INSECT UPDATE

No insect problems have been reported in the last week in northeast Nebraska, but that doesn’t mean we shouldn’t be on the lookout.  Now that corn is emerging it is time to look for wireworm, white grub and cutworm damage.  Wireworm damage usually appears first as gaps in emerging stands.  Wireworms are segmented, hard shelled orange to yellow worms with very small legs, and will be found feeding on the seed early in the emergence process, or on the emerging plant below the soil surface (Fig. 1).  They are rarely seen above ground.  Wireworms cannot be controlled by post-emergence treatments so the only remedy is to replant if necessary, using a seed treatment to protect the stand.  Cutworms usually feed on the seedlings first, then begin to cut plants as the worms grow larger (Fig. 2).  They can be controlled with post-emergence treatments (see last week’s newsletter). White grubs generally damage corn in the 3 to 6 leaf stage (sometimes even later) as they move closer to the soil surface after spending the winter up to three feet or more below ground (Fig 3).  White grub damaged plants first begin to wilt, turn purple, and may die as the root systems are destroyed or severely pruned.  Unfortunately, like wireworms, white grubs cannot be controlled with post-emergence treatments, and replanting of lost stand is the only alternative.

There have been reports of an insect often confused with cutworms in the field.  Crane fly larvae (Tipulidae) are the immatures of a thin bodied, long legged fly that are often called “giant mosquitoes” because of their resemblance to the mosquito.  These larvae are often found in cool, damp areas where they feed on organic matter.  They do not harm crops.  The difference between cutworms and crane fly larvae is cutworms have a head capsule and 6 visible legs, whereas the crane fly larvae do not have legs and have fleshy protuberances on the rear end (sometimes described as a “collar” (Fig 4).  Make sure you have a proper ID before treating any field for suspected insect problems.

Alfalfa appears to be in good shape without any insect problems reported.

Fig. 1. Wireworm	Fig. 2. White grub	Fig. 3. Crane fly larvae

SPRING BLACK STEM AND LEAF SPOT
Recent weather conditions have favored the development of spring black stem in the northern third of Nebraska.  This disease is “sneaky” in that it can cause significant loss without being noticed.  It is also often confused with insect damage.  Following is a description of spring black stem. 

Cause - Spring black stem and leaf spot is caused by the fungus Phoma medicaginis var. medicaginis. This fungus survives the winter as pycnidia or dormant mycelium in overwintered stem lesions or fallen leaves.

Symptoms occur primarily on stems and leaves but seedpods, crowns and upper taproots also may become infected. In spring small black spots develop on leaves, petioles and stems of new shoots. The leaf lesions are irregular, and enlarge and merge until much of the leaflet is covered Infected leaves turn yellow and drop from the plant. Lesions on stems and petioles turn black. As stem lesions enlarge and merge, most of the stem becomes black.  If the stem is girdled by the advancing lesions, it will die. The fungus also can invade the plant base and cause the crown and upper taproot to rot.

Conditions Favoring Disease Development - Pycnidia form in abundance in infected stems during late fall and early winter. In spring during wet weather spores are produced inside the pycnidia. As they ooze from the pycnidia, they are splashed by rain onto the leaves, petioles and stems of newly emerged shoots. The new shoots become infected as they grow through the residue from the previous year's crop. Some spores may be spread by wind and insects.  The foliage must be wet from dew or rain for infection and spread to occur. The disease continues to move upward in the canopy if cool, wet weather persists through spring.

The fungus also is carried in the seed.  Seedling blight can occur when infected seed is planted. The disease will often spread from these seedlings to nearby healthy seedlings. Early seedling loss creates stand establishment problems.

Spring black stem and leaf spot is usually a problem only on the first cutting. Regrowth from this cutting may become infected but disease severity is generally light. If environmental conditions between mid-April and mid-June are favorable (cool and wet) for disease development, plants may become defoliated before cutting. When severe, as much as two-thirds to three-fourths leaf defoliation can occur which results in the first cutting being primarily stems. This not only reduces yield, but significantly lowers the quality of the hay. In Nebraska the disease becomes severe after cool, wet weather.

Control - Early cutting is recommended if spring black stem and leaf spot is prevalent on the lower portion of plants. Scout fields weekly in spring to determine the extent of disease development. Decide whether to cut early based on the results of scouting fields and weather forecasts. Don't delay the decision to cut early, otherwise most of the leaves may be lost by harvest. Ideally, one-tenth bloom is the optimum growth stage for harvesting while maintaining forage quality and limiting premature leaf loss due to disease. If the field is recovering from winter injury, it may be necessary to delay the first cutting at the risk of foliage loss to spring black stem and leaf spot.

A few moderately resistant cultivars have been developed. Multi-resistant cultivars with a high level of resistance to spring black stem and leaf spot should soon be available.

