economic levels in most fields. Since many growers were in the process of harvesting the third cutting, we advised them to take the cutting and watch the regrowth.

Description

The cowpea aphid is easily distinguished from other aphids in alfalfa largely because it is the only black aphid found infesting the crop. In general, it is a relatively small aphid, less than 2 mm long. Non-winged and winged adults are usually shiny black while the smaller nymphs may appear to be a dull gray to black. The first half of the antennae is white, and the legs are usually a creamy white color with blackish tips. In alfalfa, these aphids obviously feed on young terminal growth, but can be found infesting leaves, blooms, and stems. Damage symptoms include yellowing, wilting, and dieback. In general, legumes can be seriously damaged, either by direct insect feeding or by the transmission of virus diseases.

Distribution

The cowpea aphid is generally distributed across North America and has been reported in at least 28 states and in three Canadian provinces. This aphid species also has an extensive host range with a marked preference for legumes. Other known host plants are apple, carrot, cotton, cowpea, dandelion, dock, goldenrod, kidney bean, lambsquarters, lettuce, lima bean, pinto bean, peanut, pepperweed, pigweed, red clover, shepherdspurse, vetch, wheat, white sweet clover, and yellow sweet clover. The aphid lives throughout the year without producing sexual forms and they are always parthenogenetic viviparous females (ready to produce offspring at birth).

Monitoring and treatment

Because the cowpea aphid has only recently become a problem in alfalfa, no monitoring guidelines or economic thresholds have been developed for this aphid. An Oklahoma State University entomologist provided the following information. "Normally, we do not worry much about cowpea aphid, and if temperatures increase, predators will feast heavily on them; however, if damage (yellowing and stunting) is evident, then insecticide treatment may be appropriate." Based on his observations, cowpea aphids damage alfalfa and feed on the plant similar to the pea aphid; therefore, thresholds are likely similar.

OSU Cooperative Extension recommends that on alfalfa less than 10 inches tall, 50 aphids/stem should be used as a threshold. On alfalfa taller than 10 inches, 100 aphids/stem may be used. The Texas website suggests a threshold near or below that of blue alfalfa aphid: Height less than 10 inches: 10-12 aphids per stem or 50 per sweep. Height greater than 10 inches: 40-50 aphids per stem or 200 per sweep. These thresholds have not been verified locally, but may be helpful in making treatment decisions.

Control

Very little information is available on insecticide efficacy against cowpea aphids. A glance at a University of Arizona insecticide trial showed that Warrior at 2.6 oz/acre provided the best overall control. We have established an insecticide trial at the UNL Haskell Ag Lab near Concord so more information will be available soon.

An interesting note -- we had a heavy infestation of cowpea aphids in an alfalfa field at the Haskell Ag Lab last week (over 500/sweep). Over the weekend we had over 3 inches of rain and on Monday the aphid populations had crashed. Rain and possibly an abundance of lady beetles, a major aphid predator, appear to have solved the problem for us. Time will tell if this aphid will become a more common pest of alfalfa in Nebraska.

The following websites offer more information on the cowpea aphid:

Blister beetles produce a chemical that causes blisters on your skin if you crush a beetle, hence its name. This chemical - cantharidin - is comparable to cyanide and strychnine in toxicity. Horses are especially susceptible, but cattle and sheep also may be affected.

Small amounts of cantharidin can cause colic in horses. It is absorbed through the intestine and can cause symptoms such as inflammation, colic, straining, elevated temperature, depression, increased heart rate and respiration, dehydration, sweating, and diarrhea. There is frequent urination during the first 24 hours after ingestion, accompanied by inflammation of the urinary tract. This irritation may also result in secondary infection and bleeding. In addition, calcium levels in horses may be lowered drastically and heart muscle tissues destroyed. Since animals can die within 72 hours, it is imperative to contact a veterinarian as soon as blister beetle poisoning is suspected.

