Heavy Grasshopper Infestations May Warrant Treatment
July 17, 2008
Significant populations of grasshoppers are being reported in areas bordering crop fields in several parts of Nebraska. In some situations these grasshoppers already have moved into and have begun to damage the crop. If these grasshopper species are one of the four major species that are likely to infest cropland, control may be warranted as the insects will continue to be a problem for the rest of the summer. The best approach to this problem is to try to control the grasshoppers while they are concentrated in the border areas before they spread into the crops and before they become adults and become harder to control.
Only four of the 100+ species of grasshoppers found in Nebraska normally damage field crops. These species are the twostriped, redlegged, differential and migratory grasshoppers. (For a detailed guide on identifying these four species see: Grasshopper Identification Guide for Cropland Grasshoppers Summer Feeding Species, EC1569. These species feed on a wide range of plants and are most often found in mixed habitats that include broadleaf weeds.
Because grasshoppers move into cropland generally from untilled areas surrounding crop fields, grasshopper scouting should be conducted in these adjacent untilled areas before significant damage is being seen in the crop margins. If grasshoppers have already invaded the field, also sample field areas to determine if control is warranted. The hoppers are most likely to move from these areas to adjoining crops when their food supply in these borders dries up.
Getting Accurate Grasshopper Counts
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, and varied vegetation should be included in the count area.
To sample for grasshopper densities 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. Average these estimates to get the number of grasshoppers per square yard.
When the number of grasshoppers per square yard has been estimated, use Table I to determine the potential need for treatment. The current increased crop value makes the economic return from treatment more likely; therefore, "light" populations (11-20 per square yard) in field borders are more likely to warrant treatment.
Grasshoppers are easiest to control before they become adults and have fully developed wings. Numerous insecticides are labeled and effective for grasshopper control on various crops. They are summarized on the UNL Department of Entomology Web site at http://entomology.unl.edu/grasshoppers/insecticides.shtml. Tremendous variability in control will occur after grasshoppers become 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.
Because grasshoppers do not like to enter dense plant canopies, most damage will be limited to field edges. Border treatments often are used to protect cropland from grasshoppers. In years with extreme populations, border treatments may not provide season-long control. 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 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. Be sure to read and follow harvest and grazing restrictions when spraying borders adjoining cropland.
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 only a few insecticides are labeled for both crops and the surrounding areas (rangeland/pasture or non-crop areas). Malathion and carbaryl are labeled on most crops along with range/pasture and non-crop areas, but these products are less effective on mature grasshoppers. Acephate (e.g. Orthene) is labeled for non-crop use, but the only crop it is labeled for is dry beans. Dimilin is labeled for range/pasture and 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 are esfenvalerate (e.g. Asana) and lambda-cyhalothrin (e.g. Warrior), because they are labeled for non-crop use and for use on several crops (corn, soybeans, sugar beets, dry beans, sunflowers, potatoes). Mustang is labeled for range/pasture and also can be used on most crops. Numerous other products are specifically labeled for grasshopper control in various crops (see http://entomology.unl.edu/fldcrops/).
For more information, including this year's grasshopper population predictions by USDA APHIS, see the Department of Entomology's site: Grasshoppers of Nebraska.
Panhandle REC, Scottsbluff