Alfalfa stands are sometimes burned in early spring to control alfalfa weevil. This practice also has been shown to reduce the inoculum of spring black stem. Grazing the aftermath after a hard freeze in the fall also should reduce pathogen inoculum.

Using certified seed produced in arid areas will ensure maximum stand establishment and reduce the chance of seedling blight caused by the spring black stem fungus.

Confusing Black Stem with Insects - There is a tendency to confuse black stem with insect damage.  Alfalfa weevils are usually the first suspect, but there is should be no confusion on what is causing defoliation.  Alfalfa weevils are smallish green worms with a light stripe down the back and will feed on the top, newly emerging leaves, causing a shothole appearance.  Heavy damage will cause the tops to look frosted as the damaged leaf tissue turns brown.  Weevils should be easily found with a sweep net or bucket.  Black stem starts on the lower portion of the plant and works its way up.  Potato leafhoppers also work on the upper leaves as well as the bottom leaves, and will leave a characteristic “v” shape feeding pattern of the leaves.

Regardless of what is currently affecting alfalfa, at this time the best solution is to take a cutting and scout the regrowth.  It is NOT necessary to treat with an insecticide immediately after the hay is removed.  Give the field time for normal greenup and look for insects.  If insects are holding back the regrowth, then spray.

COMPREHENSIVE NUTRIENT MANAGEMENT PLANNING UPDATE

In January of this year The Nebraska Department of Environmental Quality (DEQ) notified more than 200 holders of Livestock Waste Control Operating Permits of the requirement to complete a “land application training” on manure management. This must be completed by December 31, 2003. Several options exist. One option is to complete an open book test on the Extension Circular EC02-179, Managing Livestock Manure to Protect Environmental Quality.  This EC is available through normal NU publication procedures. To receive both the required take home test and a copy of this EC, producers must contact Dennis Heitman, DEQ, PO Box 98922, 1200 'N' St., Suite 400, Lincoln, NE 68509-8922, 402-471-4239.

A second option for meeting the “land application training” requirement is to participate in an educational program approved by DEQ. One is the Extension CNMP class that is taught each winter. This program is based on the Nebraska’s CNMP Manure Application Workbook. Another option will be a satellite broadcast that is planned for this coming December. Others may include participation at one of several field days being planned for late summer by the University of Nebraska Extension CNMP Team.

In early May 2003 another manure application resource was published and distributed. Extension Circular EC02-720 Nebraska's CNMP Manure Application Workbook is a detailed step-by-step instruction manual for preparing the non-engineering parts of a permit application to operate a livestock facility. It also includes forms and instructions for annual manure nutrient management, and other records that are needed to maintain a permit after it is approved.

These two Extension Circulars are complimentary. The Managing Livestock Manure to Protect Environmental Quality explains the science behind the procedures, while the Nebraska's CNMP Manure Application Workbook provides the working tools for hands-on management. In the near future two more CNMP workbooks will be available: The Manure Storage Workbook, and the Odor Management Plan Workbook.  (Statewide CNMP Team, Richard DeLoughery, Coordinator  (402) 370-4061, Rick Koelsch, Charles Shapiro, and Charles Wortmann)

TIMING POST-EMERGENT  WEED CONTROL IN CORN

As the season progresses, not only does your corn grow, the weeds grow as well, competing with the crop for light,  water and nutrients. The longer the weeds compete with the corn the greater the yield losses. The level of crop yield loss will depend on environmental variables and (a) weed species composition within a given field, (b) weed density and (c) time of weed emergence relative to the crop growth stage.

In addition, to decide whether or not weed control is economically worthwhile, there is a need for understanding if a given weed infestation is likely to reduce yield if left uncontrolled. This establishes the rationale for introduction of the concept of critical period of weed control (CPWC).  The CPWC is a period in the crop growth cycle during which weeds must be controlled to prevent yield losses. Weeds that emerge before or after this period may not present a threat to crop yields. This information is essential in making decisions on the need for and timing of weed control and in achieving an efficient use of herbicides.

Research at the University of Nebraska has shown that each crop has a CPWC during which weeds must be controlled to maintain maximum yields. However, we also concluded that the length of such critical period is influenced by the cropping practices, for example by the nitrogen level in corn.

CPWC  in dry-land corn as affected by nitrogen: Studies were conducted in 1999 and 2000 at Mead and Concord.  Predominant weed species at both locations/years were velvetleaf, common waterhemp and green foxtail, with the densities ranging from 80-120 plants per square yard.  Nitrogen was applied immediately prior to planting as 46-0-0 and incorporated within one hour after application.