How many beetles does it take to kill a horse? It depends. Concentration of cantharidin varies with the species and sex of the beetle. The amount of cantharidin necessary to kill a horse is estimated at 1 milligram per kilogram of horse weight. This translates to about 25 of the more toxic striped blister beetles for a 275 pound horse to over 100 for a 1200 pound animal. Much smaller numbers will cause discomfort, colic, and other symptoms.

Cantharidin is very stable and remains toxic in dead beetles. Most animals are poisoned by ingesting dead beetles in cured hay.

The best way to manage blister beetles is to keep fields from being attractive to beetles. Blister beetles are attracted to flowers, so cut on a schedule that keeps alfalfa and weeds from producing flowers. Practice good weed management to keep other flowering plants to a minimum, including along field margins.

Check fields for blister beetles before harvest. Avoid crimping or crushing hay if beetles are present. Crushed beetles remain in the hay and can poison animals. Since blister beetles tend to swarm, crushing can deposit many dead beetles into a single flake of hay.

Do not use a hay conditioner when harvesting blister beetle infested alfalfa. Sickle bar mowers and some disk mowers lay the hay down but do not crush it. If beetles are not killed by driving on the hay they will crawl out of the hay and leave as it dries. Other tips include:

• Do not drive on hay shortly after cutting
• Avoid using outer edges of fields for horse hay, especially if adjacent to weedy strips
• Harvest before hay blooms
• Use a short residue insecticide around field margins; follow directions on harvest intervals

We see much greater success establishing alfalfa stands in August when the top two to three feet of the soil profile is at at least 80% of field capacity for soil moisture. Seeds must germinate soon after planting and have moisture continuously available to support pre-winter growth.

Any planting delay could cause poorer stands. In southern Nebraska you can wait until mid-August, which is ideal. Planting after August 31 becomes risky. In central Kansas alfalfa can be planted as late as mid-September.

Seedbed preparation is crucial for late summer plantings. Good seed-to-soil contact and weed control are critical, both when seeding into prepared seedbeds or into wheat stubble. Put extra effort into creating a firm seedbed which will enhance seed-to-soil contact, limit the rate of soil drying, and help deeper moisture "wick" up toward the surface.

Whenever seeding alfalfa in August, be wary of grasshoppers. They're especially big and bad this year, and they love to eat new seedlings. Spray field margins with insecticides before planting if necessary.

One important caution -- never plant into dry soil. August plantings into dry soil may lie dormant for several weeks. Too little time then will remain for seedlings to develop good cold tolerance. Many failures occurred in recent years because folks forgot that fall rains are unreliable in our area.

If you have moisture, plant. With help from Mother Nature, good hay is just a spring away.

The roots will quickly take in the available moisture as well as lots of nitrates from the soil, causing a large spike in nitrate concentration soon after a rain. To limit potential problems, don't cut hay or silage until plants have a week to ten days to metabolize these extra nitrates.

Prussic acid poisoning also can be a problem, especially after a good rain. With increased moisture new shoots and tillers will start growing. Since these plant parts have the greatest potential for prussic acid poisoning, be prepared to watch the grazing more closely beginning about a week after the rain.

Similar problems can occur if grazing dried up alfalfa. New growth can increase the bloat hazard within a week or two.

Prussic acid, also called hydrocyanic acid and cyanide, often is overlooked when nitrate poisoning becomes a danger. Prussic acid is different from nitrates, but just as dangerous in plants under stress.

The danger of prussic acid poisoning is limited to just a few plants, most of them related to the sorghum family. Sudangrass produces the least amount of prussic acid and can be managed quite easily to prevent problems. Sorghum-sudan crosses are a bit more hazardous and forage sorghums, cane, grain sorghum, and shattercane can be very dangerous. Other summer grasses like millets and corn, as well as small grains do not produce toxic levels of prussic acid.

Prussic acid rarely is a problem in hay or silage. That's because as the plants dry or ferment much of the prussic acid disappears as a gas.

