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Because grasshoppers move into cropland from untilled hatching beds around field borders and in grasslands, grasshopper surveys should be conducted in these adjacent untilled areas early in the season. If grasshoppers have already invaded the field, they can be sampled to determine if control is warranted.
Sampling /thresholds
Estimating grasshopper densities is difficult and can only be done accurately with some practice. The best method for determining grasshopper density in field borders or hatching areas is to count the number of grasshoppers by using the square-foot method. With practice, this approach can provide good estimates of hopper density. To use this method, randomly select an area several feet away and visualize a one-square foot area around that spot. Walk toward this spot while watching this square-foot area and count the number of grasshoppers that you see in or jumping out of this area. Repeat this procedure 18 times and divide the total number of grasshoppers you saw by two. This will give you the number of grasshoppers per square yard (9 square feet). Counting sites should be chosen at random. Just after hatching, when grasshoppers are small, they will be difficult to see and you likely will underestimate the true hopper density. When sampling, vary the vegetation in the count area, and sample both north and south facing slopes.
Who's whoFor photos, illustrations and more information on identifying grasshopper species, see the June 13, 2003 CropWatch. |
To sample for grasshoppers within fields where grasshopper density will be lower, use the same method except visualize and count the hoppers in a square yard area. Because of the difficulty of seeing hoppers in this larger area, counts will be somewhat less accurate. Take 18 samples and divide the total by 18 to arrive at the average number of grasshoppers per square yard. When the number of grasshoppers per square yard has been estimated, use Table I to determine if treatment is necessary. While sampling it is important to determine the species present and the approximate stage (instar) of the grasshoppers. This is best done by using a sweep net to allow capture of a representative sample of hoppers.
Grasshopper control in cropland
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Grasshoppers are easiest to control in the 3rd and 4th instar stages before they become adults. The size of these stages will be less than ½ inch in smaller species and 3/4 inch in larger species. Numerous insecticides are labeled and effective for grasshopper control on various crops. Most of these will be effective when grasshoppers are immature. Tremendous variability in control will occur later in the summer when the grasshoppers are adults. If a range of rates is listed for a given insecticide, the higher rates generally should be used once adults are present. Always follow the recommended label rates, application directions, and restrictions.
Often border treatments are used to protect cropland from grasshoppers. However, if populations are high, border treatments may not provide season long control. The size of the border treatment needed is difficult to determine. It may be effective with as little as 150 feet or as much as 1/4-1/2 mile may be needed if the grasshopper source area is large. A border spray should be effective for at least 7-14 days, depending on re-infestation pressure. Also, the residual activity of the treatments will vary with the chemical and environmental conditions. It is important to monitor the border areas and crop margins after treatment to make sure grasshoppers do not re-enter the field. When spraying borders adjoining cropland, be sure to read and follow harvest and grazing restrictions.
When treating borders, it is often necessary to treat the edge of the crop to reduce hopper numbers that have already moved into the field margin. One of the biggest problems with these treatments is that few insecticides are labeled for treating both crops and the surrounding areas, whether it be rangeland/pasture or non-crop areas. Malathion (e.g. Atrapa) and carbaryl (e.g. Sevin) are labeled on most crops along with range/pasture and non-crop areas. Acephate (e.g. Orthene) is labeled for non-crop use, but the only crop it is labeled for is dry beans. Dimilin is labeled on range/pasture and for non-crop use, but it's only additional label is on soybeans. One advantage of Dimilin as a border spray around corn would be the lowered impact on natural enemies, especially those effective on spider mites. Perhaps the best products for this type of treatment would be Asana or Warrior, because they are labeled for non-crop use and for use on several other crops. Of these products, Asana and Warrior will provide better control once grasshoppers have become adults, but again control of adults may be variable.
Further information
Current lists of pesticide registrations for various crops can be found at the University of Nebraska Department of Entomology Web site at http://entomology.unl.edu/fldcrops/pestipm.htm The following Web sites contain extensive information on grasshoppers and grasshopper management:
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| Published by University of Nebraska-Lincoln Extension in the Institute of Agriculture and Natural Resources Cooperating with the counties and the U.S. Department of Agriculture. | ||
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