GRASSHOPPER UPDATE

Visits to Knox and Holt counties last Friday have confirmed that there are areas in northeast Nebraska, particularly in the western counties, where grasshopper potential is still very high, particularly in pastures and forages. Some fields with 20 or more newly hatched hoppers per square foot were seen in several fields. Many fields had experienced a hatch of grasshoppers over two weeks ago but much of this first hatch have suffered mortality due to the cool moist weather (an exception is Holt Co. which up to Monday had not received as much rainfall as other areas). While some fields in the drier areas are nearing decision-making time, most growers will be able to use the next two weeks to assess populations and make plans. Not every area will have high populations.

The recent rain of the night of June 9^th will likely have a favorable impact, at least as far as hoppers go. Heavy rains should cause substantial mortality to 1^st and 2^nd instar grasshoppers. Therefore next week will be a time to begin reevaluating earlier surveys. We will review scouting, economic thresholds and control alternatives next week. (KJ)

EUROPEAN CORN BORER MOTHS ARE FLYING

European corn borer moths have been flying for a couple weeks in the southern half of Nebraska and about a week in northeast Nebraska. 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, we should not be complacent about the European corn borer in non-Bt cornfields.

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 genetically engineered Bt (Bacillus thuringiensis) 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 or only partially controlled by Bt 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. The weather of June 9^th in northeast Nebraska should have caused high mortality of moths and small larvae.

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

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. In this example assume that 50 percent of the corn plants in a field are damaged with an average of 4 larvae/damaged plant, that final yield expectation is 125 bu/acre, and that corn is worth $2.75/bu. Insecticide costs are $8.00/acre, and that application costs are another $4.00/acre (total = $12.00/acre).

Average number of larvae/plant (percent of injured plants X number of larvae/injured plant) 50% x 4 = 2	2
Potential yield loss if all larvae survive (number of larvae/plant X 5% loss/borer/plant) 5 x 2	10 %
Potential bushel loss (potential yield loss X yield potential) 125 x 10%	12.5 bu
Potential dollar loss (potential bushel loss X estimated price of corn) 12.5 X $2.75	$34.38
Preventable loss (potential dollar loss X proportion of larval controlled $34.38 x 75%	$25.78

If preventable loss is greater than the cost of an insecticide treatment, the treatment is economically justified (in this case a 13.78 dollar an acre advantage to treat $25.78 - $12.00).

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.

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.

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. 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)

TIMING OF POST-EMERGENT WEED CONTROL IN SOYBEAN

With the advances of herbicide tolerant soybean (eg. Roundup-Ready) there is still a constant dilemma on how to " time " post-emergence weed control. 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 row spacing in soybean.

Time of weed removal as affected by soybean row spacing:

Critical time of weed removal is a time in the crop growth cycle when weed control needs to begin in order to prevent yield losses. Studies were conducted in 1999 at Mead, 2000 and 2001 at Mead and Concord. Predominant weed species at both locations/years were velvetleaf, common waterhemp and green foxtail, with the densities ranging from 70-100 plants per square yard.

The critical time of weed removal was significantly influenced by row spacing. Generally, an increase in row spacing resulted in a need for earlier weed removal, thus a less competitive crop. For example, beginning of the CPWC in the wide-row soybean (30") was approximately at the 1^rd trifoliate stage, based on a 5% acceptable yield loss level (Table 1). This suggests that in the wide-row soybeans the control measure should start early in the season (at the 1^st trifoliate stage). Beginning of the CPWC in the 15" rows was delayed and corresponded approximately to the 2^nd trifoliate stage, compared to the 3^rd trifoliate stage in soybean grown in the 7.5" rows (Table 1).

Table 1: The beginning of CPWC in soybean based on 5% yield loss expressed as crop leaf stage (eg.V1) and days after crop emergence (DAE) as affected by the row spacing, at two locations in 1999, 2000 and 2001.

	Row spacing		Time to control weeds	Time to control weeds
	inches		Soybean leaf stage	Days after crop emergence
	7.5		V3	19
	15		V2	15
	30		V1	9

This data implies that reducing row spacing delayed the timing of weed control and increased the tolerance of soybean to weed presence. The mechanism of soybean tolerance needs to be determined yet, although we believe it is related to the crop shading effects. The speculation is that even though the weeds are present in the narrow row soybeans they are not growing as vigorously and they are not as competitive against the crop, due to crop shading effects. Furthermore, from a practical standpoint, these results indicate that a reduction in soybean row spacing increases soybean tolerance to weeds and it may require less intensive weed management programs (eg. weed control measure applied perhaps once or twice).

Cost of delaying weed control: The commonly asked question among producers is "how much is it going to cost me if I delay weed control". Possible reasons for delayed weed control may include weather constraints (rainfall, wind), and time constraints due to large acreages to spray. In order to answer the above question the yield loss data from the above studies were pooled among years-locations and graphed against the crop growth stage at the time of weed removal in corn and soybean (Figure 1).

The 2 percent yield loss per every leaf stage of delay past the critical stage of weed control, was determined as the cost of delaying weed control in soybean. For example, the time to control weeds in 7.5 inch rows soybean is the V3 stage (third trifoliate-Table 1), if weed control is delayed to the V4 (fourth trifoliate), it will cost a producer about 2 percent in yield losses due to prolonged competition from weeds. The same is true if weed control is delayed past the recommended critical time in other soybean row spacings (Figure 1). This recommendation is applicable up to the R3 stage in soybean (beginning pod). If the weed control is delayed further than these indicated stages the yield losses will be much higher than suggested.

In terms of actual economic losses in soybean, it will be about $5 per acre for every soybean leaf stage of delay, assuming a price of $5 bushel and a yield goal of 40 bushels.

WEED SIZE:

Weed size at the time of weed control measure is another concern. If the weeds emerge 4-5 days before the crop or they are taller than crop 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 5-10 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.

The size of weed species will effect the herbicide use rates too, especially the rates of Roundup or any generic glyphosate 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 - 60 oz) will be needed to control species such as ivy-leaf morning-glory, sweet clover, field bindweed, Venice mellow and various smartweed species (lady's thumb, Pennsylvania smartweed, wild buckwheat, etc).

Practical use of the CPWC and timing of weed control in herbicide tolerant crops: Roundup-Ready soybeans have received high levels of acceptance in our state. The concept of critical period of weed control is an important part of integrated weed management in answering a fundamental question IF and WHEN to apply post-emergence herbicide.

A generally sound strategy in Roundup-Ready soybeans will be to apply Roundup tank-mixed with a residual herbicide at the beginning of the critical period, which will provide adequate weed control for 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)

Figure 1: Soybean yield loss and beginning of CPWC as influenced by the timing of weed removal and row spacing.