When grazing these plants, please be extra cautious this year. New shoots and tillers and very young leaves contain the highest poisoning potential. This is bad news since your cattle are likely to prefer these plant parts when grazing. Limit their opportunity to select just these dangerous plant parts by waiting to graze until plants are tall enough to have enough older leaves to prevent animals from just picking their favorite parts. That means 18 inches for sudangrass and 24 inches for sorghum-sudan hybrids. During drought I don't even want to think about grazing cane or milo.

Be sure to fill animals with hay or grain before first turning into these pastures. And with a little care, your animals will be safe.

Another rumor is that nitrates will decline to safe levels if you just let the crop stand in the field until winter. Yes, nitrates usually decline as a plant matures if it has some new growth, like grain or leaves, that will use up the nitrates. But plants that are essentially dormant or have no more growth potential often retain high nitrate levels for many, many months. Again, test to know for sure.

Finally, rumor has it that if you wait about a week after a hard freeze, prussic acid will not be a problem any more. A hard freeze kills leaves and as they dry, much of their potential prussic acid disappears as a gas. But when tops are killed, sometimes new shoots or suckers begin growing at the plant base. These often have very high poisoning potential. Watch for new growth and potential problems.

Don't become so afraid of nitrates and prussic acid that you avoid your crops altogether, but make sure you know the true hazards, and then use them wisely and safely.

The Republican River Basin Irrigation Management Project focuses on showing farmers and crop consultants how time of application and amount of water applied affects yield. It provides for demonstrations of research-based irrigation management strategies in farmer fields.

Field tours showcasing corn irrigated with one of three strategies (fully watered, water miser best management practice, and deficit irrigation) will be August 13 near Holbrook and Aug. 15 near Culbertson. Both will begin at 7 p.m. Other water-saving methods to be covered include improving irrigation systems, reducing tillage, growing crops that use less water, etc. The new EQIP cost share program provides money for most of these methods and will also be discussed.

To reach the Aug. 13 tour, from the east edge of Holbrook on U.S. Highways 6 and 34, drive 0.5 mile north on the county road. The plot is on the west side of the road and Gene Glanzer is the farmer cooperator. The second tour, Thursday, August 15, is 1.5 miles east of Culbertson on U.S. Highways 6 and 34 or 8 miles west of McCook. The plot is on the north side of the road. Ron Hoyt is the farmer cooperator.

The three types of irrigation management strategies being spotlighted include:

Fully watered

The traditional Best Management Practice irrigation management strategy focuses on keeping soil-water at a high enough level to prevent moisture stress from being a yield-limiting factor. The goal of the strategy is to maintain plant available soil-water (in the active root zone) between field capacity and 50% depletion from planting through maturity. Usually the soil is kept at 0.5-1 inch below field capacity to allow for storage of rain. After the hard dough stage the soil is allowed to dry to 60% depletion.

Water miser

The Water Miser Best Management Practice focuses on saving water during the less sensitive vegetative growth stages and watering fully during the critical reproductive growth stages. Irrigation is delayed until about two weeks before tassel emergence for corn unless soil-water becomes 70% depleted (in the active root zone). Once the crop reaches the reproductive growth stage, the plant available soil-water (in the active root zone) is maintained in a range between field capacity and 40% depletion. Usually the soil is kept 0.5-1 inch below field capacity to allow for storage of rain. After the hard dough stage the soil is allowed to dry down to 60% depletion.

Deficit irrigation

The deficit irrigation management strategy focuses on correctly timing the application of a restricted quantity of water both within the growing season as well as over a several year period. The intent is to stabilize yields between years by applying irrigation based on soil-water depletions.

The strategy is to delay applying water until about two weeks before tassel emergence for corn unless soil-water becomes 70% depleted. Once the crop reaches the reproductive growth stage, the plant available soil-water (in the active root zone) is maintained between 30% and 60% depleted during the early reproductive stage and is allowed to dry down to 70% depleted after the hard dough stage.

For more information about the field tours or the Republican River Basin Irrigation Management Project, contact Steve Melvin at (308) 367-4424 or smelvin2@unl.edu.