 | Fall Grasshopper Control
Gary Hein, UNL Dept. of Entomology, |
The abundance of grasshoppers in western Nebraska this fall is likely to cause problems as the winter wheat crop becomes established. Now is the time for wheat growers to evaluate their options for controlling grasshoppers in wheat fields and to plan accordingly.
Thresholds for grasshopper control in fields and field margins indicate that control would likely be needed if populations are above seven grasshoppers per square yard in the field or above 20 per square yard in field margins. These thresholds need to be modified for grasshopper control in winter wheat because the amount of leaf material available for the grasshoppers to eat is quite minimal. Also, at this time grasshoppers are full grown and can consume a great deal of plant material in a short time.
Because of these factors, light to moderate infestations in the field and borders can cause stand reduction or loss along the borders of
winter wheat fields. In most years, treating wheat borders or the area surrounding the wheat is adequate. A border treatment of 150 feet should be adequate, but narrower border treatments also will provide some benefit. However, this year with the extreme grasshopper populations in some areas, control may be difficult and multiple border treatments may be required. Several options are
available for controlling grasshoppers along the margins of these winter wheat fields. Timing and preparation are important to all of these options.
The most common method of grasshopper control along wheat field borders is to spray the borders for grasshoppers just before the wheat emerges. Timing is critical to optimum control. If the application is too early there will be no residual activity in the borders when the wheat emerges and grasshopper populations may build back too quickly. If it is applied too late some of the earliest emerging wheat may already be damaged. A border spray should be effective for 10-14 days. The residual of the treatments will vary, but it is important to monitor the wheat borders and the margins after treatment to make sure the grasshoppers do not re-enter the field and cause significant damage. A repeat application after 7-14 days may be required if grasshopper pressure persists. The best products for controlling grasshoppers in non-cropland borders (ditches and other waste areas) are Asana XL and Orthene 75S. Orthene also can be used for treating adjacent rangeland. These treatments can only be used to treat the areas adjacent to wheat and cannot be used on growing wheat.
Another time when grasshopper pressure may increase is when a frost kills the foliage on broadleaf weeds like sunflowers in areas adjacent to the wheat. Grasshoppers losing this forage source may
move quickly into wheat. However, a heavy frost also may reduce the grasshopper numbers.
The most effective treatments to control grasshoppers on growing wheat include dimethoate (Cygon), Furadan and Lorsban. Refer to the chemical labels for the proper rates, safety precautions and restrictions. When treating wheat, adjacent pasture or range, make sure to read and follow all label restrictions on grazing. If rate ranges are listed for these chemicals, generally the higher rates are used at this time of year because of the difficulty in controlling the larger adult grasshoppers.
Bait provides yet another option for grasshopper control. Sevin insecticide can be impregnated on bran bait at a rate of 2% or 5% to provide good control when properly timed just before wheat emergence. The key to this method is to evenly distribute bait and to reapply if the bait is no longer attractive to the grasshoppers. Attractiveness of the bait will be reduced substantially by moisture (rain or heavy dew).
Another option for controlling grasshoppers is to apply an insecticide around the field borders with your drill at planting time. This option will reduce the need to monitor your wheat fields but requires specialized equipment to apply the materials. Thimet and DiSyston are registered for use at planting on winter wheat. The biggest concern with these products is the capability to meter out the chemical at the proper rate. Adding products to the planter box is not acceptable because of safety concerns and the small granules rapidly sift to the bottom making metering very erratic. Also, this method will increase the possibility of phytotoxicity, especially with Thimet. Grass seeder attachments can be used to meter the granules, but this method can be quite variable. The use of some type of metering system (e.g. Gandy boxes) is the best method of applying these granules at planting. Some erratic results have also been seen with these granules during dry conditions. There may be enough moisture for the wheat seed to germinate and grow, but not enough moisture to activate the granules so that the wheat plants take up the chemical.
A second method of applying insecticides at planting is with the use of Furadan 4F. This product has a 24c registration and can be applied as a direct microtube injection into the seed furrow or it can be mixed with liquid fertilizer. The registered rate for this application is 0.25 to 0.5 fluid ounces per 1,000 row feet. It can only be applied as a border treatment up to 60 feet in width. Mixing Furadan with liquid fertilizer can improve effectiveness of the product, but problems of compatibility need to be addressed. The best application method with liquid fertilizer is with a closed injection system. Furadan is injected into the fertilizer line just ahead of the manifold where the product is mixed. The apparatus for direct injection for the microtube method and injection into the liquid fertilizer line can be obtained from Agri-Inject Inc. (Yuma, CO 303-848-5336).
Tank mixes with Furadan need to be undertaken cautiously. This product is highly toxic and any method that increases its handling will significantly increase risk to the handler. Compatibility must be checked and constant agitation maintained to keep the product well mixed. One formulation designated as Furadan 4F-Code 2837 is specifically made for mixing with liquid fertilizer. Anyone considering this method should check with their dealer to make sure they take all the proper precautions in mixing and handling it.
These treatments should give adequate control of low to moderate grasshopper populations. If grasshopper populations are high, complete control may be difficult. In this case the borders can be planted at an extra high (perhaps double) wheat density to allow for possible plant loss. This would probably only be needed in the first one or two passes with the drill on the edges.
The key to managing this problem is in being prepared. This includes having access to the right equipment and applying treatments at the right time. Fields should be monitored regularly even after treatment to monitor its effects and to determine when it is no longer providing control. Under extreme conditions a combination of methods or multiple applications may be necessary.
For more information on grasshoppers, click HERE for "A Guide to Grasshopper Control in Cropland", NebFact 97-328.