Late Summer Grasshopper Numbers May Require Treatment - UNL CropWatch, August 1, 2011
August 1, 2011
With current crop values, treatment of even light populations (10-20 per square yard) may pay.
Figure 1. Above: Adult differential grasshopper
Both grasshopper species have yellow bodies with black markings, but the redlegged species is usually slightly smaller and its hind legs are bright red. The upper part of the differential's hind legs has a series of chevron-like markings.
(Photos by Jeff Bradshaw)
Significant populations of grasshoppers are being reported in areas bordering crop fields in several parts of Nebraska. If these grasshopper species are one of the four major species that are likely to infest cropland, control may be warranted. These insects will continue to be a problem for the rest of the summer.
It’s best 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 more than 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 Grasshopers 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, scout and, if necessary, treat these adjacent untilled areas first. 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.
Estimating grasshopper densities is difficult and can only be done accurately with some practice. The best method for field borders or hatching areas is the square foot 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 you see in or jumping out of this area. Repeat this procedure 18 times and divide the total number of grasshoppers by two. This will give you the number of grasshoppers per square yard (9 square feet). Counting sites should be chosen at random and include varied vegetation.
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 you see in or jumping out of this area. Repeat this procedure 18 times and divide the total number of grasshoppers 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 1 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 (Figure 1). Numerous insecticides are labeled and effective for grasshopper control on various crops. They are summarized on the UNL Department of Entomology grasshopper web page at http://entomology.unl.edu/grasshoppers/insecticides.shtml. After grasshoppers become adults control success will be much more variable. If a range of rates is listed for a given insecticide, generally it’s best to use the higher rates 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 reenter 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, and it is only active against immature grasshoppers. 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 MAX is labeled for grass forage/fodder/hay crops and also can be used on field corn, seed corn, popcorn, alfalfa, sunflower, sorghum, soybeans and wheat. Numerous other products are specifically labeled for grasshopper control in various crops (see http://entomology.unl.edu/grasshoppers/gh_insecticides.htm).
For more information, see
- The UNL Department of Entomology's website, Grasshoppers of Nebraska. It includes the 2010 USDA APHIS grasshopper fall population survey data and the Nebraska Cooperative Rangeland Grasshopper and Mormon Cricket Suppression and Outreach Programs for 2011.
- UNL Extension Publications. This site offers publications on grasshopper identification and control in crops, range, yards, and gardens, as well as the book, Grasshoppers of Nebraska, a field guide to the many species of grasshoppers living in the state.
Bob Wright and Jeff Bradshaw