CPWC in corn was affected by the level of nitrogen fertilizer. Generally, a reduction in nitrogen fertilizer resulted in a longer CPWC, thus corn was the less tolerant crop to weed presence. For example, at zero N level, CPWC ranged from approximately 1^st to 11^th leaf stage of corn, based on a 5% acceptable yield loss (Table 1). This suggests that when no N-fertilizer is applied, the timing of weed control measure should start early in the season (at the 1^st leaf stage of corn) and needs to be maintained through the 11th leaf stage,  approximately the time of crop canopy closure.

Table 1: Critical period of weed control in corn based on 5% yield loss expressed as crop leaf stage (eg.V1)  and  days after crop emergence as affected by the level of nitrogen fertilizer.

	Nitrogen-Level		Time to Control Weeds	Time to Control Weeds
	lbs / acre		Corn leaf stage	Approximated Days After Crop Emergence
	N = 0		V1 - V11	8-45
	N = 55		V3 - V10	10-42
	N = 110		V4 - V9	15-39
	N = 210		V6 - V9	20-39

This data implies that an increase in N fertilizer delayed the timing of weed control and increased the corn tolerance to weed presence.  From a practical standpoint, an insufficient N can reduce corn tolerance to weeds and it can widen the window of a CPWC. Furthermore, from a nitrogen restriction-use and a regulatory perspective, anticipated restrictions on the level of  N use in corn may require more intensive weed management programs.

Cost of delaying weed control in corn crop: -            A common question among producers is “how much is it going to cost me if I delay weed control”.  In order to answer such question we graphed the yield loss data against the crop growth stage at the time of weed removal  (Figure 5).  In a practical situation one may decide to select , for example,  2%, 5% or 10% yield loss to signify the beginning of the critical period (time of weed removal).  This range will allow to adjust CPWC depending on the risk one is willing to take.  In our study, an arbitrary level of 5% yield loss was used to determine the beginning of CPWC in both crops  (see the 5% yield-loss-line at the Figure 5).

In order to determine the cost of delaying weed control, the curve above the arbitrarily selected  point (the beginning of CPWC) should be used. For example, if an arbitrarily selected point of CPWC is 5%, the 5% yield loss will occur if the weeds are removed at the 2^nd leaf stage in 0-N-level (Figure 5).  Delaying weed control to the 3^rd leaf stage will cause about 7% yield loss, in essence costing producer a 2 % of yield loss. Similar trend is observed for the later leaf stages at each of the four curves (Figure 5).

Therefore,  we conclude that delaying the time of weed removal, after the starting point of CPWC will cost a producer an average of 2 % in yield loss per every leaf stage of delay .  This recommendation is applicable up to canopy closure in corn (about 11 fully developed leaves).

To determine the actual economics of the cost of delayed control, the producer will have to convert the percentage yield loss of the actual target yield on his farm.  For example, if a target yield for corn is 100 bushels per acre, delaying weed control for every leaf stage of crop will cost producers about 2 bushels per acre of yield (thus 2% of 100 bushels per acre). In terms of actual economic loss,  it will be about $4 per acre for  every crop leaf stage of delay, assuming a price of $2 bushel for corn.

WEED SIZE:   Weed  size at the time of weed control measure is another concern.  In the corn study, the weeds were about the same size as the crop at the time of their removal except for the Mead site in 2000.   If the weeds are taller than corn they will shade the crop so the control should be initiated 4-5 days (1-2 leaves) prior to the beginning of CPWC.   If the weeds emerge 5-8 days after the crop they will not shade the crop that early in the season so the control can be initiated 5-10 days (2-3 leaves) after the beginning of critical period, as it is shown with the later start of the CPWC at Mead in 2000.                                         

The size of weed species will effect the herbicide use rates too, especially the rates of Roundup or various generic glyphosates  in Roundup-Ready soybeans.  It is well known that Roundup has much better activity on grassy than broad leaf species. Therefore the rates of 16 to 24 oz should provide control of most common annual grassy species (foxtails, barnyardgrass, field sandbur, woolly cupgrass, panicums) that are  3-8 inches tall. The same rates should control annual broadleaves (velvetleaf, lambsquarters, pigweeds, mustards) that are less than 6 inches tall. For taller grasses and  broadleaf species a full rate (32 oz) will be required. Higher rates of Roundup (40 oz  - 60oz) will be needed to control species such as  ivy-leaf morning-glory,  sweet clover, field bindweed, Venice mellow and various smartweeds (lady’s thumb, Pennsylvania smartweed, wild buckwheat,  etc).

Practical use of the CPWC and timing of weed control in herbicide tolerant crops:  A generally sound strategy, for example in Roundup-Ready corn will be to apply  Roundup tank-mixed with a residual herbicide at the beginning of the critical period, which will provide adequate weed control the entire critical period. In order to select appropriate herbicide mixtures for the weed spectrum at your farm, we suggest to consult the herbicide efficacy tables from the Guide for Weed Management in Nebraska (Extension Publication, EC-130)   (SK)