2003 Newsletters

The weather has really helped us out this year as far as preventing a widespread grasshopper infestation.  However, there are some “hotspots” scattered around with large numbers.  This is the last week for reasonably easy control of grasshoppers.  Waiting to treat after this week will make it more difficult as the hoppers leave their hatching beds and spread out into crops, and get larger.

Western Bean Cutworm

After the small, dark brown larvae hatch on corn plants; they move to the whorl or tassel to feed on the tender yellow leaf tissue or on the tassel itself. Once the tassel emerges or if it has already emerged when the eggs hatch, the larvae will move to the green silks. The developing larvae will feed on the green silks moving down the silk channel until they reach the ear tip. The larvae will feed in the ear tip until they are fully developed. If the infestation on one ear tip is so great that the larvae become crowded, a few individuals may move outside the ear and begin to feed on the side of the ear.

Even though field scouting for western bean cutworm in field corn should begin when the first moths are caught, control decisions should be made shortly after the moth flight peaks. The moth flight usually peaks between July 10 and July 24.  When scouting for western bean cutworms in corn, check 20 plants in at least five areas of each field. Look for eggs on the top surface of the upper most leaf or look for larvae in the tassel. If 8% of field corn plants, 5% of seed corn plants or 5% of popcorn plants have egg masses or larvae, consider applying an insecticide.

Western bean cutworm moths prefer to lay eggs in corn plants that are in the late whorl stage compared to those that have completely tasseled. Pay particular attention to later planted fields or those with uneven development. Western bean cutworm eggs that hatch when corn plants are in the whorl stage of growth have a high rate of survival. The larvae are well protected in the whorl or tassel.

If an insecticide treatment is warranted in corn, it should be made when 95% of the plants in a field have tasseled. This timing of the application increases the chance that the worms will be exposed to the insecticide resulting in better control.  Chemigation has provided very good control of this insect, even at lowest labeled rates. Asana, Ambush, Baythroid, Pounce, Lorsban, Capture, Mustang, Penncap-M, Warrior, and Seven are all labeled for control of western bean cutworm.

Need Western Bean Cutworm Eggs

We are looking for western bean cutworm eggs for research purposes.  If you have a field where we could come in and gather eggs please let us know.  Call Keith, Tom or Jerry at the Haskell Ag Lab, 402-584-2261.

Glyphosate sometimes fails to control weeds adequately. Generally, failures are related to climatic conditions such as rainfall before the herbicide is rainfast, drought, failure to use AMS, too high water volume, stress on weeds growing in dense stands, and cold temperatures, especially under cloudy conditions. The glyphosate rate should be selected based on weed species, stage of growth, vigor and size of weeds, dust, and cost.

Weeds under drought stress caused by lack of rainfall or high weed density, or covered with dust are difficult to control, especially if low rates of glyphosate are used. Approved AMS should be used with all glyphosate products. Diseases, insects, and either too little or too much water also may cause stress. Do not add other surfactants, wetting agents, or buffering agents unless the label requires them. Glyphosate efficiency may be reduced on some weed species when mixed with fertilizer solutions.

Several factors can contribute to less than desirable control, depending on the situation and time of year glyphosate is applied. Glyphosate works best under good conditions for plant growth, especially when weeds are small. Labels give specific information on weed height and rate to use for different species. Weeds differ in the sensitivity to glyphosate. Table 1 lists those weeds that are tougher to control with glyphosate. Unless a pre-emergence herbicide is added, two applications of glyphosate may not be sufficient to control weeds season long in Roundup Ready crops. Some weeds such as carpetweed, tumble pigweed and puncture vine may emerge after the last application. Perennial weeds may escape the glyphosate rate used in Roundup Ready crops because rates are too low.

Table 1. Weeds that may escape glyphosate in Roundup Ready crops or fallow in Nebraska. 
Species; Mechanism

Glyphosate should not be allowed to stay in the tank for more than 24 hours because glyphosate degrades and additional glyphosate would need to be added to compensate for the loss

Reasons for slow control in the early spring or late fall: Speed of kill depends on the rate used and air temperature. It takes longer to kill volunteer wheat, downy brome, and jointed goatgrass after September 20 than before. In the spring, cool temperatures usually slow the kill of volunteer wheat, annual bromes, and jointed goatgrass. However, suggested rates usually end with the same degree of control if applied early or late. Tough to control weeds, such as prickly lettuce or kochia, may need to be tank-mixed with 2,4-D or dicamba or a commercial formulation containing these herbicides should be used. This spring, glyphosate failed to control kochia in thick stands under drought stress, but did control scattered plants. The glyphosate label states that kochia should not be treated in the button stage.

Reasons for poor weed control in Roundup Ready soybean or Roundup Ready corn: Poor control is related to stage of weed growth, weed species, and weather conditions. Sometimes glyphosate + Pursuit may not control emerged common lambsquarters. Cloudy cool weather may lead to reduced control of some Russian thistle and kochia biotypes. Dust along gravel roads has reduced control with glyphosate. Velvetleaf control has diminished with evening applications of glyphosate.

Reasons for poor weed control in stubble: Glyphosate performance in wheat stubble fields is affected by many factors. These include spray volume, time of day when sprayed, temperature, rainfall, weed species, stage of weed growth, weed density, water logging, drought stress, wind, climatic conditions, water hardness, distribution of crop residues, cropping history, and vigor of the winter wheat crop. Other factors reducing weed control with herbicides include cutting off tops of weeds during harvest, glyphosate-tolerant weeds, thin wheat stands, dust, and poor straw and chaff distribution. Sometimes large weeds intercept the herbicide so that it doesn’t reach shorter weeds. In the case of dense stubble, consider increasing the rate. This may be why glyphosate labels suggest increasing the glyphosate rate in no-till systems.

Tank mixes of glyphosate and atrazine have led to reduced control of barnyardgrass and yellow foxtail. There is some evidence that evening spraying reduces the antagonism caused by atrazine.

If weeds are cut off when harvesting, you may need to allow sufficient regrowth to occur before applying glyphosate. With broadleaf weeds it may be advisable to add 2,4-D or dicamba to the mix. Weeds must be sprayed before they produce seeds or use to much soil water.

Operators becoming more knowledgeable about sprayers, herbicides, weeds, and conditions that improve performance can overcome many of these factors. Simply adding ammonium sulfate to the water before adding glyphosate can improve control, especially when weeds are under stress. Increasing the glyphosate rate can solve many of the poor weed control problems. Using 100% overlap will decrease strips from plugged tips.

Climatic conditions that affect weed control: Conditions at the time of herbicide application are important in obtaining maximum control. Glyphosate is the most popular herbicide to use after wheat harvest. High temperatures (95oF or higher) zero to three days before spraying reduces control with glyphosate. Rain within 24 hours reduces control of some weed species with some formulations of glyphosate. Some labels state they are rainfast within two hours; however, some species of weeds, such as barnyardgrass, under drought stress may not be controlled adequately. Most applicators wait until dew is off the plants before spraying. Time of day may make a difference with some weeds. Poor control of velvetleaf has been observed with glyphosate when applications are made in the evening. In surveys taken after wheat harvest we have not seen differences in weed control. However, in research plots control of barnyardgrass decreases with lower rates applied in the evening, especially when under drought stress. Rain up to a week after spraying stressed weeds reduces control, probably due to inadequate translocation of glyphosate.                                                        

Gail A. Wicks

04-10-03
04-30-03
05-21-03
05-28-03
06-04-03
06-11-03
6-17-03
6-25-03
7-8-03