EXTENSION 
INSTITUTE OF AGRICULTURE AND NATURAL RESOURCES         
UNIVERSITY OF NEBRASKA-LINCOLN 
NORTHEAST RESEARCH & EXTENSION CENTER       
601 EAST BENJAMIN AVENUE, SUITE 104
NORFOLK, NE 68701-0812
PHONE: (402) 370-4000 
FAX (402) 370-4010

Scouting For Corn Rootworm Larvae

Corn rootworms should be in the second instar stage by now in most of northeast Nebraska . For the few that use post-emergence treatments for rootworm control, cultivation application of granules, or post applications of Furadan 4F should be applied now for optimum corn rootworm control. Lorsban 4E applications through a center pivot can also be applied at this time. When applying Lorsban through a pivot, remember to use enough water to wet the soil to at least 4 inches down to move the Lorsban through the root zone. Once the initial movement has stopped, the Lorsban will bind tightly and will not be moved much further with additional moisture.

Rootworm larvae are white, with a brown head capsule and a dark spot on the rear end. This is the only insect in the soil with this "double-headed" appearance. Three pairs of small legs will be found just behind the head. Other insects in the soil are ground beetle larvae, with large orange to brown heads and jaws. Nematodes are thin, white and hair-like in appearance, often curling in loops. These nematodes visible to the naked eye feed on organic matter and are not harmful to the corn plant.

Most people can find larvae in rootworm Bt corn fields and fields previously treated with an insecticide, and want to know if a rescue treatment is necessary. This is a difficult decision to make, as many factors are involved in the decision. No insecticide or Bt corn will give 100% control. The goal is to protect the root mass from extensive damage, and some larvae will survive. All treatments can perform poorly at one time or another, mostly due to extreme environmental conditions over which we have no control. All continuous corn fields should be scouted for rootworm larvae. If 3 or more larvae are found on average per plant, a rescue treatment may be necessary. This threshold is very subjective and allowance should be made for the skill of the scout. Experienced consultants will find the larvae much easier than beginners and may want to raise their threshold based on their previous experience. Yield losses due to rootworm feeding are difficult to correlate with larva numbers and will vary depending on environmental conditions, variety, root regrowth, etc. Lodged plants may impede harvest but not necessarily reduce yield if adequate nutrients and water are readily available to the plant. Some entomologists now feel that a corn plant can lose a whole node of roots without yield reduction.

To check for larvae, dig a 6 - 9 inch cube of soil centered on the corn plant. Sample a minimum of 2 consecutive corn plants at each of five different sites in a field. Carefully search through the soil and plant roots for the creamy white, slender worm with a dark brown head and brown tail plate (the double head appearance). Visible larvae will range from about 1/8 to 2 inch long. Placing the root-soil mass on a dark surface such as a black trash bag may help spot the larva as they fall from the dirt.

Any of the insecticides registered for cultivation application can be applied to the soil at the base of the plant and cultivated in. Under irrigated conditions, flying the material on and then watering may provide good results. If plants are too large for cultivation, apply by air and water in if possible. Rescue treatments will not guarantee total root protection but should provide a barrier between rootworms and the root mass, allowing developing roots to establish, and injured roots to recover faster. (KJ)

Grasshoppers Hatching, Plan Ahead For Management and Control

Grasshoppers have begun hatching for the last couple of weeks now in northeast Nebraska . The dry weather will favor survival but infestations will be spotty with some areas with high populations and others nearly untouched. Populations will likely increase as you move further west in the state. Now is the time to get serious about keeping on top of grasshopper populations in northeast Nebraska . Areas that had problems the last several years are probably more at risk, but farmers should be aware of the potential problem and begin planning now to prevent serious damage to pasture, alfalfa, and row crops.

Grasshopper primarily damage wheat, alfalfa and corn, but during high population years may feed on any other crops, trees, and shrubs. Four grasshopper species cause nearly all damage to crops in Nebraska . They are the migratory, differential, two-striped, and red-legged. These hoppers prefer habitats with a variety of host plants, including both grasses and broadleaf weeds. As a result, they prefer cropland settings with undisturbed areas such as roadside ditches, crop borders, abandoned cropland, and overgrazed pastures or rangeland.

Most grasshoppers overwinter in the egg stage. Females may lay up to 300-400 eggs during the summer and fall under favorable conditions. Eggs are laid in pods of between 8 to 30 eggs. Pods are deposited in the upper few inches of undisturbed soil, such and grasslands, pastures, ditches, field margins, etc. Some grasshoppers prefer to lay eggs in soil surrounded by roots of grasses; other species select open areas with accumulations of surface debris. A few species of grasshoppers overwinter as nymphs but are normally few in number and do not cause economic damage. Grasshopper nymphs go through 5 larval instars. After each instar they shed their skins and grow larger. It takes 6 to 8 weeks for nymphs to develop into adults. Only adults have wings and move readily out of hatching areas.

Hatch of each species may be spread over a month or more. The earliest hatching problem hopper is the two-striped, which begin to hatch from mid-May to early June. The migratory will begin to hatch about a week after the two-striped, and the red-legged and differential will begin to hatch about three weeks after the two-striped.

Since most grasshoppers lay their eggs in untilled areas, to infest a crop they have to migrate from field margins or other areas of untilled soil. One exception is the lesser migratory which may deposit eggs throughout a field, particularly alfalfa.

Grasshopper Management - Grasshoppers are easiest to control before they become adults. The best time to control grasshoppers is during the 3rd and 4th instars. This generally occurs in late June through mid-July, when most of the eggs will have hatched. Scout for hoppers my moving through an area and estimating the number of hoppers per square yard that jump out as you move through. The following table is a guideline for treatment.

Grasshopper Population Category	Field	Field Margin	Treatment Necessary?
Non-economic	0-2	5-10	No
Light	3-7	11-20	Depends on size, species, type of crop
Moderate	8-14	20-40	Probably
Abundant	15 or more	41 or more	Yes

Border treatments should be adequate for grasshopper control providing the timing of application is before the hoppers move from hatching areas into crops. A border treatment of 150 feet should be adequate depending on the size of the grasshopper source area. Multiple border treatments may be necessary in areas with higher populations. A border spray should be effective for 7 - 14 days. You must keep monitoring crop margins after treatments to make sure grasshopper do not reenter the field.

If a range of rates is listed for a chemical, higher rates should generally be used later in the season when hoppers are larger. Several insecticides are effective for grasshopper control. Follow the recommended label rates, application directions, and restrictions. Some commonly used insecticides labeled for corn, soybeans and alfalfa are (chemical name in parentheses, these are available as generics):

Asana, Baythroid, Cygon (dimethoate), Lorsban (chlorpyrifos) Furadan, Mustang Max, Penncap M, Proaxis, Warrior (lambda cyhalothrin), and Sevin. (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 st 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 constrains (rainfall, wind), and time constrains due to large acreage 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 passed 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 (Table 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 - 60oz) 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 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)

 

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